ALMA (Atacama Large Millimeter/submillimeter Array)
The ALMA Observatory is an international astronomy facility, a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and NINS (National Institutes of Natural Sciences) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the NRC (National Research Council) of Canada and the NSC (National Science Council) of Taiwan and by NINS of Japan in cooperation with the Academia Sinica (AS) in Taiwan, and KASI (Korea Astronomy and Space Science Institute) Korea. 1) 2)
ALMA construction and operations are led on behalf of Europe by ESO on behalf of its Member States; by NRAO (National Radio Astronomy Observatory), managed by AUI (Associated Universities, Inc.), on behalf of North America; and by NAOJ (National Astronomical Observatory of Japan) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
Figure 1: Global partnerships of the ALMA Program (image credit: ALMA partnership) 3)
ALMA isthe largest astronomical project in existence, it is a single telescope of revolutionary design, composed of 66 high precision antennas (forming a sparse array of antennas) of 12 m and 7 m in diameter. ALMA is located at a truly unique and unusual place: the Chilean Atacama desert. While the astronomers will operate the telescope from the OSF (Operations Support Facility) Technical Building, at 2,900 m above sea level, the array of antennas will be located at the Altiplano de Chajnantor, a plateau at an altitude of 5,000 m altitude. This location was selected because of many well justified scientific reasons, particularly dryness and altitude. The ALMA site with the average annual rainfall below 100 mm is the perfect place for a new telescope capable of detecting radio waves just millimeters in wavelength. Indeed, radio waves penetrate a lot of the gas and dust in space, and can pass through the Earth’s atmosphere with little distortion. However, if the atmosphere above ALMA contained water, the radio signals would be heavily absorbed – the tiny droplets of water scatter the radio waves in all directions before they reach the telescope, and would degrade the quality of the observations.
Furthermore, the flat and wide land at the ALMA site is suitable for the construction of a large-scale array. Considering these aspects, the ALMA Observatory will not only be unique because of its ambitious scientific goals, and the unprecedented technical requirements, it will also be unique because of the very specific, harsh environment and living conditions in which the most challenging radio telescope array will operate with high efficiency and accuracy.
ALMA is an international astronomy facility, a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led on behalf of Europe (ESO), North America (NRAO/AUI), and East Asia (NAOJ). The JAO (Joint ALMA Observatory) provides the unified leadership and management of the construction, commissioning and operation of ALMA. The JAO coordinates the ALMA Development Program in order to effectively manage the technological evolution of the ALMA facility. Periodically, solicitations (“calls”) are issued by each of the international partners to identify and fund development initiatives (“upgrades”) which will enhance the performance of the ALMA facility. The implementation of ALMA upgrades are assigned on a competitive basis.
Figure 2: A state-of-the-art telescope to study light with wavelengths of about one millimeter, shining from some of the coldest objects in the Universe, ALMA is a cooperation of the European Southern Observatory (ESO), together with its international partners. The site of ALMA is the 5000-m altitude Chajnantor plateau in northern Chile, one of the driest places on Earth (video credit: ESO, ALMA (ESO/NAOJ/NRAO), C. Malin , P. Horálek, Liam Young, B. Tafreshi, J. J. Tobin (University of Oklahoma/Leiden University), M. Kaufman, Theofanis N. Matsopoulos, H. H. Heyer, S. Argandoña and H. Zodet. Music by Movetwo, Published on Dec 7, 2016)
Figure 3: The Atacama Large Millimeter/submillimeter Array is an astronomical interferometer of radio telescopes in the Atacama desert of northern Chile. Since a high and dry site is crucial to millimeter and submillimeter wavelength operations, the array has been constructed on the Chajnantor plateau at 5,000 metres altitude, near Llano de Chajnantor Observatory and Atacama Pathfinder Experiment (video credit: ESO, Published on Oct 7, 2017)
Major ALMA Facilities
ALMA will be the world’s most powerful telescope for studying the Universe at submillimeter and millimeter wavelengths, on the boundary between infrared light and the longer radio waves. However, ALMA does not resemble many people’s image of a giant telescope. It does not use the shiny, reflective mirrors of visible- and infrared-light telescopes; it is instead comprised of many “antennas” that look like large metallic satellite dishes. 15)
Several antennas have already been installed in the harsh conditions of the 5000 m altitude Chajnantor plateau, and more are under construction at the 2900 m altitude OSF (Operations Support Facility). When ALMA is fully operational, visitors to Chajnantor will encounter 66 antennas, 54 of them with 12 m diameter dishes, and 12 smaller ones, with a diameter of 7 m each.
Figure 4: Photo of the first ALMA 12 m antenna, manufactured by Mitsubishi Electric Corporation (image credit: ALMA ,ESO/NAOJ/NRAO)
The most visible part of each antenna is the dish, a large reflecting surface. Most of ALMA’s dishes have a diameter of 12 m. Each dish plays the same role as the mirror of an optical telescope: it collects radiation coming from distant astronomical objects, and focuses it into a detector that measures the radiation. The difference between the two types of telescopes is the wavelength of the radiation detected. Visible light, captured by optical telescopes, is just a small part of the spectrum of electromagnetic radiation, with wavelengths between roughly 380 and 750 nm. ALMA, in contrast, will probe the sky for radiation at longer wavelengths from a few hundred µm to about 1 mm (about one thousand times longer than visible light). This is known, perhaps unsurprisingly, as mm and sub-mm radiation, and lies at the very short-wavelength end of radio waves.
This longer wavelength range is the reason, why ALMA’s dishes are not mirrors, but have a surface of metallic panels. The reflecting surfaces of any telescope must be virtually perfect: if they have any defects that are larger than a few percent of the wavelength to be detected, the telescope won’t produce accurate measurements. The longer wavelengths that ALMA’s antennas detect mean that although the surfaces are accurate to within 25 µm — much less than the thickness of a single sheet of paper, the dishes do not need the mirror finish used for visible-light telescopes. So although ALMA’s dishes look like giant metallic satellite dishes, to a submillimeter-wavelength photon (light-particle), they are almost perfectly smooth reflecting surfaces, focusing the photons with great precision.
Not only are the dish surfaces carefully controlled, but the antennas can be steered very precisely and pointed to an angular accuracy of 0.6 arcseconds (one arcsecond is 1/3600 of a degree). This is accurate enough to pick out a golf ball at a distance of 15 km.
ALMA will combine the signals from its array of antennas as an interferometer — acting like a single giant telescope as large as the whole array. Thanks to the two antenna transporter vehicles, astronomers will be able to reposition the antennas according to the kind of observations needed. So, unlike a telescope that is constructed and remains in one place, the antennas are robust enough to be picked up and moved between concrete foundation pads without this affecting their precision engineering.
In addition, the antennas achieve all this without the protection of a telescope dome or enclosure. The dishes are exposed to the harsh environmental conditions of the high altitude Chajnantor plateau, with strong winds, intense sunlight, and temperatures between ±20 ºC. Despite Chajnantor being in one of the driest regions on the planet, there is even sometimes snow here, but ALMA’s antennas are designed to survive all these hardships.
The production of the antennas is being shared between the ALMA partners. ESO has ordered twentyfive 12 m antennas, with an option for an additional seven, from the AEM Consortium (Alcatel Alenia Space France, Alcatel Alenia Space Italy, European Industrial Engineering S.r.L., MT Aerospace). The North American partners have placed an order of the same size with Vertex RSI, while the four 12 m and twelve 7 m antennas comprising ALMA’s ACA (Atacama Compact Array) have been ordered by NAOJ from MELCO (Mitsubishi Electric Corporation).
Apart from the obvious difference in size between the 12-meter and 7-meter antennas, careful observers will spot subtle differences in the antenna design from each partner. However, all the antennas are designed to meet the stringent technical specifications, and work together smoothly as parts of the whole. These state-of-the-art dishes, combined in a single revolutionary telescope, reflect the cooperative nature of the global ALMA project.
Figure 5: Photo of the ALMA antenna array (image credit: ALMA partnership, Ref. 3)
Figure 6: The 12th 7 m antenna developed by Japan was delivered to the high site in Chajnantor on April 29, 2013. Now all the 16 antennas of the ACA (Atacama Compact Array) are installed at the Array Operations Site at an altitude of 5,000 m, waiting to unveil secrets of the universe (image credit: ALMA partnership) 16)
Figure 7: The final antenna of the ALMA project is here seen arriving to the high site at the ALMA Observatory, 5000 m above sea level. Its arrival completes the complement of 66 ALMA antennas on the Chajnantor Plateau in the Atacama Desert of northern Chile (image credit: ALMA, ESO/NAOJ/NRAO, A. Marinkovic) 17)
ALMA Front End Integration Centers: A construction project like ALMA, involving several partners in four different continents, requires consensus on several organizational and managerial decisions concerning the actual execution of certain construction activities. Several different scenarios for assembling and integrating the Front End components were extensively studied. This study revealed that the best solution was a “parallel approach”, installing half of the Front End in Europe and the other half in North America with identical and parallel procedures. This scenario was preferred in view of logistics, organization and program risks. Mainly based on considerations of risk mitigation, the parallel FEIC (Front End Integration Centers) was selected. The European FEIC is located at Rutherford Appleton Laboratory (UK) and the North American FEIC at NRAO. A third FEIC is installed in Taiwan to carry out the integration of Front End assemblies required for the antennas supplied by NAOJ.
Figure 8: Aerial view of the ALMA OSF (Operation Support Facility) at 2,900 m altitude (image credit: ALMA partnership)
A world-class observatory site in the desert: 18)
The ALMA Observatory is operated at two distinct sites, far away from comfortable living conditions of modern civilization. The ALMA OSF is the base camp for the every-day, routine operation of the observatory. It is located at an altitude of about 2900 m, quite high compared to standard living conditions, but still quite acceptable for scientific projects in astronomy of similar scope. However, the OSF will not only serve as the location for operating the Joint ALMA Observatory, it is also the AIV (Assembly, Integration and Verification) station for all the high technology equipment before being moved to the AOS (Array Operations Site), located at 5000 m altitude. Antenna assembly is done at the OSF site at three separate areas, one each for the antennas provided by North America (VERTEX), Japan (MELCO), and Europe (AEM Consortium).
The OSF is also the center for activities associated with commissioning and science verification as well as Early Science operation. During the operations phase of the observatory it is the workplace of the astronomers and of the teams responsible for maintaining proper functioning of all the telescopes.
The construction of the OSF and AOS sites and their access required substantial efforts of the ALMA project. Obviously, there was no access to these two remote locations (Figure 9). The OSF site, located at 2900 m altitude, is about 15 km away from the closest public road, the Chilean highway No. 23. The AOS is another 28 km away from the OSF site. Thus, one of the first projects to be accomplished by ALMA was to construct an access road not only to the OSF but also to the AOS road, 43 km in length, not only at high altitudes, but also with sufficient width to regularly transport a large number of large radio telescopes with a diameter of 12 m.
The geographical location of ALMA (at Altiplano de Chajnantor) is latitude: -23.029° ; longitude: -67.755°
ALMA Front End System
The ALMA Front End system is the first element in a complex chain of signal receiving, conversion, processing and recording. The Front End is designed to receive signals of ten different frequency bands. 19)
The ALMA Front End is far superior to any existing systems. Indeed, spin offs of the ALMA prototypes are leading to improved sensitivities in existing millimeter and submillimeter observatories around the world. The Front End units are comprised of numerous elements, produced at different locations in Europe, North America, East Asia and Chile.
ALMA Cryostats: The largest single element of the Front End system is the cryostat (vacuum vessel) with the cryo-cooler attached. The cryostats will house the receivers, which are assembled in cartridges and can relatively easily be installed or replaced. The corresponding warm optics, windows and infrared filters were delivered by the IRAM (Institut de Radio Astronomie Millimétrique) of France. The operating temperature of the cryostats will be as low as 4 K (equivalent to -269ºC).
ALMA Receiver Bands: In the initial phase of operations, the antennas will be equipped with at least four receiver bands: Band 3 (3 mm), Band 6 (1 mm), Band 7 (0.85 mm), Band 9 (0.45 mm). It is planned to equip the antennas with the missing bands at a later stage of ALMA operations. The development programs were successful, as the requirements could be met – and sometimes the performance is even better than defined in the specifications.
Table 2: The 10 frequency bands of the ALMA antennas
Modular Cryogenic Receiver Concept. The complete front end unit will have a diameter of 1 m, be about 1m high and have a mass of about 750 kg. The cryostat will be cooled down to ~4 K by a 3-stage commercial closed-cycle cryocooler based on the Gifford – McMahon cooling cycle. The individual frequency bands are implemented in the form of modular cartridges that will be inserted in a large common cryostat. This cartridge concept allows for a great flexibility in construction and operation of the array. Figure 10 shows an example of such a receiver cartridge. Another advantage of the cartridge layout with well-defined interfaces is the fact that different cartridges can be developed and built by different groups within the ALMA Project with a large degree of independence but without the risk of incompatibility between them. 20)
Figure 10: Example of a, Band 6, receiver cartridge. The larger diameter metal plate in the middle is the boundary between cooled receiver electronics inside the cryostat (right hand side) and the room temperature electronics (left hand side), image credit: ALMA partnership)
Figure 11: Photo of one typical receiver cartridge built for ALMA ((image credit: ALMA partnership)
Band 5 — July 17, 2015: After more than five years of development and construction, ALMA successfully opened its eyes on another frequency range after obtaining the first fringes with a Band 5 receiver, specifically designed to detect water in the local Universe. Band 5 will also open up the possibility of studying complex molecules in star-forming regions and protoplanetary discs, and detecting molecules and atoms in galaxies in the early Universe, looking back about 13 billion years (Ref. 11).
“Band 5 will open up new possibilities to explore the Universe and bring new discoveries,” explains ESO’s Gianni Marconi, who is responsible for the integration of Band 5. “The frequency range of this receiver includes an emission line of water that ALMA will be able to study in nearby regions of star formation. The study of water is, of course, of intense interest because of its role in the origin of life.” With Band 5, ALMA will also be able to probe the emission from ionized carbon from objects seen soon after the Big Bang, opening up the possibility of probing the earliest epoch of galaxy formation. “This band will also enable astronomers to study young galaxies in the early Universe about 500 million years after the Big Bang,” added Gianni Marconi.
The Band 5 receivers were originally designed and prototyped by Onsala Space Observatory's Group for Advanced Receiver Development (GARD) at Chalmers University of Technology in Sweden, in collaboration with the Rutherford Appleton Laboratory, UK, and ESO, under the European Commission supported Framework Program FP6 (ALMA Enhancement). After having successfully tested the prototypes, the first production-type receivers were built and delivered to ALMA by a consortium of NOVA and GARD in the first half of 2015. Two receivers were used for the first light. The remainder of the 73 receivers ordered, including spares, will be delivered between now and 2017.
Figure 12: Photo of one of the Band 5 receiver cartridges built for ALMA. Extremely weak signals from space are collected by the ALMA antennas and focussed onto the receivers, which transform the faint radiation into an electrical signal (image credit: ALMA partnership)
ALMA Back End and Correlator
The ALMA Back End systems deliver signals generated by Front End units installed in each antenna to the Correlator installed in the AOS (Array Operations Site) Technical Building, located at an altitude of 5,000 m. Signal processing and data transfer is schematically shown in Figure 13. Analog data, produced by the Front End electronics, is processed and digitized before entering into the data encoder, followed by the optical transmitter units and multiplexers. All these elements are installed in the receiver cabins of each antenna. Optical signals are then transmitted by fibers to the AOS Technical Building. The total distance is, in one antenna configuration, about 15 km. At the Technical Building the incoming optical signals are de-multiplexed and de-formatted before entering the Correlator. 21) 22) 23)
ALMA main array Correlator: The ALMA main array Correlator, to be installed in the AOS Technical Building, is the last component in the receiving end of the data transmission. It is a very large data processing system, composed of four quadrants, each of which can process data coming from up to 504 pairs of antennas. The complete correlator will have 2912 printed circuit boards, 5200 interface cables, and more than 20 million solder points. Integral parts of the Correlator are TFB (Tunable Filter Bank) cards. The layout is such that four TFB cards are needed for the data coming from a single antenna. The TFB cards have been developed and optimized by the University of Bordeaux over the last few years.
ACA (Atacama Compact Array) Correlator: The ACA Correlator is designed to process the signals detected by the Atacama Compact Array (ACA). This correlator consists of 52 modules connected with each other through optical-fiber cables. All the modules are installed in 8 racks in the AOS Technical Building. The power spectra issued from the correlation are transferred to the ACA data processing computers.
• May 15, 2020: The contract has been signed for the production of the final set of receivers to be installed on the Atacama Large Millimeter/submillimeter Array (ALMA). Of the originally foreseen ten receiver bands, eight have already been installed, and the ninth, Band 1, is currently in production in East-Asia. Now, contracts have been signed to start the production of the final band in the original ALMA definition — Band 2, led by ESO. Exceeding the originally defined frequency range for this Band (69-90 GHz), the proposed receiver will operate at the full 67-116 GHz frequency window. The hugely successful Band 3 receiver has already opened up the 84-116 GHz frequency range years ago, but the new Band 2 will allow for observations across the entire 67-116 GHz atmospheric window using a single receiver. The project will involve multiple international partners as detailed below. 24)
Figure 14: The figure shows the typical contributions of continuum emission sources with respect to the ALMA Band 2 frequency coverage (purple shaded region), and the Band 1 frequency coverage (pink shaded region). ALMA Band 2 will on average span one of the cleanest mm or submm-wave windows available. The grey bands depict the frequency bands of the Planck satellite, which covered similar frequencies as ALMA. Figure adapted from Planck Collaboration et al 2016 (A&A 594, A10 (2016)), image credit: ESO
Following successful tests of a prototype Band 2 receiver (Yagoubov et al. 2020, A&A 634, A46), the ALMA board has approved the pre-production of a series of six cartridges, with the goal of eventually moving into the production of the full set, one for each of the ALMA antennas. This will depend on the verification of the performance and series production readiness based on the pre-production receiver cartridges.
The 67-116 GHz atmospheric window is rich in strong molecular lines with less crowding than other atmospheric windows. Due to the relatively low excitation energies, these lines are exceptionally suited for studying the dense molecular gas and early phases of star formation. Furthermore, the window is rich in complex organic molecules (COMs), tying in directly with the new fundamental science drivers defined in the ALMA 2030 Development Roadmap. Band 2 will also fill a gap in ALMA's ability to observe the molecular reservoir in redshifted galaxies. Finally, Band 2 continuum measurements have the lowest intrinsic background, as thermal dust emission, CMB emission, free-free emission and synchrotron emission conspire to form a local emission minimum in frequency space.
The production of the Band 2 receiver cartridges will be undertaken by a consortium comprising the Netherlands Research School for Astronomy (NOVA), Chalmers University, Gothenburg, Sweden, and the Italian National Institute for Astrophysics (INAF). The National Astronomical Observatory of Japan (NAOJ) will contribute to the production and testing of receiver optics as an East Asia contribution to the ALMA Development Program. The National Radio Astronomy Observatory (NRAO) and the University of Chile have been involved in the development and production of some components of the receivers, which will be sent to ESO for testing and integration.
ALMA links with other observatories to create an Earth-size telescope
November 2015: ALMA continues to expand its power and capabilities by linking with other millimeter-wavelength telescopes in Europe and North American in a series of VLBI (Very Long Baseline Interferometry) observations. In VLBI, data from two or more telescopes are combined to form a single virtual telescope that spans the geographic distance between them. The most recent of these experiments with ALMA formed an Earth-size telescope with extraordinarily fine resolution. 25)
Figure 15: ALMA combined its power with IRAM and VLBA in VLBI separated observations (image credit: A. Angelich, NRAO/AUI/NSF)
These experiments are an essential step in including ALMA in the EHT (Event Horizon Telescope), a global network of millimeter-wavelength telescopes that will have the power to study the supermassive black hole at the center of the Milky Way in unprecedented detail.
Before ALMA could participate in VLBI observations, it first had to be upgraded adding a new capability known as a phased array. This new version of ALMA allows its 66 antennas to function as a single radio dish 85 m in diameter, which then becomes one element in a much larger VLBI telescope.
• The first test of ALMA’s VLBI capabilities occurred on 13 January 2015, when ALMA successfully linked with the APEX (Atacama Pathfinder Experiment Telescope), which is about two kilometers from the center of the ALMA array.
• On 30 March 2015, ALMA reached out much further by linking with IRAM (Institut de Radioastronomie Millimetrique), the 30 m radio telescope in the Sierra Nevada of southern Spain. Together they simultaneously observed the bright quasar 3C 273. Data from this observation were combined into a single observation with a resolution of 34 µarcsec (1 microarcsecond = 2.8º x 10-10). This is equivalent to distinguish an object of less than 10 cm on the Moon, seen from Earth. - The March observations were made during an observing campaign of the EHT at a wavelength of 1.3 mm.
• The most recent VLBI observing run was performed on 1–3 August 2015 with six of the VLBA (Very Long Baseline Array) antennas of NRAO (National Radio Astronomy Observatory). This combined instrument formed a virtual Earth-size telescope and observed the quasar 3C 454.3, which is one of the brightest radio beacons on the sky, despite lying at a distance of 7.8 billion light-years. These data were first processed at NRAO and MIT-Haystack in the United States and further post-processing analysis is being performed at the MPIfR (Max Planck Institute for Radio Astronomy) in Bonn, Germany.
- The VLBA is an array of 10 antennas spread across the United States from Hawaii to St. Croix. For this observation, six antennas were used: North Liberty, IA; Fort Davis, TX; Los Alamos, NM; Owens Valley, CA; Brewster, WA; and Mauna Kea, HI. The observing wavelength was 3 mm.
• The new observations are a further step towards global interferometric observations with ALMA in the framework of the Global mm-VLBI Array and the EHT (Event Horizon Telescope), with ALMA as the largest and the most sensitive element. The addition of ALMA to millimeter VLBI will boost the imaging sensitivity and capabilities of the existing VLBI arrays by an order of magnitude.
Note: As of 19 March 2020, the previously single large ALMA file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the ALMA project mission and its imagery in the period 2020 and 2019, in addition to some of the mission milestones.
Some selected observation imagery provided by ALMA in 2020-2019
• June 16, 2020: An international team of astronomers has created the most detailed map yet of the atmosphere of the red supergiant star Antares. The unprecedented sensitivity and resolution of both the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) revealed the size and temperature of Antares’ atmosphere from just above the star’s surface, throughout its chromosphere, and all the way out to the wind region. 28) 29)
- Red supergiant stars, like Antares and its more well-known cousin Betelgeuse, are huge, relatively cold stars at the end of their lifetime. They are on their way to run out of fuel, collapse, and become supernovae. Through their vast stellar winds, they launch heavy elements into space, thereby playing an important role in providing the essential building blocks for life in the universe. But it is a mystery how these enormous winds are launched. A detailed study of the atmosphere of Antares, the closest supergiant star to Earth, provides a crucial step towards an answer.
- The ALMA and VLA map of Antares is the most detailed radio map yet of any star, other than the Sun. ALMA observed Antares close to its surface (its optical photosphere) in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. As seen in visible light, Antares’ diameter is approximately 700 times larger than the Sun. But when ALMA and the VLA revealed its atmosphere in radio light, the supergiant turned out to be even more gigantic.
- “The size of a star can vary dramatically depending on what wavelength of light it is observed with,” explained Eamon O’Gorman of the Dublin Institute for Advanced Studies in Ireland and lead author of the study published in the June 16 edition of the journal Astronomy & Astrophysics. “The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”
Figure 16: Radio images of Antares with ALMA and the VLA. ALMA observed Antares close to its surface in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. In the VLA image a huge wind is visible on the right, ejected from Antares and lit up by its smaller but hotter companion star Antares B (image credit: ALMA (ESO/NAOJ/NRAO), E. O’Gorman; NRAO/AUI/NSF, S. Dagnello)
- The radio telescopes measured the temperature of most of the gas and plasma in Antares’ atmosphere. Most noticeable was the temperature in the chromosphere. This is the region above the star’s surface that is heated up by magnetic fields and shock waves created by the vigorous roiling convection at the stellar surface – much like the bubbling motion in a pot of boiling water. Not much is known about chromospheres, and this is the first time that this region has been detected in radio waves.
Figure 17: Artist impression of the atmosphere of Antares. As seen with the naked eye (up until the photosphere), Antares is around 700 times larger than our sun, big enough to fill the solar system beyond the orbit of Mars (Solar System scale shown for comparison). But ALMA and VLA showed that its atmosphere, including the lower and upper chromosphere and wind zones, reaches out 12 times farther than that (image credit: NRAO/AUI/NSF, S. Dagnello)
- Thanks to ALMA and the VLA, the scientists discovered that the star’s chromosphere extends out to 2.5 times the star’s radius (our Sun’s chromosphere is only 1/200th of its radius). They also found that the temperature of the chromosphere is lower than previous optical and ultraviolet observations have suggested. The temperature peaks at 3,500 degrees Celsius (6,400 degrees Fahrenheit), after which it gradually decreases. As a comparison, the Sun’s chromosphere reaches temperatures of almost 20,000 degrees Celsius.
- “We found that the chromosphere is ‘lukewarm’ rather than hot, in stellar temperatures,” said O’Gorman. “The difference can be explained because our radio measurements are a sensitive thermometer for most of the gas and plasma in the star’s atmosphere, whereas past optical and ultraviolet observations were only sensitive to very hot gas and plasma.”
- “We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere,” said co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile who previously observed Antares’ atmosphere in infrared light. “Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individually, but we want to try that in future studies.”
- In the ALMA and VLA data, astronomers for the first time saw a clear distinction between the chromosphere and the region where winds start to form. In the VLA image, a huge wind is visible, ejected from Antares and lit up by its smaller but hotter companion star Antares B.
- “When I was a student, I dreamt of having data like this,” said co-author Graham Harper of the University of Colorado, Boulder. “Knowing the actual sizes and temperatures of the atmospheric zones gives us a clue of how these huge winds start to form and how much mass is being ejected.”
- “Our innate understanding of the night sky is that stars are just points of light. The fact we can map the atmospheres of these supergiant stars in detail, is a true testament to technological advances in interferometry. These tour de force observations bring the universe close, right into our own backyard,” said Chris Carilli of the National Radio Astronomy Observatory, who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998.
• June 9, 2020: High-resolution observations of a young star forming system clearly unveil a pair of protostars at their earliest stages of evolution deeply embedded within the source IRAS 16293-2422 in the Ophiuchus molecular cloud. The team led by the MPE (Max Planck Institute for Extraterrestrial Physics) in Garching, Germany, used the ALMA interferometer not only to pin down the source configuration, but also to measure the gas and stellar kinematics, determining the mass of the young binary. The two close protostars are somewhat heavier than previously thought and they revolve around each other once in about 400 years. 30)
- The system called IRAS 16293-2422 is one of the brightest star-forming regions in our neighborhood. It is located in the Ophiuchus molecular cloud at a distance of about 460 light-years and has been widely studied, also because it shows strong emission of numerous complex organic molecules, building blocks of pre-biotic species. However, until now the detailed configuration of the region was unclear, with observations at different wavelengths showing multiple compact sources at slightly different locations. This confusion was due to the large amount of material in front of the nascent protostars, expected at these earliest stages of formation.
- An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics (MPE) has now obtained high-resolution radio observations with the ALMA interferometer, which clearly reveals two compact sources A1 and A2 in addition to the well-known protostar B (see Figure 18). “Our observations confirm the location of the two close protostars and reveal that each is surrounded by a very small dust disk. Both, in turn, are in turn embedded in a large amount of material showing complex patterns” remarks María José Maureira from MPE, the lead author of the study. 31)
Figure 18: Zoom into the Ophiuchus molecular cloud, highlighting the star forming system IRAS 16293-2422 with the protostar B in the upper right corner and the now clearly identified protostars A1 and A2 on the bottom left. The binary system is shown also in a further zoom-in panel (image credit: MPE; background: ESO/Digitized Sky Survey 2, Davide De Martin)
Figure 19: Detailed view of the binary protostar system with a size comparison to our solar system. The separation between the sources A1 and A2 is roughly the diameter of the Pluto orbit. The size of the disk around A1 (unresolved) is about the diameter of the asteroid belt. The size of the disk around A2 is about the diameter of the Saturn orbit (image credit: MPE)
- The source A1 has a mass of a bit less than 1 solar mass and is embedded in a small dust disk about the size of the asteroid belt; the source A2 has a mass of about 1.4 solar masses and is embedded in a somewhat larger disk (see Figure 19). Interestingly, this disk around A2 also appears at an angle compared to the overall orientation of the larger cloud structure, while the disk around the source B – at a much larger distance – is seen face-on, indicating a rather chaotic formation history.
- In addition to direct imaging of the dust emission, the team also obtained information on the motion of the gas around the stars through observations of spectral lines of organic molecules, which well trace the high-density region surrounding the discovered binary system. This allowed them to get an independent mass measurement and to confirm that A1 and A2 form a bound pair.
- Combining their latest observations with data collected over the past 30 years, the team found that the two stars orbit each other once every 360 years at a distance similar to the extent of Pluto’s orbit, where the orbit is tilted by about 60° (see Figure 20). “This is the first time that we were able to derive the full orbital parameters of a binary system at this early stage of star formation,” points out Jaime Pineda from MPE, who contributed to the modelling.
- “With these results we are finally able to dive into one of the most embedded and youngest proto-stellar systems, unveiling its dynamical structure and complex morphology, where we clearly see filamentary material connecting the circumstellar disks to the surrounding region and likely to the cirbumbinary disk. The small disks are probably still being fed and growing!” emphasizes Paola Caselli, director at MPE and head of the Center for Astrochemical Studies. “This was only possible thanks to the great sensitivity of ALMA and the observations of molecules which uniquely trace these dense regions. Molecules send us signals at very specific frequencies, and, following changes of such frequencies across the region (due to internal motions) one can reconstruct the complex kinematics of the system. This is the power of astrochemistry.”
Figure 20: Relative motion of A1 (blue) with respect to A2 (red) overlaid on the ALMA continuum observation. The visual impression that A1 orbits around A2 is confirmed through a detailed analysis of the motion of the protostars over a 30-year period (image credit: MPE)
• May 22, 2020: Astronomers of NAOJ (National Astronomy Observatory, Japan) and Keio University using the Atacama Large Millimeter/submillimeter Array (ALMA) found quasi-periodic flickers in millimeter-waves from the center of the Milky Way, Sagittarius (Sgr) A*. The team interpreted these blinks to be due to the rotation of radio spots circling the supermassive black hole with an orbit radius smaller than that of Mercury. This is an interesting clue to investigate space-time with extreme gravity. 32)
- “It has been known that Sgr A* sometimes flares up in millimeter wavelength,” tells Yuhei Iwata, the lead author of the paper published in the Astrophysical Journal Letters and a graduate student at Keio University, Japan. “This time, using ALMA, we obtained high-quality data of radio-wave intensity variation of Sgr A* for 10 days, 70 minutes per day. Then we found two trends: quasi-periodic variations with a typical time scale of 30 minutes and hour-long slow variations.”
- Astronomers presume that a supermassive black hole with a mass of 4 million suns is located at the center of Sgr A*. Flares of Sgr A* have been observed not only in millimeter wavelength, but also in infrared light and X-ray. However, the variations detected with ALMA are much smaller than the ones previously detected, and it is possible that these levels of small variations always occur in Sgr A*.
- The black hole itself does not produce any kind of emission. The source of the emission is the scorching gaseous disk around the black hole. The gas around the black hole does not go straight to the gravitational well, but it rotates around the black hole to form an accretion disk.
- The team focused on short timescale variations and found that the variation period of 30 minutes is comparable to the orbital period of the innermost edge of the accretion disk with the radius of 0.2 astronomical units (1 astronomical unit corresponds to the distance between the Earth and the Sun: 150 million kilometers). For comparison, Mercury, the solar system’s innermost planet, circles around the Sun at a distance of 0.4 astronomical units. Considering the colossal mass at the center of the black hole, its gravity effect is also extreme in the accretion disk.
- “This emission could be related with some exotic phenomena occurring at the very vicinity of the supermassive black hole,” says Tomoharu Oka, a professor at Keio University.
Figure 21: Artist’s impression of the gaseous disk around the supermassive black hole. Hot spots circling around the black hole could produce the quasi-periodic millimeter emission detected with ALMA (image credit: Keio University)
- Their scenario is as follows. Hot spots are sporadically formed in the disk and circle around the black hole, emitting strong millimeter waves. According to Einstein’s special relativity theory, the emission is largely amplified when the source is moving toward the observer with a speed comparable to that of light. The rotation speed of the inner edge of the accretion disk is quite large, so this extraordinary effect arises. The astronomers believe that this is the origin of the short-term variation of the millimeter emission from Sgr A*.
- The team supposes that the variation might affect the effort to make an image of the supermassive black hole with the Event Horizon Telescope. “In general, the faster the movement is, the more difficult it is to take a photo of the object,” says Oka. “Instead, the variation of the emission itself provides compelling insight for the gas motion. We may witness the very moment of gas absorption by the black hole with a long-term monitoring campaign with ALMA.” The researchers aim to draw out independent information to understand the mystifying environment around the supermassive black hole. 33)
• May 20, 2020: In our 13.8 billion-year-old universe, most galaxies like our Milky Way form gradually, reaching their large mass relatively late. But a new discovery made with the Atacama Large Millimeter/submillimeter Array (ALMA) of a massive rotating disk galaxy, seen when the universe was only ten percent of its current age, challenges the traditional models of galaxy formation. This research appears on 20 May 2020 in the journal Nature. 34) 35)
- Galaxy DLA0817g, nicknamed the Wolfe Disk after the late astronomer Arthur M. Wolfe, is the most distant rotating disk galaxy ever observed. The unparalleled power of ALMA made it possible to see this galaxy spinning at 170 miles (272 kilometers) per second, similar to our Milky Way.
- “While previous studies hinted at the existence of these early rotating gas-rich disk galaxies, thanks to ALMA we now have unambiguous evidence that they occur as early as 1.5 billion years after the Big Bang,” said lead author Marcel Neeleman of the Max Planck Institute for Astronomy in Heidelberg, Germany.
How did the Wolfe Disk form?
- The discovery of the Wolfe Disk provides a challenge for many galaxy formation simulations, which predict that massive galaxies at this point in the evolution of the cosmos grew through many mergers of smaller galaxies and hot clumps of gas.
- “Most galaxies that we find early in the universe look like train wrecks because they underwent consistent and often ‘violent’ merging,” explained Neeleman. “These hot mergers make it difficult to form well-ordered, cold rotating disks like we observe in our present universe.”
- In most galaxy formation scenarios, galaxies only start to show a well-formed disk around 6 billion years after the Big Bang. The fact that the astronomers found such a disk galaxy when the universe was only ten percent of its current age, indicates that other growth processes must have dominated.
- “We think the Wolfe Disk has grown primarily through the steady accretion of cold gas,” said J. Xavier Prochaska, of the University of California, Santa Cruz and coauthor of the paper. “Still, one of the questions that remains is how to assemble such a large gas mass while maintaining a relatively stable, rotating disk.”
Figure 22: Artist's impression of the Wolfe Disk, a massive rotating disk galaxy in the early, dusty universe. The galaxy was initially discovered when ALMA examined the light from a more distant quasar (top left), image credit: NRAO/AUI/NSF, S. Dagnello
- The team also used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and the NASA/ESA Hubble Space Telescope to learn more about star formation in the Wolfe Disk. In radio wavelengths, ALMA looked at the galaxy’s movements and mass of atomic gas and dust while the VLA measured the amount of molecular mass – the fuel for star formation. In UV-light, Hubble observed massive stars. “The star formation rate in the Wolfe Disk is at least ten times higher than in our own galaxy,” explained Prochaska. “It must be one of the most productive disk galaxies in the early universe.”
A ‘normal’ galaxy
- The Wolfe Disk was first discovered by ALMA in 2017. Neeleman and his team found the galaxy when they examined the light from a more distant quasar. The light from the quasar was absorbed as it passed through a massive reservoir of hydrogen gas surrounding the galaxy – which is how it revealed itself. Rather than looking for direct light from extremely bright, but more rare galaxies, astronomers used this ‘absorption’ method to find fainter, and more ‘normal’ galaxies in the early universe.
- “The fact that we found the Wolfe Disk using this method, tells us that it belongs to the normal population of galaxies present at early times,” said Neeleman. “When our newest observations with ALMA surprisingly showed that it is rotating, we realized that early rotating disk galaxies are not as rare as we thought and that there should be a lot more of them out there.”
- “This observation epitomizes how our understanding of the universe is enhanced with the advanced sensitivity that ALMA brings to radio astronomy,” said Joe Pesce, astronomy program director at the National Science Foundation, which funds the telescope. “ALMA allows us to make new, unexpected findings with almost every observation.”
- The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Figure 23: ALMA radio image of the Wolfe Disk, seen when the universe was only ten percent of its current age (image credit: ALMA (ESO/NAOJ/NRAO), M. Neeleman; NRAO/AUI/NSF, S. Dagnello)
• March 19, 2020: Astronomers using ALMA have found striking orbital geometries in protoplanetary disks around binary stars. While disks orbiting the most compact binary star systems share very nearly the same plane, disks encircling wide binaries have orbital planes that are severely tilted. These systems can teach us about planet formation in complex environments. 36)
- In the last two decades, thousands of planets have been found orbiting stars other than our Sun. Some of these planets orbit two stars, just like Luke Skywalker’s home Tatooine. Planets are born in protoplanetary disks – we now have wonderful observations of these thanks to ALMA – but most of the disks studied so far orbit single stars. ‘Tatooine’ exoplanets form in disks around binary stars, so-called circumbinary disks.
- Studying the birthplaces of ‘Tatooine’ planets provides a unique opportunity to learn about how planets form in different environments. Astronomers already know that the orbits of binary stars can warp and tilt the disk around them, resulting in a circumbinary disk misaligned relative to the orbital plane of its host stars. For example, in a 2019 study led by Grant Kennedy of the University of Warwick, UK, ALMA found a striking circumbinary disk in a polar configuration.
- “With our study, we wanted to learn more about the typical geometries of circumbinary disks,” said astronomer Ian Czekala of the University of California at Berkeley. Czekala and his team used ALMA data to determine the degree of alignment of nineteen protoplanetary disks around binary stars. “The high resolution ALMA data was critical for studying some of the smallest and faintest circumbinary disks yet,” said Czekala.
- “We see a clear overlap between the small disks, orbiting compact binaries, and the circumbinary planets found with the Kepler mission,” Czekala said. Because the primary Kepler mission lasted 4 years, astronomers were only able to discover planets around binary stars that orbit each other in fewer than 40 days. And all of these planets were aligned with their host star orbits. A lingering mystery was whether there might be many misaligned planets that Kepler would have a hard time finding. “With our study, we now know that there likely isn’t a large population of misaligned planets that Kepler missed, since circumbinary disks around tight binary stars are also typically aligned with their stellar hosts,” added Czekala.
- Still, based on this finding, the astronomers conclude that misaligned planets around wide binary stars should be out there and that it would be an exciting population to search for with other exoplanet-finding methods like direct imaging and microlensing. (NASA’s Kepler mission used the transit method, which is one of the ways to find a planet.)
Figure 24: The astronomers compared the ALMA data of the circumbinary disks with the dozen ‘Tatooine’ planets that have been found with the Kepler space telescope. To their surprise, the team found that the degree to which binary stars and their circumbinary disks are misaligned is strongly dependent on the orbital period of the host stars. The shorter the orbital period of the binary star, the more likely it is to host a disk in line with its orbit. However, binaries with periods longer than a month typically host misaligned disks [image credit: ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello]
- Czekala now wants to find out why there is such a strong correlation between disk (mis)alignment and the binary star orbital period. “We want to use existing and coming facilities like ALMA and the next generation Very Large Array to study disk structures at exquisite levels of precision,” he said, “and try to understand how warped or tilted disks affect the planet formation environment and how this might influence the population of planets that form within these disks.”
- “This research is a great example of how new discoveries build on previous observations,” said Joe Pesce, National Science Foundation Program Officer for NRAO and ALMA. “Discerning trends in the circumbinary disk population was only made possible by building on the foundation of archival observational programs undertaken by the ALMA community in previous cycles.”
- The astronomers published their results in The Astrophysical Journal. 37)
Figure 25: Two examples of aligned and misaligned protoplanetary disks around binary stars (circumbinary disks), observed with ALMA. Binary star orbits are added for clarity. Left: in star system HD 98800 B, the disk is misaligned with inner binary stars. The stars are orbiting each other (in this view, towards and away from us) in 315 days. Right: in star system AK Sco, the disk is in line with the orbit of its binary stars. The stars are orbiting each other in 13.6 days [image credit: ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello]
• March 5, 2020: An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) captured the very moment when an old star first starts to alter its environment. The star has ejected high-speed bipolar gas jets which are now colliding with the surrounding material; the age of the observed jet is estimated to be less than 60 years. These are key features to understand how the complex shapes of planetary nebulae are formed. 38)
- Sun-like stars evolve to puffed-up Red Giants in the final stage of their lives. Then, the star expels gas to form a remnant called a planetary nebula. There is a wide variety in the shapes of planetary nebulae; some are spherical, but others are bipolar or show complicated structures. Astronomers are interested in the origins of this variety, but the thick dust and gas expelled by an old star obscure the system and make it difficult to investigate the inner-workings of the process.
- To tackle this problem, a team of astronomers led by Daniel Tafoya in Chalmers University of Technology, Sweden, pointed ALMA at W43A, an old star system in the constellation Aquila, the Eagle.
- Thanks to ALMA’s high resolution, the team obtained a very detailed view of the space around W43A. “The most notable structures are its small bipolar jets,” says Tafoya, the lead author of the research paper published by the Astrophysical Journal Letters. The team found that the velocity of the jets is as high as 175 km per second, which is much higher than previous estimations. Based on this speed and the size of the jets, the team calculated the age of the jets to be less than a human life-span. 39)
- “Considering the youth of the jets compared to the overall lifetime of a star, it is safe to say we are witnessing the ‘exact moment’ that the jets have just started to shove through the surrounding gas,” explains Tafoya. “When the jets carve through the surrounding material in some 60 years, a single person can watch the progress in their life.”
- In fact, the ALMA image clearly maps the distribution of dusty clouds entrained by the jets, which is telltale evidence that it is impacting on the surroundings.
- The team assumes that this entrainment is the key to form a bipolar-shaped planetary nebula. In their scenario, the aged star originally ejects gas spherically and the core of the star loses its envelope. If the star has a companion, gas from the companion pours onto the core of the dying star, and a portion of this new gas forms the jets. Therefore, whether or not the old star has a companion is an important factor to determine the structure of the resulting planetary nebula.
Figure 26: ALMA image of the old star system W43A. The high velocity bipolar jets ejected from the central aged star are seen in blue, low velocity outflow is shown in green, and dusty clouds entrained by the jets are shown in orange (image credit: ALMA (ESO/NAOJ/NRAO), Tafoya et al.)
- “W43A is one of the peculiar so called ‘water fountain’ objects,” says Hiroshi Imai at Kagoshima University, Japan, a member of the team. “Some old stars show characteristic radio emissions from water molecules. We suppose that spots of these water emissions indicate the interface region between the jets and the surrounding material. We named them ‘water fountains,’ and it could be a sign that the central source is a binarity system launching a new jet.”
- “There are only 15 ‘water fountain’ objects identified to date, despite the fact that more than 100 billion stars are included in our Milky Way Galaxy,” explains José Francisco Gómez at Instituto de Astrofísica de Andalucía, Spain. “This is probably because the lifetime of the jets is quite short, so we are very lucky to see such rare objects.”
• February 20, 2020: An international team of astronomers used two of the most powerful radio telescopes in the world to create more than three hundred images of planet-forming disks around very young stars in the Orion Clouds. These images reveal new details about the birthplaces of planets and the earliest stages of star formation. 40)
- Most of the stars in the Universe are accompanied by planets. These planets are born in rings of dust and gas, called protoplanetary disks. Even very young stars are surrounded by these disks. Astronomers want to know exactly when these disks start to form, and what they look like. But young stars are very faint, and there are dense clouds of dust and gas surrounding them in stellar nurseries. Only highly sensitive radio telescope arrays can spot the tiny disks around these infant stars amidst the densely packed material in these clouds.
- For this new research, astronomers pointed both the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) to a region in space where many stars are born: the Orion Molecular Clouds. This survey, called VLA/ALMA Nascent Disk and Multiplicity (VANDAM), is the largest survey of young stars and their disks to date.
- Very young stars, also called protostars, form in clouds of gas and dust in space. The first step in the formation of a star is when these dense clouds collapse due to gravity. As the cloud collapses, it begins to spin – forming a flattened disk around the protostar. Material from the disk continues to feed the star and make it grow. Eventually, the left-over material in the disk is expected to form planets.
- Many aspects about these first stages of star formation, and how the disk forms, are still unclear. But this new survey provides some missing clues as the VLA and ALMA peered through the dense clouds and observed hundreds of protostars and their disks in various stages of their formation.
Young planet-forming disks
- “This survey revealed the average mass and size of these very young protoplanetary disks,” said John Tobin of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, and leader of the survey team. “We can now compare them to older disks that have been studied intensively with ALMA as well.”
- What Tobin and his team found, is that very young disks can be similar in size, but are on average much more massive than older disks. “When a star grows, it eats away more and more material from the disk. This means that younger disks have a lot more raw material from which planets could form. Possibly bigger planets already start to form around very young stars.”
Four special protostars
- Among hundreds of survey images, four protostars looked different than the rest and caught the scientists’ attention. “These newborn stars looked very irregular and blobby,” said team member Nicole Karnath of the University of Toledo, Ohio (now at SOFIA Science Center). “We think that they are in one of the earliest stages of star formation and some may not even have formed into protostars yet.”
- It is special that the scientists found four of these objects. “We rarely find more than one such irregular object in one observation,” added Karnath, who used these four infant stars to propose a schematic pathway for the earliest stages of star formation. “We are not entirely sure how old they are, but they are probably younger than ten thousand years.”
- To be defined as a typical (class 0) protostar, stars should not only have a flattened rotating disk surrounding them, but also an outflow – spewing away material in opposite directions – that clears the dense cloud surrounding the stars and makes them optically visible. This outflow is important, because it prevents stars from spinning out of control while they grow. But when exactly these outflows start to happen, is an open question in astronomy.
- One of the infant stars in this study, called HOPS 404, has an outflow of only two kilometers (1.2 miles) per second (a typical protostar-outflow of 10-100 km/s or 6-62 miles/s). “It is a big puffy sun that is still gathering a lot of mass, but just started its outflow to lose angular momentum to be able to keep growing,” explained Karnath. “This is one of the smallest outflows that we have seen and it supports our theory of what the first step in forming a protostar looks like.”
Combining ALMA and VLA
- The exquisite resolution and sensitivity provided by both ALMA and the VLA were crucial to understand both the outer and inner regions of protostars and their disks in this survey. While ALMA can examine the dense dusty material around protostars in great detail, the images from the VLA made at longer wavelengths were essential to understand the inner structures of the youngest protostars at scales smaller than our solar system.
- “The combined use of ALMA and the VLA has given us the best of both worlds,” said Tobin. “Thanks to these telescopes, we start to understand how planet formation begins.”
- The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
- This research was presented in two papers:
a) ”The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. A Statistical Characterization of Class 0 and I Protostellar Disks,” by J. Tobin et al., The Astrophysical Journal, Volume 890, No 2, Published: 20 February 2020, URL: https://doi.org/10.3847/1538-4357/ab6f64
b) “Detection of Irregular, Sub-mm Opaque Structures in the Orion Molecular Clouds: Protostars within 10000 years of formation?,” by N. Karnath et al., The Astrophysical Journal, Volume 890, No 2, Published: 20 February 2020, https://doi.org/10.3847/1538-4357/ab659e
- The original press release was published by the National Radio Astronomy Observatory (NRAO), an ALMA partner on behalf of North America.
Figure 27: VANDAM survey: ALMA and the VLA observed more than 300 protostars and their young protoplanetary disks in Orion. This image shows a subset of stars, including a few binaries. The ALMA and VLA data compliment each other: ALMA sees the outer disk structure (visualized in blue), and the VLA observes the inner disks and star cores (orange), image credit: ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello
Figure 28: Observed protostars in Orion Molecular Clouds: This image shows the Orion Molecular Clouds, the target of the VANDAM survey. Yellow dots are the locations of the observed protostars on a blue background image made by Herschel. Side panels show nine young protostars imaged by ALMA (blue) and the VLA (orange) image credit: ALMA (ESO/NAOJ/NRAO), J. Tobin; NRAO/AUI/NSF, S. Dagnello; Herschel/ESA
Figure 29: Schematic showing the formation of protostars: This schematic shows a proposed pathway (top row) for the formation of protostars, based on four very young protostars (bottom row) observed by VLA (orange) and ALMA (blue). Step 1 represents the collapsing fragment of gas and dust. In step 2, an opaque region starts to form in the cloud. In step 3, a hydrostatic core starts to form due to an increase in pressure and temperature, surrounded by a disk-like structure and the beginning of an outflow. Step 4 depicts the formation of a class 0 protostar inside the opaque region, that may have a rotationally supported disk and more well-defined outflows. Step 5 is a typical class 0 protostar with outflows that have broken through the envelope (making it optically visible), an actively accreting, rotationally supported disk. In the bottom row, white contours are the protostar outflows as seen with ALMA (image credit: ALMA (ESO/NAOJ/NRAO), N. Karnath; NRAO/AUI/NSF, B. Saxton and S. Dagnello)
Figure 30: Star chart of constellation Orion and observed protostars: The Orion Molecular Clouds (blue, as seen with Herschel) are located in the constellation Orion. Red dots show the locations of the observed protostars in the VANDAM survey (image credit: IAU; Sky & Telescope magazine; NRAO/AUI/NSF, S. Dagnello; Herschel/ESA; ALMA (ESO/NAOJ/NRAO), J. Tobin)
• February 5, 2020: Like humans, stars change with age and ultimately die. For the Sun and stars like it, this change will take it through a phase where, having burned all the hydrogen in its core, it swells up into a large and bright red-giant star. Eventually, the dying Sun will lose its outer layers, leaving behind its core: a hot and dense star called a white dwarf. 41)
- “The star system HD101584 is special in the sense that this ‘death process’ was terminated prematurely and dramatically as a nearby low-mass companion star was engulfed by the giant,” said Hans Olofsson of the Chalmers University of Technology, Sweden, who led a recent study, published in Astronomy & Astrophysics, of this intriguing object. 42)
- Thanks to new observations with ALMA, complemented by data from the ESO-operated Atacama Pathfinder EXperiment (APEX), Olofsson and his team now know that what happened in the double-star system HD101584 was akin to a stellar fight. As the main star puffed up into a red giant, it grew large enough to swallow its lower-mass partner. In response, the smaller star spiralled in towards the giant’s core but didn’t collide with it. Rather, this maneuver triggered the larger star into an outburst, leaving its gas layers dramatically scattered and its core exposed.
- The team says the complex structure of the gas in the HD101584 nebula is due to the smaller star’s spiralling towards the red giant, as well as to the jets of gas that formed in this process. As a deadly blow to the already defeated gas layers, these jets blasted through the previously ejected material, forming the rings of gas and the bright bluish and reddish blobs seen in the nebula.
- A silver lining of a stellar fight is that it helps astronomers to better understand the final evolution of stars like the Sun. “Currently, we can describe the death processes common to many Sun-like stars, but we cannot explain why or exactly how they happen. HD101584 gives us important clues to solve this puzzle since it is currently in a short transitional phase between better studied evolutionary stages. With detailed images of the environment of HD101584 we can make the connection between the giant star it was before, and the stellar remnant it will soon become,” says co-author Sofia Ramstedt from Uppsala University, Sweden.
- Co-author Elizabeth Humphreys from ESO in Chile highlighted that ALMA and APEX, located in the country’s Atacama region, were crucial to enabling the team to probe “both the physics and chemistry in action” in the gas cloud. She added: “This stunning image of the circumstellar environment of HD101584 would not have been possible without the exquisite sensitivity and angular resolution provided by ALMA.”
- While current telescopes allow astronomers to study the gas around the binary, the two stars at the center of the complex nebula are too close together and too far away to be resolved. ESO’s Extremely Large Telescope, under construction in Chile’s Atacama Desert, “will provide information on the ‘heart’ of the object,” says Olofsson, allowing astronomers a closer look at the fighting pair.
Figure 31: This new ALMA image shows the outcome of a stellar fight: a complex and stunning gas environment surrounding the binary HD101584. The colors represent speed, going from blue — gas moving the fastest towards us — to red — gas moving the fastest away from us. Jets, almost along the line of sight, propel the material in blue and red. The stars in the binary are located at the single bright dot at the center of the ring-like structure shown in green, which is moving with the same velocity as the system as a whole along the line of sight. Astronomers believe this ring has its origin in the material ejected as the lower mass star in the binary spiralled towards its red-giant partner (image credit: ALMA (ESO/NAOJ/NRAO), Olofsson et al. Acknowledgement: Robert Cumming)
• January 21, 2020: Phosphorus, present in our DNA and cell membranes, is an essential element for life as we know it. But how it arrived on the early Earth is something of a mystery. Astronomers have now traced the journey of phosphorus from star-forming regions to comets using the combined powers of ALMA and the European Space Agency’s probe Rosetta. Their research shows, for the first time, where molecules containing phosphorus form, how this element is carried in comets, and how a particular molecule may have played a crucial role in starting life on our planet. 43)
- “Life appeared on Earth about 4 billion years ago, but we still do not know the processes that made it possible,” says Víctor Rivilla, the lead author of a new study published today in the journal Monthly Notices of the Royal Astronomical Society. The new results from the Atacama Large Millimeter/Submillimeter Array(ALMA), in which the European Southern Observatory (ESO) is a partner, and from the ROSINA instrument on board Rosetta, show that phosphorus monoxide is a key piece in the origin-of-life puzzle. 44)
- With the power of ALMA, which allowed a detailed look into the star-forming region AFGL 5142, astronomers could pinpoint where phosphorus-bearing molecules, like phosphorus monoxide, form. New stars and planetary systems arise in cloud-like regions of gas and dust in between stars, making these interstellar clouds the ideal places to start the search for life’s building blocks.
- The ALMA observations showed that phosphorus-bearing molecules are created as massive stars are formed. Flows of gas from young massive stars open up cavities in interstellar clouds. Molecules containing phosphorus form on the cavity walls, through the combined action of shocks and radiation from the infant star. The astronomers have also shown that phosphorus monoxide is the most abundant phosphorus-bearing molecule in the cavity walls.
- After searching for this molecule in star-forming regions with ALMA, the European team moved on to a Solar System object: the now-famous comet 67P/Churyumov–Gerasimenko. The idea was to follow the trail of these phosphorus-bearing compounds. If the cavity walls collapse to form a star, particularly a less-massive one like the Sun, phosphorus monoxide can freeze out and get trapped in the icy dust grains that remain around the new star. Even before the star is fully formed, those dust grains come together to form pebbles, rocks and ultimately comets, which become transporters of phosphorus monoxide.
- ROSINA, which stands for Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, collected data from 67P for two years as Rosetta orbited the comet. Astronomers had found hints of phosphorus in the ROSINA data before, but they did not know what molecule had carried it there. Kathrin Altwegg, the Principal Investigator for Rosina and an author in the new study, got a clue about what this molecule could be after being approached at a conference by an astronomer studying star-forming regions with ALMA: “She said that phosphorus monoxide would be a very likely candidate, so I went back to our data and there it was!”
- This first sighting of phosphorus monoxide on a comet helps astronomers draw a connection between star-forming regions, where the molecule is created, all the way to Earth.
- “The combination of the ALMA and ROSINA data has revealed a sort of chemical thread during the whole process of star formation, in which phosphorus monoxide plays the dominant role,” says Rivilla, who is a researcher at the Arcetri Astrophysical Observatory of INAF, Italy’s National Institute for Astrophysics.
- “Phosphorus is essential for life as we know it,” adds Altwegg. “As comets most probably delivered large amounts of organic compounds to the Earth, the phosphorus monoxide found in comet 67P may strengthen the link between comets and life on Earth.”
- This intriguing journey could be documented because of the collaborative efforts between astronomers. “The detection of phosphorus monoxide was clearly thanks to an interdisciplinary exchange between telescopes on Earth and instruments in space,” says Altwegg.
- Leonardo Testi, ESO astronomer and ALMA European Operations Manager, concludes: “Understanding our cosmic origins, including how common the chemical conditions favorable for the emergence of life are, is a major topic of modern astrophysics. While ESO and ALMA focus on the observations of molecules in distant young planetary systems, the direct exploration of the chemical inventory within our Solar System is made possible by ESA missions, like Rosetta. The synergy between world leading ground-based and space facilities, through the collaboration between ESO and ESA, is a powerful asset for European researchers and enables transformational discoveries like the one reported in this paper.”
Figure 32: This ALMA image shows a detailed view of the star-forming region AFGL 5142. A bright, massive star in its infancy is visible at the center of the image. The flows of gas from this star have opened up a cavity in the region, and it is in the walls of this cavity (shown in color), that phosphorus-bearing molecules like phosphorus monoxide are formed. The different colors represent material moving at different speeds (image credit: ALMA (ESO/NAOJ/NRAO), Rivilla et al.)
Figure 33: This infographic shows the key results from a study that has revealed the interstellar thread of phosphorus, one of life’s building blocks. Thanks to ALMA, astronomers could pinpoint where phosphorus-bearing molecules form in star-forming regions like AFGL 5142. The background of this infographic shows a part of the night sky in the constellation of Auriga, where the star-forming region AFGL 5142 is located. The ALMA image of this object is on the top left of the infographic, and one of the locations where the team found phosphorus-bearing molecules is indicated by a circle. The most common phosphorus-bearing molecule in AFGL 5142 is phosphorus monoxide, represented in orange and red in the diagram on the bottom left. Another molecule found was phosphorus nitride, represented in orange and blue. -Using data from the ROSINA instrument onboard ESA’s Rosetta, astronomers also found phosphorus monoxide on comet 67P/Churyumov–Gerasimenko, shown on the bottom right. This first sighting of phosphorus monoxide on a comet helps astronomers draw a connection between star-forming regions, where the molecule is created, all the way to Earth, where it played a crucial role in starting life (image credit: ALMA (ESO/NAOJ/NRAO), Rivilla et al.; ESO/L. Calçada; ESA/Rosetta/NAVCAM; Mario Weigand, www.SkyTrip.de)
Figure 34: Mosaic of comet 67P/Churyumov–Gerasimenko, created using images taken on 10 September 2014 when ESA’s Rosetta spacecraft was 27.8 km from the comet (image credit: ESA/Rosetta/NAVCAM)
Figure 35: This wide-field view shows the region of the sky, in the constellation of Auriga, where the star-forming region AFGL 5142 is located. This view was created from images forming part of the Digitized Sky Survey 2 (image credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)
Figure 36: This chart shows the location of the star-forming region AFGL 5142, recently observed with ALMA, in the constellation of Auriga. The map shows most of the stars visible to the unaided eye under good conditions, and AFGL 5142 itself is highlighted with a red circle on the image (image credit: ESO, IAU and Sky & Telescope)
• December 11, 2019: Astronomers using ALMA (Atacama Large Millimeter/submillimeter Array), have spotted the light of a massive galaxy seen only 970 million years after the Big Bang. This galaxy, called MAMBO-9, is the most distant dusty star-forming galaxy that has ever been observed without the help of a gravitational lens. 45)
- Dusty star-forming galaxies are the most intense stellar nurseries in the universe. They form stars at a rate up to a few thousand times the mass of the Sun per year (the star-forming rate of our Milky Way is just three solar masses per year) and they contain massive amounts of gas and dust. Such monster galaxies are not expected to have formed early in the history of the universe, but astronomers have already discovered several of them as seen when the cosmos was less than a billion years old. One of them is galaxy SPT0311-58, which ALMA observed in 2018.
- Because of their extreme behavior, astronomers think that these dusty galaxies play an important role in the evolution of the universe. But finding them is easier said than done. “These galaxies tend to hide in plain sight,” said Caitlin Casey of the University of Texas at Austin and lead author of a study published in The Astrophysical Journal. “We know they are out there, but they are not easy to find because their starlight is hidden in clouds of dust.” 46)
- MAMBO-9’s light was already detected ten years ago by co-author Manuel Aravena, using the Max-Planck Millimeter BOlometer (MAMBO) instrument on the IRAM 30-meter telescope in Spain and the Plateau de Bure Interferometer in France. But these observations were not sensitive enough to reveal the distance of the galaxy. “We were in doubt if it was real, because we couldn’t find it with other telescopes. But if it was real, it had to be very far away,” says Aravena, who was at that time a PhD student in Germany and is currently working for the Universidad Diego Portales in Chile.
- Thanks to ALMA’s sensitivity, Casey and her team have now been able to determine the distance of MAMBO-9. “We found the galaxy in a new ALMA survey specifically designed to identify dusty star-forming galaxies in the early universe,” said Casey. “And what is special about this observation, is that this is the most distant dusty galaxy we have ever seen in an unobstructed way.”
Figure 37: ALMA radio image of the dusty star-forming galaxy called MAMBO-9. The galaxy consists of two parts, and it is in the process of merging (image credit: ALMA (ESO/NAOJ/NRAO), C. M. Casey et al.; NRAO/AUI/NSF, B. Saxton)
- The light of distant galaxies is often obstructed by other galaxies closer to us. These galaxies in front work as a gravitational lens: they bend the light from the more distant galaxy. This lensing effect makes it easier for telescopes to spot distant objects (this is how ALMA could see galaxy SPT0311-58). But it also distorts the image of the object, making it harder to make out the details.
- In this study, the astronomers saw MAMBO-9 directly, without a lens, and this allowed them to measure its mass. “The total mass of gas and dust in the galaxy is enormous: ten times more than all the stars in the Milky Way. This means that it has yet to build most of its stars,” Casey explained. The galaxy consists of two parts, and it is in the process of merging.
- Casey hopes to find more distant dusty galaxies in the ALMA survey, which will give insight into how common they are, how these massive galaxies formed so early in the universe, and why they are so dusty. “Dust is normally a by-product of dying stars,” she said. “We expect one hundred times more stars than dust. But MAMBO-9 has not produced that many stars yet and we want to find out how dust can form so fast after the Big Bang.”
- “Observations with new and more capable technology can produce unexpected findings like MAMBO-9,” said Joe Pesce, National Science Foundation Program Officer for NRAO and ALMA. “While it is challenging to explain such a massive galaxy so early in the history of the universe, discoveries like this allow astronomers to develop an improved understanding of, and ask ever more questions about, the universe.”
Figure 38: Artist impression of what MAMBO-9 would look like in visible light. The galaxy is very dusty and it has yet to build most of its stars (image credit: NRAO/AUI/NSF, B. Saxton)
- The light from MAMBO-9 travelled about 13 billion years to reach ALMA’s antennas (the universe is approximately 13.8 billion years old today). That means that we can see what the galaxy looked like in the past (Watch this video to learn how ALMA works as a time-machine). Today, the galaxy would probably be even bigger, containing one hundred times more stars than the Milky Way, residing in a massive galaxy cluster.
- The National Radio Astronomy Observatory is a facility of the National Science Foundation (NSF), operated under cooperative agreement by Associated Universities, Inc.
• November 14, 2019: Two peacock-shaped gas clouds were revealed in the Large Magellanic Cloud (LMC) by observations with ALMA. A team of astronomers found several massive baby stars in the complex filamentary clouds, which agrees well with computer simulations of giant collisions of gas clouds. The researchers interpret this to mean that the filaments and young stars are telltale evidence of violent interactions between the LMC and the SMC (Small Magellanic Cloud) 200 million years ago. 47) 48)
- Astronomers know that stars are formed in collapsing clouds in space. However, the formation processes of giant stars, 10 times or more massive than the Sun, are not well understood because it is difficult to pack such a large amount of material into a small region. Some researchers suggest that interactions between galaxies provide a perfect environment for massive star formation. Due to the colossal gravity, clouds in the galaxies are stirred, stretched, and often collide with each other. A huge amount of gas is compressed in an unusually small area, which could form the seeds of massive stars.
- A research team used ALMA to study the structure of dense gas in N159, a bustling star formation region in the LMC. Thanks to ALMA’s high resolution, the team obtained a very detailed map of the clouds in two sub-regions, N159E-Papillon Nebula and N159W South.
- Interestingly, the cloud structures in the two regions look very similar: fan-shaped filaments of gas extending to the north with the pivots in the southernmost points. The ALMA observations also found several massive baby stars in the filaments in the two regions.
- “It is unnatural that in two regions separated by 150 light-years, clouds with such similar shapes were formed and that the ages of the baby stars are similar in two regions separated 150 light years,” says Kazuki Tokuda, a researcher at Osaka Prefecture University and the National Astronomical Observatory of Japan. “There must be a common cause of these features. Interaction between the LMC and SMC is a good candidate.”
- In 2017, Yasuo Fukui, a professor at Nagoya University and his team revealed the motion of hydrogen gas in the LMC and found that a gaseous component right next to N159 has a different velocity than the rest of the clouds. They suggested a hypothesis that the starburst is caused by a massive flow of gas from the SMC to the LMC, and that this flow originated from a close encounter between the two galaxies 200 million years ago.
- The pair of the peacock-shaped clouds in the two regions revealed by ALMA fits nicely with this hypothesis. Computer simulations show that many filamentary structures are formed in a short time scale after a collision of two clouds, which also backs this idea.
- “For the first time, we uncovered the link between massive star formation and galaxy interactions in very sharp detail,” says Fukui, the lead author of one of the research papers. “This is an important step in understanding the formation process of massive star clusters in which galaxy interactions have a big impact.”
Figure 39: ALMA images of two molecular clouds N159E-Papillon Nebula (left) and N159W South (right). Red and green show the distribution of molecular gas in different velocities seen in the emission from 13CO. The blue region in N159E-Papillon Nebula shows the ionized hydrogen gas observed with the Hubble Space Telescope. The blue part in N159W South shows the emission from dust particles obtained with ALMA [image credit: ALMA (ESO/NAOJ/NRAO)/Fukui et al./Tokuda et al./NASA-ESA Hubble Space Telescope]
• October 15, 2019: At the center of a galaxy called NGC 1068, a supermassive black hole hides within a thick doughnut-shaped cloud of dust and gas. When astronomers used ALMA ( Atacama Large Millimeter/submillimeter Array) to study this cloud in more detail, they made an unexpected discovery that could explain why supermassive black holes grew so rapidly in the early Universe. 49)
Figure 40: Artist's impression of the heart of galaxy NGC 1068, which harbors an actively feeding supermassive black hole, hidden within a thick doughnut-shaped cloud of dust and gas. ALMA discovered two counter-rotating flows of gas around the black hole. The colors in this image represent the motion of the gas: blue is material moving toward us, red is moving away (image credit: NRAO/AUI/NSF, S. Dagnello)
- “Thanks to the spectacular resolution of ALMA, we measured the movement of gas in the inner orbits around the black hole,” explains Violette Impellizzeri of the National Radio Astronomy Observatory (NRAO), working at ALMA in Chile and lead author on a paper published in the Astrophysical Journal. “Surprisingly, we found two disks of gas rotating in opposite directions.” 50)
- Supermassive black holes already existed when the Universe was young, just a billion years after the Big Bang. But how these extreme objects, whose masses are up to billions of times the mass of the Sun, had time to grow so fast, is an outstanding question among astronomers. This new ALMA discovery could provide a clue. “Counter-rotating gas streams are unstable, which means that clouds fall into the black hole faster than they do in a disk with a single rotation direction,” said Impellizzeri. “This could be a way in which a black hole can grow rapidly.”
- NGC 1068 (also known as Messier 77) is a spiral galaxy approximately 47 million light-years from Earth in the direction of the constellation Cetus. At its center is an active galactic nucleus, a supermassive black hole that is actively feeding itself from a thin, rotating disk of gas and dust, also known as an accretion disk.
Figure 41: ALMA image showing two disks of gas moving in opposite directions around the black hole in galaxy NGC 1068. The colors in this image represent the motion of the gas: blue is material moving toward us, red is moving away. The white triangles are added to show the accelerated gas that is expelled from the inner disk – forming a thick, obscuring cloud around the black hole (image credit: ALMA (ESO/NAOJ/NRAO), V. Impellizzeri; NRAO/AUI/NSF, S. Dagnello)
- Previous ALMA observations revealed that the black hole is gulping down material and spewing out gas at incredibly high speeds. This gas that gets expelled from the accretion disk likely contributes to hiding the region around the black hole from optical telescopes.
- Impellizzeri and her team used ALMA’s superior zoom lens ability to observe the molecular gas around the black hole. Unexpectedly, they found two counter-rotating disks of gas. The inner disk spans 2-4 light-years and follows the rotation of the galaxy, whereas the outer disk (also known as the torus) spans 4-22 light-years and is rotating the opposite way.
- “We did not expect to see this, because gas falling into a black hole would normally spin around it in only one direction,” said Impellizzeri. “Something must have disturbed the flow because it is impossible for a part of the disk to start rotating backward all on its own.”
- Counter-rotation is not an unusual phenomenon in space. “We see it in galaxies, usually thousands of light-years away from their galactic centers,” explained co-author Jack Gallimore from Bucknell University in Lewisburg, Pennsylvania. “The counter-rotation always results from the collision or interaction between two galaxies. What makes this result remarkable is that we see it on a much smaller scale, tens of light-years instead of thousands from the central black hole.”
- The astronomers think that the backward flow in NGC 1068 might be caused by gas clouds that fell out of the host galaxy, or by a small passing galaxy on a counter-rotating orbit captured in the disk.
- At the moment, the outer disk appears to be in a stable orbit around the inner disk. “That will change when the outer disk begins to fall onto the inner disk, which may happen after a few orbits or a few hundred thousand years. The rotating streams of gas will collide and become unstable, and the disks will likely collapse in a luminous event as the molecular gas falls into the black hole. Unfortunately, we will not be there to witness the fireworks,” said Gallimore.
• October 4, 2019: The two baby stars were found in the [BHB2007] 11 system – the youngest member of a small stellar cluster in the Barnard 59 dark nebula, which is part of the clouds of interstellar dust called the Pipe nebula. Previous observations of this binary system showed the outer structure. Now, thanks to the high resolution of the ALMA (Atacama Large Millimeter/submillimeter Array) and an international team of astronomers led by scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, we can see the inner structure of this object. 51)
- “We see two compact sources that we interpret as circumstellar disks around the two young stars,” explains Felipe Alves from MPE who led the study. A circumstellar disk is the ring of dust and gas that surrounds a young star. The star accrete matter from the ring to grow bigger. “The size of each of these disks is similar to the asteroid belt in our Solar System and the separation between them is 28 times the distance between the Sun and the Earth,” notes Alves.
- The two circumstellar disks are surrounded by a bigger disk with a total mass of about 80 Jupiter masses, which displays a complex network of dust structures distributed in spiral shapes – the pretzel loops. “This is a really important result,” stresses Paola Caselli, managing director at MPE, head of the Centre of Astrochemical Studies and co-author of the study. “We have finally imaged the complex structure of young binary stars with their feeding filaments connecting them to the disk in which they were born. This provides important constraints for current models of star formation.”
- The baby stars accrete mass from the bigger disk in two stages. The first stage is when mass is transferred to the individual circumstellar disks in beautiful twirling loops, which is what the new ALMA image showed. The data analysis also revealed that the less-massive but brighter circumstellar disk — the one in the lower part of the image — accretes more material. In the second stage, the stars accrete mass from their circumstellar disks. “We expect this two-level accretion process to drive the dynamics of the binary system during its mass accretion phase,” adds Alves. “While the good agreement of these observations with theory is already very promising, we will need to study more young binary systems in detail to better understand how multiple stars form.”
- This research was presented in a paper published on 3 October 2019 in the journal Science. 52)
- The team is composed of F. O. Alves (Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics, Garching, Germany), P. Caselli (Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics, Germany), J. M. Girart (Institut de Ciències de l’Espai, Consejo Superior de Investigaciones Científicas, Spain and Institut d’Estudis Espacials de Catalunya, Spain), D. Segura-Cox (Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics, Garching, Germany), G. A. P. Franco (Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Brazil), A. Schmiedeke (Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics, Garching, Germany) and B. Zhao (Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics, Garching, Germany).
Figure 42: ALMA captured this unprecedented image of two circumstellar disks, in which baby stars are growing, feeding with material from their surrounding birth disk. The complex network of dust structures distributed in spiral shapes remind of the loops of a pretzel. These observations shed new light on the earliest phases of the lives of stars and help astronomers determine the conditions in which binary stars are born (image credit: ALMA (ESO/NAOJ/NRAO), Alves et al.)
• September 30, 2019: Using the detailed eyes of the ALMA (Atacama Large Millimeter/submillimeter Array) and ESO’s VLT (Very Large Telescope), astronomers have mapped the intense tails of a cosmic jellyfish: a number of knotty streams of gas spewing outwards from a spiral galaxy named ESO 137-001. 53)
Figure 43: This celestial cnidarian is shown here in beautiful detail. The various elements making up this image were captured by different telescopes. The galaxy and its surroundings were imaged by the NASA/ESA Hubble Space Telescope; its tails, which trace streams of hydrogen and show up in hues of bright purple, by the MUSE instrument mounted on the VLT; and bright hotspots of carbon dioxide emission from within the system, which show up as flares of orange-red, were spotted by ALMA [image credit: ALMA (ESO/NAOJ/NRAO), P. Jachym (Czech Academy of Sciences) et al.]
- These tails are caused by a dramatic phenomenon known as ram-pressure stripping. The space between galaxies in a cluster is not empty, but full of material that acts like a viscous fluid. As a galaxy travels through this resistant environment, gas is stripped out of the galaxy to form a wake that creates beautiful, intricate systems such as that seen here around ESO 137-001 (which resides in the Norma galaxy cluster). The direction and position of the tail shed light on the way in which the galaxy is moving — with galaxies usually falling towards the center of the cluster itself.
- This image offers the first high-resolution map of the cold molecular gas lurking within a ram-pressure stripped system. ESO 137-001 is one of the nearest jellyfish galaxies to Earth, and is particularly interesting because its long, extended tails of gas contain features known as ‘fireballs’: bursts of star formation. The precise mechanisms governing how stars form within jellyfish tails are mysterious, and this map thus provides a new window onto the conditions needed for new stars to form in such intense, changeable environments.
- The ALMA array comprises 66 antennas, and is located on the Chajinator plateau in the Chilean Atacama Desert at an altitude of 5000 meters. ALMA observes the night sky from this remote location to unlock the secrets of how the Universe — and its weird and wonderful residents, ESO 137-001 included — formed and evolved, revealing more about our cosmic origins.
• September 16, 2019: Star clusters are formed by the condensation of molecular clouds, masses of cold, dense gas that are found in every galaxy. The physical properties of these clouds in our own galaxy and nearby galaxies have been known for a long time. But are they identical in distant galaxies that are more than 8 billion light-years away? For the first time, an international team led by the University of Geneva (UNIGE) has been able to detect molecular clouds in a Milky Way progenitor, thanks to the unprecedented spatial resolution achieved in such a distant galaxy. 54)
- These observations, published in Nature Astronomy, show that the distant clouds have a higher mass, density and internal turbulence than the clouds hosted in nearby galaxies and that they produce far more stars. The astronomers attribute these differences to the ambient interstellar conditions in distant galaxies, which are too extreme for the molecular clouds typical of nearby galaxies to survive. 55)
- Molecular clouds consist of dense, cold molecular hydrogen gas that swirls around at supersonic velocities, generating density fluctuations that condense and form stars. In nearby galaxies, such as the Milky Way, a molecular cloud produces between 10+3 and 10+6 stars. In far-off galaxies, however, located more than 8 billion light-years away, astronomers have observed gigantic star clusters containing up to 100 times more stars. Why is there such a difference?
Exceptional observation made possible using a cosmic magnifying glass
- To answer this question, the astronomers were able to make use of a natural telescope - the gravitational lens phenomenon - in combination with ALMA (Atacama Large Millimeter / Submillimeter Array), an interferometer made up of 50 millimetric radio antennas that reconstruct the entire image of a galaxy instantly.
- "Gravitational lenses are a natural telescope that produces a magnifying-glass effect when a massive object is aligned between the observer and the distant object," explains Miroslava Dessauges, a researcher in the Department of Astronomy in UNIGE's Faculty of Science and first author of the study.
- "With this effect, some parts of distant galaxies are stretched on the sky and can be studied at an unrivalled resolution of 90 light-years!" ALMA, meanwhile, can be employed to measure the level of carbon monoxide, which acts as a tracer of molecular hydrogen gas that constitutes the cold clouds.
- This resolution made it possible to characterize the molecular clouds individually in a distant galaxy, nicknamed the "Cosmic Snake", 8 billion light-years away. "It's the first time we've been able to pinpoint molecular clouds one from each other," says Daniel Schaerer, professor in UNIGE's Department of Astronomy.
Figure 44: Molecular clouds detected at the unprecedented resolution of 90 light-years in the Cosmic Snake, located more than 8 billion light-years away, a typical progenitor of our galaxy (left). Observed at resolutions 50,000 times better, each of these clouds resembles the very tormented gas of the Carina nebula located only 7500 light-years away, a veritable nursery of emerging stars (right) image credit: © UNIGE, Dessauges et NASA, ESA
- The astronomers were therefore able to compare the mass, size, density and internal turbulence of molecular clouds in nearby and distant galaxies. "It was thought that the clouds had the same properties in all galaxies at all times, continues the Geneva-based researcher, but our observations have demonstrated the opposite!"
Molecular clouds resistant to extreme environments
- These new observations revealed that the molecular clouds in distant galaxies had a mass, density and turbulence 10 to 100 times higher than those in nearby galaxies. "Such values had only been measured in clouds hosted in nearby interacting galaxies, which have interstellar medium conditions resembling those of distant galaxies," adds Miroslava Dessauges.
- The researchers could link the differences in the physical properties of the clouds with the galactic environments, which are more extreme and hostile in far-off galaxies than in closer galaxies.
- "A molecular cloud typically found in a nearby galaxy would instantly collapse and be destroyed in the interstellar medium of distant galaxies, hence its enhanced density and turbulence guarantee its survival and equilibrium," explains Miroslava Dessauges.
- "The characteristic mass of the molecular clouds in the Cosmic Snake appears to be in perfect agreement with the predictions of our scenario of fragmentation of turbulent galactic disks. As a result, this scenario can be put forward as the mechanism of formation of massive molecular clouds in distant galaxies," adds Lucio Mayer, a professor at the Centre for Physical and Cosmological Theory at the University of Zurich.
- The international team also discovered that the efficiency of star formation in the Cosmic Snake galaxy is particularly high, likely triggered by the highly supersonic internal turbulence of the clouds. "In nearby galaxies, a molecular cloud forms about 5% of its mass in stars. In distant galaxies, this number climbs to 30%," observes Daniel Schaerer.
- The astronomers will now study other distant galaxies in order to confirm their observational results obtained for the Cosmic Snake. Miroslava Dessauges says in conclusion: "We'll also push the resolution even further by taking advantage of the unique performance of the ALMA interferometer. In parallel, we need to understand in more detail the ability of molecular clouds in distant galaxies to form stars so efficiently."
• September 13, 2019: Astronomers using one of the most advanced radio telescopes (ALMA) have discovered a rare molecule in the dust and gas disc around a young star — and it may provide an answer to one of the conundrums facing astronomers. 56)
- The star, named HD 163296, is located 330 light years from Earth and formed over the last six million years.
- It is surrounded by a disc of dust and gas — a so-called protoplanetary disc. It is within these discs that young planets are born. Using a radio telescope in the Atacama Desert in Chile, researchers were able to detect an extremely faint signal showing the existence of a rare form of carbon monoxide — known as an isotopologue (13C17O).
- The detection has allowed an international collaboration of scientists, led by the University of Leeds, to measure the mass of the gas in the disc more accurately than ever before. The results show that disc is much heavier — or more 'massive' — than previously thought.
- Alice Booth, a PhD researcher at Leeds who led the study, said: "Our new observations showed there was between two and six times more mass hiding in the disc than previous observations could measure.
- "This is an important finding in terms of the birth of planetary systems in discs — if they contain more gas, then they have more building material to form more massive planets."
- The study — The first detection of 13C17O in a protoplanetary disk: a robust tracer of disk gas mass — was published (12/09/2019) in the Astrophysical Journal Letters. 57)
- The scientists' conclusions are well timed. Recent observations of protoplanetary discs have perplexed astronomers because they did not seem to contain enough gas and dust to create the planets observed.
- Dr John Ilee, a researcher at Leeds who was also involved in the study, added: "The disc-exoplanet mass discrepancy raises serious questions about how and when planets are formed. However, if other discs are hiding similar amounts of mass as HD 163296, then we may just have underestimated their masses until now."
- "We can measure disc masses by looking at how much light is given off by molecules like carbon monoxide. If the discs are sufficiently dense, then they can block the light given off by more common forms of carbon monoxide -- and that could result in scientists underestimating the mass of the gas present. - This study has used a technique to observe the much rarer 13C17O molecule — and that's allowed us to peer deep inside the disc and find a previously hidden reservoir of gas."
- The researchers made use of one of the most sophisticated radio telescopes in the world — the ALMA (Atacama Large Millimeter/submillimeter Array) — high in the Atacama Desert.
Figure 45: The inner red regions represent the dust in the disc, thought to be shaped into rings by forming planets. The wider blue region is the carbon monoxide (CO) gas in the disc. The inner green region shows the rarer 13C17O gas that the researchers have detected for the first time (image credit: ALMA (ESO/NAOJ/NRAO), Booth and colleagues, University of Leeds)
- ALMA is able to observe light that is invisible to the naked eye, allowing astronomers to view what is known as the 'cold universe' — those parts of space not visible using optical telescopes.
- Booth said: "Our work shows the amazing contribution that ALMA is making to our understanding of the Universe. It is helping build a more accurate picture of the physics leading to the formation of new planets. This of course then helps us understand how the Solar System and Earth came to be."
- The researchers are already planning the next steps in their work.
- Booth added: "We suspect that ALMA will allow us to observe this rare form of CO in many other discs. By doing that, we can more accurately measure their mass, and determine whether scientists have systematically been underestimating how much matter they contain."
• August 22, 2019: Swirling clouds, big colorful belts, giant storms. The beautiful and incredibly turbulent atmosphere of Jupiter has been showcased many times. But what is going on below the clouds? What is causing the many storms and eruptions that we see on the ‘surface’ of the planet? However, to study this, visible light is not enough. We need to study Jupiter using radio waves. 58) 59)
- New radio wave images made with the ALMA (Atacama Large Millimeter/submillimeter Array) observatory provide a unique view of Jupiter’s atmosphere down to fifty kilometers below the planet’s visible (ammonia) cloud deck.
- “ALMA enabled us to make a three-dimensional map of the distribution of ammonia gas below the clouds. And for the first time, we were able to study the atmosphere below the ammonia cloud layers after an energetic eruption on Jupiter,” said Imke de Pater of the University of California, Berkeley (EE. UU.).
- The atmosphere of giant Jupiter is made out of mostly hydrogen and helium, together with trace gases of methane, ammonia, hydrosulfide, and water. The top-most cloud layer is made up of ammonia ice. Below that is a layer of solid ammonia hydrosulfide particles, and deeper still, around 80 kilometers below the upper cloud deck, there likely is a layer of liquid water. The upper clouds form the distinctive brown belts and white zones seen from Earth.
- Many of the storms on Jupiter take place inside those belts. They can be compared to thunderstorms on Earth and are often associated with lightning events. Storms reveal themselves in visible light as small bright clouds, referred to as plumes. These plume eruptions can cause a major disruption of the belt, which can be visible for months or years.
- The ALMA images were taken a few days after amateur astronomers observed an eruption in Jupiter’s South Equatorial Belt in January 2017. A small bright white plume was visible first, and then a large-scale disruption in the belt was observed that lasted for weeks after the eruption.
- De Pater and her colleagues used ALMA to study the atmosphere below the plume and the disrupted belt at radio wavelengths and compared these to UV-visible light and infrared images made with other telescopes at approximately the same time.
- “Our ALMA observations are the first to show that high concentrations of ammonia gas are brought up during an energetic eruption,” said de Pater. “The combination of observations simultaneously at many different wavelengths enabled us to examine the eruption in detail. Which led us to confirm the current theory that energetic plumes are triggered by moist convection at the base of water clouds, which are located deep in the atmosphere. The plumes bring up ammonia gas from deep in the atmosphere to high altitudes, well above the main ammonia cloud deck,” she added.
Figure 46: Radio image of Jupiter made with ALMA. Bright bands indicate high temperatures and dark bands low temperatures. The dark bands correspond to the zones on Jupiter, which are often white at visible wavelengths. The bright bands correspond to the brown belts on the planet. This image contains over 10 hours of data, so fine details are smeared by the planet’s rotation (image credit: ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello)
- “These ALMA maps at millimeter wavelengths complement the maps made with the National Science Foundation’s Very Large Array in centimeter wavelengths,” said Bryan Butler of the NRAO (National Radio Astronomy Observatory). “Both maps probe below the cloud layers seen at optical wavelengths and show ammonia-rich gases rising into and forming the upper cloud layers (zones), and ammonia-poor air sinking down (belts).”
- “The present results show superbly what can be achieved in planetary science when an object is studied with various observatories and at various wavelengths”. Explains Eric Villard, an ALMA astronomer part of the research team. “ALMA, with its unprecedented sensitivity and spectral resolution at radio wavelengths, worked together successfully with other major observatories around the world, to provide the data to allow a better understanding of the atmosphere of Jupiter.”
Figure 47: Flat map of Jupiter in radio waves with ALMA (top) and visible light with the Hubble Space Telescope (bottom). The eruption in the South Equatorial Belt is visible in both images (image credit: ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello; NASA/Hubble)
Figure 48: Spherical ALMA map of Jupiter showing the distribution of ammonia gas below Jupiter’s cloud deck (image credit: ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello)
• June 17, 2019: Researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) observed the earliest combined signals of oxygen, carbon, and dust from a galaxy in the Universe, 13 billion years ago. By comparing the different signals, the team determined that the galaxy is, in fact, two merging galaxies, making it the earliest example of merging galaxies yet discovered. 60)
- Takuya Hashimoto at Waseda University, Japan, and his team used ALMA to observe B14-65666, an object located 13 billion light-years away in the constellation Sextans. Because of the finite speed of light, the signals we receive from B14-65666 today had to travel for 13 billion years to reach us. In other words, they show us the image of what the galaxy looked like 13 billion years ago, less than 1 billion years after the Big Bang. 61)
- ALMA achieved the earliest observation of radio emissions from oxygen, carbon, and dust in B14-65666. The detection of multiple signals allows astronomers to retrieve complementary information.
- Data analysis showed that the emissions are divided into two blobs. Previous observations with the Hubble Space Telescope (HST) had revealed two-star clusters in B14-65666. Now, with the three emission signals detected by ALMA, the team was able to show that the two blobs do in-fact form a single system, but with different speeds; which indicates that the blobs are two merging galaxies. The earliest known example of merging galaxies. The research team estimated that the total stellar mass of B14-65666 is less than 10% that of the Milky Way, meaning that it’s in its earliest phases of evolution. Despite its youth, B14-65666 is producing stars 100 times more actively than the Milky Way. Such active star-formation rate is another signature of galactic mergers because the gas compression in colliding galaxies naturally leads to bursty star-formation.
- “With rich data from ALMA and HST, combined with advanced data analysis, we could put the pieces together to show that B14-65666 is a pair of merging galaxies in the earliest era of the Universe,” explains Hashimoto. “Detection of radio waves from three components in such a distant object demonstrates ALMA’s high capability to investigate the distant Universe.”
- Present galaxies like our Milky Way have experienced countless, often violent, mergers. Sometimes a more massive galaxy swallowed a smaller one. In rare cases, galaxies with similar sizes merged to form a new, larger galaxy. Mergers are essential for galaxy evolution, attracting many astronomers eager to trace back them.
- “Our next step is to search for nitrogen, another major chemical element, and even the carbon monoxide molecule,” said Akio Inoue, a professor at Waseda University. “Ultimately, we hope to observationally understand the circulation and accumulation of elements and material in the context of galaxy formation and evolution.”
Figure 49: Composite image of B14-65666 showing the distributions of dust (red), oxygen (green), and carbon (blue), observed by ALMA and stars (white) observed by the Hubble Space Telescope (image credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Hashimoto et al.)
Figure 50: Artist’s impression of the merging galaxies B14-65666 located 13 billion light-years away (image credit: NAOJ)
• May 10, 2019: Astronomers map the substance aluminum monoxide (AlO) in a cloud around a distant young star—Origin Source I. The finding clarifies some important details about how our solar system, and ultimately we, came to be. The cloud's limited distribution suggests AlO gas rapidly condenses to solid grains, which hints at what an early stage of our solar evolution looked like. 62) 63) 64)
- Professor Shogo Tachibana of the University of Tokyo Organization for Planetary and Space Science has a passion for space. From small things like meteorites to enormous things like stars and nebulae—huge clouds of gas and dust in space—he is driven to explore our solar system's origins.
- "I have always wondered about the evolution of our solar system, of what must have taken place all those billions of years ago," he said. "This question leads me to investigate the physics and chemistry of asteroids and meteorites."
- Space rocks of all kinds greatly interest astronomers as these rocks can remain largely unchanged since the time our sun and planets formed from a swirling cloud of gas and dust. They contain records of the conditions at that time—generally considered to be 4.56 billion years ago—and their properties such as composition can tell us about these early conditions.
- "On my desk is a small piece of the Allende meteorite, which fell to Earth in 1969. It's mostly dark but there are some scattered white inclusions (foreign bodies enclosed in the rock), and these are important," continued Tachibana. "These speckles are calcium and aluminum-rich inclusions (CAIs), which were the first solid objects formed in our solar system."
- Minerals present in CAIs indicate that our young solar system must have been extremely hot. Physical techniques for dating these minerals reveal a fairly specific age for the solar system. However, Tachibana and colleagues wished to expand on the details of this stage of evolution.
Figure 51: The white inclusions called CAIs are among the oldest solid matter in the solar system (image credit: Rohan Mehra, Division for Strategic Public Relations)
- "There are no time machines to explore our own past, so we wanted to see a young star that could share traits with our own," said Tachibana. "With the ALMA (Atacama Large Millimeter/submillimeter Array), we found the emission lines—a chemical fingerprint—for AlO in outflows from the circumstellar disk (gas and dust surrounding a star) of the massive young star candidate Orion Source I. It's not exactly like our sun, but it's a good start."
- ALMA was the ideal tool as it offers extremely high resolution and sensitivity to reveal the distribution of AlO around the star. No other instrument can presently make such observations.
- The team now plans to explore gas and solid molecules around other stars to gather data useful to further refine solar system models.
Figure 52: The Orion Nebula where the distant young star Origin Source I resides (image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)
• March 19, 2019: Researchers have detected a radio signal from abundant interstellar dust in MACS0416_Y1, a galaxy 13.2 billion light-years away in the constellation Eridanus. Standard models can’t explain this much dust in a galaxy this young, forcing us to rethink the history of star formation. Researchers now think MACS0416_Y1 experienced staggered star formation with two intense starburst periods 300 million and 600 million years after the Big Bang with a quiet phase in between. 65) 66)
- Stars are the main players in the Universe, but they are supported by the unseen backstage stagehands: stardust and gas. Cosmic clouds of dust and gas are the sites of star formation and masterful storytellers of the cosmic history.
- “Dust and relatively heavy elements such as oxygen are disseminated by the deaths of stars,” said Yoichi Tamura, an associate professor at Nagoya University and the lead author of the research paper, “Therefore, a detection of dust at some point in time indicates that a number of stars have already formed and died well before that point.”
- Using ALMA (Atacama Large Millimeter/submillimeter Array), Tamura and his team observed the distant galaxy MACS0416_Y1. Because of the finite speed of light, the radio waves we observe from this galaxy today had to travel for 13.2 billion years to reach us. In other words, they provide an image of what the galaxy looked like 13.2 billion years ago, which is only 600 million years after the Big Bang.
- The astronomers detected a weak but telltale signal of radio emissions from dust particles in MACS0416_Y1. The HST (Hubble Space Telescope), the Spitzer Space Telescope, and the European Southern Observatory’s VLT (Very Large Telescope) have observed the light from stars in the galaxy; and from its color they estimate the stellar age to be 4 million years.
- “It ain’t easy,” said Tamura half-lost in a moonage daydream. “The dust is too abundant to have been formed in 4 million years. It is surprising, but we need to hang onto ourselves. Older stars might be hiding in the galaxy, or they may have died out and disappeared already.”
- “There have been several ideas proposed to overcome this dust budget crisis,” said Ken Mawatari, a researcher at the University of Tokyo. “However, no one is conclusive. We made a new model which doesn’t need any extreme assumptions diverging far from our knowledge of the life of stars in today’s Universe. The model well explains both the color of the galaxy and the amount of dust.” In this model, the first burst of star formation started at 300 million years and lasted 100 million years. After that, the star formation activity went quiet for a and then restarted at 600 million years. The researchers think ALMA observed this galaxy at the beginning of its second generation of star formation.
- “Dust is a crucial material for planets like Earth,” explains Tamura. “Our result is an important step forward for understanding the early history of the Universe and the origin of dust.”
Figure 53: ALMA and Hubble Space Telescope (HST) image of the distant galaxy MACS0416_Y1. Distribution of dust and oxygen gas traced by ALMA are shown in red and green, respectively, while the distribution of stars captured by HST is shown in blue [image credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Tamura, et al.]
Figure 54: Artist’s impression of the distant galaxy MACS0416_Y1. Based on the observations with ALMA and HST, researchers assume that this galaxy contains stellar clusters with a mix of old and young stars. The clouds of gas and dust are illuminated by starlight (image credit: National Astronomical Observatory of Japan)
• March 18, 2019: Scientists from the RIKEN Cluster for Pioneering Research in Japan,the Chalmers University of Technology in Sweden,and the University of Virginia in the USA and collaborators used ALMA (Atacama Large Millimeter/submillimeter Array) to observe a molecular cloud that is collapsing to form two massive protostars that will eventually become a binary star system. 67)
- While it is known that most massive stars possess orbiting stellar companions it has been unclear how this comes about – for example, are the stars born together from a common spiraling gas disk at the center of a collapsing cloud, or do they pair up later by chance encounters in a crowded star cluster.
- Understanding the dynamics of forming binaries has been difficult because the protostars in these systems are still enveloped in a thick cloud of gas and dust that prevents most light from escaping. Fortunately, it is possible to see them using radio waves, as long as they can be imaged with sufficiently high spatial resolution.
- In the current research, published in Nature Astronomy, the researchers led by Yichen Zhang of the RIKEN Cluster for Pioneering Research and Jonathan C. Tan at the Chalmers University,and the University of Virginia, used ALMA to observe, at high spatial resolution, a star-forming region known as IRAS07299-1651, which is located 1.68 kpc (kiloparsec), or about 5,500 light years, away.
- The observations showed that already at this early stage, the cloud contains two objects, a massive “primary” central star and another “secondary” forming star, also of high mass. For the first time, the research team was able to use these observations to deduce the dynamics of the system. The observations showed that the two forming stars are separated by a distance of about 180 astronomical units—a unit approximately the distance from the earth to the sun. Hence, they are quite far apart. They are currently orbiting each other with a period of at most 600 years and have a total mass at least 18 times that of our Sun. 68)
Figure 55: ALMA’s view of the IRAS-07299 star-forming region and the massive binary system at its center. The background image shows dense, dusty streams of gas (shown in green) that appear to be flowing towards the center. Gas motions, as traced by the methanol molecule, that are towards us are shown in blue; motions away from us in red. The inset image shows a zoom-in view of the massive forming binary, with the brighter, primary protostar moving toward us is shown in blue and the fainter, secondary protostar moving away from us shown in red. The blue and red dotted lines show an example of orbits of the primary and secondary spiraling around their center of mass (marked by the cross) image credit: Riken & Study Team
Figure 56: Movie composed of images taken by ALMA showing the gas streams, as traced by the methanol molecule, with different line-of-sight color-coded velocities, around the massive binary protostar system. The grey background image shows the overall distribution, from all velocities, of dust emission from the dense gas streams (image credit: Riken & Study Team)
• March 13, 2019: Researchers have spotted the formation sites of planets around a young star resembling our Sun. Two rings of dust around the star, at distances comparable to the asteroid belt and the orbit of Neptune in our Solar System, suggest that we are witnessing the formation of a planetary system similar to our own. 69) 70)
- The Solar System is thought to have formed from a cloud of cosmic gas and dust 4.6 billion years ago. By studying young planetary systems forming around other stars, astronomers hope to learn more about our own origins.
- Tomoyuki Kudo, an astronomer at the National Astronomical Observatory of Japan (NAOJ), and his team observed the young star DM Tau using the Atacama Large Millimeter/submillimeter Array (ALMA). Located 470 light-years away in the constellation Taurus, DM Tau is about half the mass of the Sun and estimated to be three to five million years old.
- "Previous observations inferred two different models for the disk around DM Tau," said Kudo. "Some studies suggested the radius of the ring is about where the Solar System's asteroid belt would be. Other observations put the size out where Neptune would be. Our ALMA observations provided a clear answer: both are right. DM Tau has two rings, one at each location."
- The researchers found a bright patch in the outer ring. This indicates a local concentration of dust, which would be a possible formation site for a planet like Uranus or Neptune.
- "We are also interested in seeing the details in the inner region of the disk, because the Earth formed in such an area around the young Sun," commented Jun Hashimoto, a researcher at the Astrobiology Center, Japan. "The distribution of dust in the inner ring around DM Tau will provide crucial information to understand the origin of planets like Earth."
Figure 57: ALMA image of the dusty disk around the young star DM Tau. You can see two concentric rings where planets may be forming (image credit: ALMA (ESO/NAOJ/NRAO), Kudo et al.)
• February 28, 2019: Astronomers have detected a stealthy black hole from its effects on an interstellar gas cloud. This intermediate mass black hole is one of over 100 million quiet black holes expected to be lurking in our galaxy. These results provide a new method to search for other hidden black holes and help us understand the growth and evolution of black holes. 71) 72)
- Black holes are objects with such strong gravity that everything, including light, is sucked in and cannot escape. Because black holes do not emit light, astronomers must infer their existence from the effects their gravity produce in other objects. Black holes range in mass from about 5 times the mass of the Sun to supermassive black holes millions of times the mass of the Sun. Astronomers think that small black holes merge and gradually grow into large ones, but no one had ever found an intermediate mass, hundreds or thousands of times the mass of the Sun.
- A research team led by Shunya Takekawa at the National Astronomical Observatory of Japan noticed HCN–0.009–0.044, a gas cloud moving strangely near the center of the Galaxy 25,000 light-years away from Earth in the constellation Sagittarius. They used ALMA (Atacama Large Millimeter/submillimeter Array) to perform high resolution observations of the cloud and found that it is swirling around an invisible massive object.
- Takekawa explains, “Detailed kinematic analyses revealed that an enormous mass, 30,000 times that of the Sun, was concentrated in a region much smaller than our Solar System. This and the lack of any observed object at that location strongly suggests an intermediate-mass black hole. By analyzing other anomalous clouds, we hope to expose other quiet black holes.”
- Tomoharu Oka, a professor at Keio University and coleader of the team, adds, “It is significant that this intermediate mass black hole was found only 20 light-years from the supermassive black hole at the Galactic center. In the future, it will fall into the supermassive black hole; much like gas is currently falling into it. This supports the merger model of black hole growth.”
Figure 58: Artist’s impression of a gas cloud swirling around a black hole [image credit: NAOJ (National Astronomical Observatory of Japan)]
• February 26, 2019: Astronomers have unveiled the enigmatic origins of two different gas streams from a baby star. Using ALMA, they found that the slow outflow and the high speed jet from a protostar have misaligned axes and that the former started to be ejected earlier than the latter. The origins of these two flows have been a mystery, but these observations provide telltale signs that these two streams were launched from different parts of the disk around the protostar. 73)
Figure 59: ALMA image of the protostar MMS5/OMC-3. The protostar is located at the center and the gas streams are ejected to the east and west (left and right). The slow outflow is shown in orange and the fast jet is shown in blue. It is obvious that the axes of the outflow and jet are misaligned (image credit: ALMA (ESO/NAOJ/NRAO) Matsushita et al.)
- Stars in the Universe have a wide range of masses, ranging from hundreds of times the mass of the Sun to less than a tenth of that of the Sun. To understand the origin of this variety, astronomers study the formation process of the stars, that is the aggregation of cosmic gas and dust.
- Baby stars collect the gas with their gravitational pull, however, some of the material is ejected by the protostars. This ejected material forms a stellar birth cry which provides clues to understand the process of mass accumulation.
- Yuko Matsushita, a graduate student at Kyushu University and her team used ALMA to observe the detailed structure of the birth cry from the baby star MMS5/OMC-3 and found two different gaseous flows: a slow outflow and a fast jet. There have been a handful of examples with two flows seen in radio waves, but MMS5/OMC-3 is exceptional.
- “Measuring the Doppler shift of the radio waves, we can estimate the speed and lifetime of the gas flows,” said Matsushita, the lead author of the research paper that appeared in the Astrophysical Journal. “We found that the jet and outflow were launched 500 years and 1300 years ago, respectively. These gas streams are quite young.”
- More interestingly, the team found that the axes of the two flows are misaligned by 17 degrees. The axis of the flows can be changed over long time periods due to the precession of the central star. But in this case, considering the extreme youth of the gas streams, researchers concluded that the misalignment is not due to precession but is related to the launching process.
Figure 60: Artist’s impression of the baby star MMS5/OMC-3. ALMA observations identified two gas streams from the protostar, a collimated fast jet and a wide-angle slow outflow, and found that the axes of the two gas flows are misaligned (image credit: NAOJ)
- There are two competing models for the formation mechanism of the protostellar outflows and jets. Some researchers assume that the two streams are formed independently in different parts of the gas disk around the central baby star, while others propose that the collocated jet is formed first, then it entrains the surrounding material to form the slower outflows. Despite extensive research, astronomers had not yet reached a conclusive answer.
- A misalignment in the two flows could occur in the ‘independent model,’ but is difficult in the ‘entrainment model.’ Moreover, the team found that the outflow was ejected considerably earlier than the jet. This clearly backs the ‘independent model.’
- “The observation well matches the result of my simulation,” said Masahiro Machida, a professor at Kyushu University. A decade ago, he performed pioneering simulation studies using a supercomputer operated by the National Astronomical Observatory of Japan. In the simulation, the wide-angle outflow is ejected from the outer area of the gaseous disk around a protostar, while the collimated jet is launched independently from the inner area of the disk. Machida continues, “An observed misalignment between the two gas streams may indicate that the disk around the protostar is warped.”
- “ALMA’s high sensitivity and high angular resolution will enable us to find more and more young, energetic outflow-and-jet-systems like MMS 5/OMC-3,” said Satoko Takahashi, an astronomer at the National Astronomical Observatory of Japan and the Joint ALMA Observatory and co-author of the paper. “They will provide clues to understand the driving mechanisms of outflows and jets. Moreover studying such objects will also tell us how the mass accretion and ejection processes work at the earliest stage of star formation.” 74)
• February 25, 2019: Red giants are old stars that eject gaseous material and solid particles through a stellar wind. Some red giants appeared to lose an exceptionally large amount of mass this way. However, new observations reveal that this is not quite the case. The stellar wind is not more intense than normal, but is affected by a partner that was overlooked until now—a second star that circles the red giant. These are the results of an international study led by Belgian university KU (Katholieke Universiteit) Leuven. 75)
- Humans don't live long enough to realize it, but stars are also born, they age, and they die. It's a process that takes billions of years. As a star gets older, it becomes bigger, colder, and redder - hence the name 'red giants'. Our sun will also become such a red giant in four and a half billion years.
- In the final stage of their life, red giants eject their mass - gas and other matter - in the form of a stellar wind. Earlier observations confirmed that red giants lose a lot of mass this way.
- Twelve mass-loss rate record holders, in particular, have been baffling scientists for decades. These red giants supposedly eject the equivalent of 100 earths per year for 100 to 2,000 years on end. Even astronomically speaking, that's a lot of matter in a short amount of time.
- This was difficult to explain, says Professor Leen Decin from the KU Leuven Institute of Astronomy: "If you look at the mass of such a star in the next phase of its life, the intense stellar wind doesn't last long enough to account for the mass loss that we've seen. It was also statistically improbable that we had discovered twelve of these red giants, knowing that what we were seeing was a phase that lasted only hundreds or thousands of years compared with their billion-year-long life. It's like finding a needle in a haystack twelve times."
- New observations from the ALMA telescope in Chile shed light on what was happening with two of these red giants. "For these stars, the stellar wind forms a spiral. It's an indirect indication that the red giant is not alone, but part of a binary star system. The red giant is the main star with a second star circling it. Both stars affect each other and their environment gravitationally in two ways: on the one hand, the stellar wind is pulled in the direction of the second star and, on the other hand, the red giant itself also wiggles slightly. These movements give the stellar wind a spiral shape."
- The discovery of a partner star made everything fall into place, says Decin: "We believed that these red giants were record holders for mass-loss rate, but that's not the case. It only seemed as though they were losing a lot of mass because there's an area between the two stars where the stellar wind is much more concentrated due to the gravity of the second star. These red giants don't lose the equivalent of 100 earths per year, but rather 10 of them - just like the regular red giants. As such, they also die a bit more slowly than we first assumed. To rephrase in a positive way: these old stars live longer than we thought."
- The astronomers are now investigating whether a system with a binary star could also be the explanation for other special red giants. "We believed that many stars lived alone, but we will probably have to adjust this idea. A star with a partner is likely to be more common than we thought," Decin concludes.
Figure 61: Thanks to new observations from the ALMA telescope in Chile, it became clear that the stellar wind of this red giant forms a spiral. This is an indirect indication that the star is not alone, but part of a binary star (image credit: ALMA (ESO/NAOJ/NRAO)/L. Decin et al.) 76)
• February 7, 2019: A team of astronomers and chemists using ALMA has detected the chemical fingerprints of sodium chloride (NaCl) and other similar salty compounds emanating from the dusty disk surrounding Orion Source I, a massive, young star in a dusty cloud behind the Orion Nebula. 77)
- “It’s amazing we’re seeing these molecules at all,” said Adam Ginsburg, a Jansky Fellow of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, and lead author of a paper accepted for publication in the Astrophysical Journal. “Since we’ve only ever seen these compounds in the sloughed-off outer layers of dying stars, we don’t fully know what our new discovery means. The nature of the detection, however, shows that the environment around this star is very unusual.”
- To detect molecules in space, astronomers use radio telescopes to search for their chemical signatures – telltale spikes in the spread-out spectra of radio and millimeter-wavelength light. Atoms and molecules emit these signals in several ways, depending on the temperature of their environments.
- The new ALMA observations contain a bristling array of spectral signatures – or transitions, as astronomers refer to them – of the same molecules. To create such strong and varied molecular fingerprints, the temperature differences where the molecules reside must be extreme, ranging anywhere from 100 kelvin to 4,000 kelvin (about -175 Celsius to 3700 Celsius). An in-depth study of these spectral spikes could provide insights about how the star is heating the disk, which would also be a useful measure of the luminosity of the star.
- “When we look at the information ALMA has provided, we see about 60 different transitions – or unique fingerprints – of molecules like sodium chloride and potassium chloride coming from the disk. That is both shocking and exciting,” said Brett McGuire, a chemist at the NRAO in Charlottesville, Virginia, and co-author on the paper.
- The researchers speculate that these salts come from dust grains that collided and spilled their contents into the surrounding disk. Their observations confirm that the salty regions trace the location of the circumstellar disk.
- “Usually when we study protostars in this manner, the signals from the disk and the outflow from the star get muddled, making it difficult to distinguish one from the other,” said Ginsburg. “Since we can now isolate just the disk, we can learn how it is moving and how much mass it contains. It also may tell us new things about the star.”
- The detection of salts around a young star is also of interest to astronomers and astrochemists because some of constituent atoms of salts are metals – sodium and potassium. This suggests there may be other metal-containing molecules in this environment. If so, it may be possible to use similar observations to measure the amount of metals in star-forming regions. “This type of study is not available to us at all presently. Free-floating metallic compounds are generally invisible to radio astronomy,” noted McGuire.
Figure 62: Artist impression of Orion Source I, a young, massive star about 1,500 light-years away. New ALMA observations detected a ring of salt — sodium chloride, ordinary table salt — surrounding the star. This is the first detection of salts of any kind associated with a young star. The blue region (about 1/3 the way out from the center of the disk) represents the region where ALMA detected the millimeter-wavelength “glow” from the salts (image credit: NRAO/AUI/NSF; S. Dagnello)
- The salty signatures were found about 30 to 60 astronomical units (AU, or the average distance between the Earth and the Sun) from the host stars. Based on their observations, the astronomers infer that there may be as much as one sextillion (a one with 21 zeros after it) kilograms of salt in this region, which is roughly equivalent to the entire mass of Earth’s oceans.
- “Our next step in this research is to look for salts and metallic molecules in other regions. This will help us understand if these chemical fingerprints are a powerful tool to study a wide range of protoplanetary disks, or if this detection is unique to this source,” said Ginsburg. “In looking to the future, the planned Next Generation VLA would have the right mix of sensitivity and wavelength coverage to study these molecules and perhaps use them as tracers for planet-forming disks.”
- Orion Source I formed in the Orion Molecular Cloud I, a region of explosive starbirth previously observed with ALMA. “This star was ejected from its parent cloud with a speed of about 10 km/s around 550 years ago,” said John Bally, an astronomer at the University of Colorado and co-author on the paper. “It is possible that solid grains of salt were vaporized by shock waves as the star and its disk were abruptly accelerated by a close encounter or collision with another star. It remains to be seen if salt vapor is present in all disks surrounding massive protostars, or if such vapor traces violent events like the one we observed with ALMA.” 78)
• February 4, 2019: Astronomers using ALMA have detected various complex organic molecules around the young star V883 Ori. A sudden outburst from this star is releasing molecules from the icy compounds in the planet forming disk. The chemical composition of the disk is similar to that of comets in the modern Solar System. Sensitive ALMA observations enable astronomers to reconstruct the evolution of organic molecules from the birth of the Solar System to the objects we see today. 79)
- The research team led by Jeong-Eun Lee (Kyung Hee University, Korea) used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect complex organic molecules including methanol (CH3OH), acetone (CH3COCH3), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and acetonitrile (CH3CN). This is the first time that acetone was unambiguously detected in a planet forming region or protoplanetary disk.
- Various molecules are frozen in ice around micrometer-sized dust particles in protoplanetary disks. V883 Ori's sudden flare-up is heating the disk and sublimating the ice, which releases the molecules into gas. The region in a disk where the temperature reaches the sublimation temperature of the molecules is called the "snow line." The radii of snow lines are about a few astronomical units (au) around normal young stars, however, they are enlarged almost 10 times around bursting stars.
- "It is difficult to image a disk on the scale of a few au with current telescopes," said Lee. "However, around an outburst star, ice melts in a wider area of the disk and it is easier to see the distribution of molecules. We are interested in the distribution of complex organic molecules as the building blocks of life."
- Ice, including frozen organic molecules, could be closely related to the origin of life on planets. In our Solar System, comets are the focus of attention because of their rich icy compounds. For example, the European Space Agency's legendary comet explorer Rosetta found rich organic chemistry around the comet Churyumov-Gerasimenko. Comets are thought to have been formed in the outer colder region of the proto-Solar System, where the molecules were contained in ice. Probing the chemical composition of ice in protoplanetary disks is directly related to probing the origin of organic molecules in comets, and the origin of the building blocks of life.
- Thanks to ALMA's sharp vision and the enlarged snow line due to the flare-up of the star, the astronomers obtained the spatial distribution of methanol and acetaldehyde. The distribution of these molecules has a ring-like structure with a radius of 60 au, which is twice the size of Neptune's orbit. The researchers assume that inside of this ring the molecules are invisible because they are obscured by thick dusty material, and are invisible outside of this radius because they are frozen in ice.
- "Since rocky and icy planets are made from solid material, the chemical composition of solids in disks is of special importance. An outburst is a unique chance to investigate fresh sublimates, and thus the composition of solids." says Yuri Aikawa at the University of Tokyo, a member of the research team.
Figure 63: The distribution of dust is shown in orange and the distribution of methanol, an organic molecule, is shown in blue (image credit: ALMA (ESO/NAOJ/NRAO), Lee et al. V883Ori)
- V883 Ori is a young star located at 1300 light-years away from the Earth. This star is experiencing a so-called FU Orionis type outburst, a sudden increase of luminosity due to a bursting torrent of material flowing from the disk to the star. These outbursts last only on the order of 100 years, therefore the chance to observe a burst is rather rare. However, since young stars with a wide range of ages experience FU Ori bursts, astronomers expect to be able to trace the chemical composition of ice throughout the evolution of young stars. 80)
• January 21, 2019: Including the powerful ALMA into an array of telescopes for the first time, astronomers have found that the emission from the supermassive black hole Sagittarius A* (Sgr A*) at the center of our Galaxy comes from a smaller region than previously thought. This may indicate that a radio jet from Sgr A* is pointed almost directly towards the Earth. 81) 82) 83)
- So far, a foggy cloud of hot gas has prevented astronomers from making sharp images of the supermassive black hole Sgr A* and causing doubt on its true nature. They have now included for the first time the powerful ALMA telescope in northern Chile into a global network of radio telescopes to peer through this fog, but the source keeps surprising them: its emission region is so small that the source may actually have to point directly at the direction of the Earth.
- Observing at a frequency of 86 GHz with the technique of VLBI (Very Long Baseline Interferometry), which combines many telescopes to form a virtual telescope the size of the Earth, the team succeeded in mapping out the exact properties of the light scattering blocking our view of Sgr A*. To remove the scattering and obtain the image, the team used a technique developed by Michael Johnson of the Harvard-Smithsonian Center for Astrophysics (CfA). "Even though scattering blurs and distorts the image of Sgr A*, the incredible resolution of these observations allowed us to pin down the exact properties of the scattering,” says Johnson. “We could then remove most of the effects from scattering and begin to see what things look like near the black hole”.
- The high quality of the unscattered image (Figure 65) has allowed the team to constrain theoretical models for the gas around Sgr A*. The bulk of the radio emission is coming from a region of a small size: a mere 300 millionth of an arc degree. “This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” explains Issaoun, who has tested several computer models against the data. “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us”.
- The German astronomer Heino Falcke, Professor of Radio Astronomy at Radboud University and PhD supervisor of Issaoun, calls this statement very unusual, but he also no longer rules it out. Last year, Falcke would have considered this a contrived model, but recently the GRAVITY team came to a similar conclusion using ESO’s Very Large Telescope Interferometer of optical telescopes and an independent technique. “Maybe this is true after all”, concludes Falcke, “and we are looking at this beast from a very special vantage point.”
Legend to Figure 64: The data were correlated at the Max Planck Institute for Radio Astronomy (MPIfR), which also operates the Global Millimeter-VLBI Array (GMVA). Data analysis software was developed at the MIT Haystack Observatory and the Smithsonian Astrophysical Observatory. Several members of the team worked in this project as part of the European Research Council funded BlackHoleCam (BHC) team. The research team is also part of the Event Horizon Telescope (EHT) consortium, an international partnership of thirteen institutes from ten countries: Germany, the Netherlands, France & Spain (via IRAM), USA, Mexico, Japan, Taiwan, Canada and China (via EAO). The participation of ALMA (Atacama Large Millimeter/submillimeter Array) through the ALMA Phasing Project has been decisive for the success of this project. The GMVA is partially supported by the European Union’s Horizon 2020 research and innovation program under grant agreement No. 730562.
- Supermassive black holes are common in the centers of galaxies and may generate the most energetic phenomena in the known universe. It is believed that, around these black holes, matter falls in a rotating disk and part of this matter is expelled in opposite directions along two narrow beams, called jets, at speeds close to the speed of light, which typically produces a lot of radio light. “Whether the radio emission seen from SgrA* originates from a symmetrical underlying structure, or is intrinsically asymmetric is a matter of intense discussion”, explains Thomas Krichbaum, member of the team.
- Sgr A* is the nearest supermassive black hole and 'weighs' about 4 million solar masses. Its apparent size on the sky is less than a 100 millionth of an arc degree, which corresponds to the size of a tennis ball on the moon as seen from the Earth. “The black hole is so small that only VLBI can provide the angular resolution needed to resolve its structure”, says Pablo Torne, astronomer in support on the observations from the IRAM 30-meter telescope. “The first observations of Sgr A* at 86 GHz date from 26 years ago, with only a handful of telescopes. Over the years, the quality of the data has improved steadily as more telescopes join,” adds J. Anton Zensus, director of the Max Planck Institute for Radio Astronomy and head of its Radio Astronomy/VLBI division.
- The findings of Issaoun and her international team including scientists from two research departments (Kramer & Zensus) at MPIfR describe the first observations at 86 GHz in which ALMA also participated, by far the most sensitive telescope at this frequency. ALMA became part of the Global Millimeter VLBI Array (GMVA), which is operated by the Max Planck Institute for Radio Astronomy, in April 2017. The participation of ALMA, made possible by the ALMA Phasing Project effort, has been decisive for the success of this project.
- The participation of ALMA in mm-VLBI is important because of its sensitivity and its location in the southern hemisphere. In addition to ALMA, twelve radio telescopes in North America and Europe also participated in the network. The resolution achieved was twice as large as in previous observations at this frequency and produced the first image of Sgr A* that is considerably reduced in interstellar scattering (an effect caused by density irregularities in the ionized material along the line of sight between Sgr A* and the Earth).
Figure 65: Sgr A* at 86 GHz: simulation (top left), added effects of scattering (top right), scattered image from observations (bottom right), unscattered image, after removing the effects of scattering in the line of sight (bottom left), image credit: S. Issaoun, M. Mościbrodzka, Radboud University (Nijmegen, The Netherlangs) / M. D. Johnson, CfA
• January 24, 2019: Using the ALMA observatory in Chile, a group of astronomers led by MPIA’s (Max Planck Institute for Astronomy, Heidelberg), Henrik Beuther has made the most detailed observation yet of the way that a giant gas cloud fragments into dense cores, which then act as the birthplaces of stars. The astronomers found that the mechanisms for fragmentation are fairly straightforward, resulting from the combination of the cloud’s pressure and gravity. More complex features, such as magnetic lines or turbulence, play a smaller role than previously thought. 84) 85)
- Stars are born when giant clouds of gas and dust collapse. Whenever one of the collapsing regions becomes hot and dense enough for nuclear fusion to set in, a star is born. For massive stars, i.e. those stars that exhibit more than eight times the mass of the Sun, that is only part of the picture, though. The biggest stars in the Universe are not born singly. They are born from massive clouds of molecular gas, which then form a cascade of fragments, with many of the fragments giving birth to a star.
- Astronomers have long wondered whether this fragmentation-mode of forming stars requires different physical mechanisms than for lower-mass stars. Proposals include turbulent gas motion, which could destabilize a region and lead to quicker collapse, or magnetic fields that could stabilize and thus delay collapse.
- The different mechanisms should leave tell-tale traces in regions where multiple stars are forming. The collapse that leads to the formation of high-mass stars takes place on a hierarchy of different levels. On the largest scales, star formation involves giant molecular clouds, which consist mostly of hydrogen gas and can reach sizes between a few dozen and more than a hundred light-years across. Within those clouds are slightly denser clumps, typically a few light-years across. Each clump contains one or more dense cores, less than a fifth of a light-year in diameter. Within each core, collapse leads to the formation of either a single star or multiple stars. Together, the stars produced in the cores of a single clump will form a star cluster.
Tell-tale scales of fragmentation
- The scales of this fragmentation at multiple levels depend on the mechanisms involved. The simplest model can be written down using no more than high school physics: An ideal gas has a pressure that depends on its temperature and density. In a simplified gas cloud, assumed to have constant density, that pressure must be strong enough everywhere to balance the force of gravity (given by Newton’s law of gravity) – even in the center of the cloud, where the inward gravitation-induced push of all the surrounding matter is strongest. Write this condition down, and you will find that any such constant-density cloud can only have a maximum size. If a cloud is larger than this maximum, which is called the Jeans length, the cloud will fragment and collapse.
- Is the fragmentation of young massive clusters really dominated by these comparatively straightforward processes? It doesn’t need to be, and some astronomers have constructed much more complex scenarios, which include the influence of turbulent gas motion and magnetic field lines. These additional mechanisms change the conditions for cloud stability, and typically increase the scales of the different types of fragment.
- Different predictions for cloud sizes offer a way of testing the simple physics scenario against its more complex competitors. That is what Henrik Beuther and his colleagues set out to do when they observed the star formation region G351.77-0.54 in the Southern constellation Scorpius (The Scorpion). Previous observations had indicated that in this region, fragmentation could be caught in the act. But none of these observations had been powerful enough to show the smallest scale of interest for answering the question of fragmentation scales: the protostellar cores, let alone their sub-structure.
ALMA takes the most detailed look yet
- Beuther and his colleagues were able to do more. They used the ALMA Observatory in the Atacama Desert in Chile. ALMA combines the simultaneous observations of up to 66 radio telescopes to achieve a resolution of down to 20 milli-arcseconds, which allows astronomers to discern details more than ten times smaller than with any previous radio telescope, and at unrivalled sensitivity – a combination that has already led to a number of breakthrough observations also in other fields.
- Beuther and his colleagues used ALMA to study the high-mass star-forming region G351.77-0.54 down to sub-core scales smaller than 50 astronomical units (in other words, less than 50 times the average distance between the Earth and the Sun). As Beuther says: “This is a prime example of how technology drives astronomical progress. We could not have obtained our results without the unprecedented spatial resolution and sensitivity of ALMA.”
Figure 66: Image of the massive star cluster NGC 3603, obtained with the VLT (Very Large Telescope). It probably has evolved in the same way as the one just forming in G351.77-0.54, the object depicted in this work (image credit: MPIA, ESO)
- Their results, together with earlier studies of the same cloud at larger scales, indicate that thermal gas physics is winning the day, even when it comes to very massive stars: Both the sizes of clumps within the cloud and, as the new observations show, of cores within the clumps and even of some core substructures are as predicted by Jeans length calculations, with no need for additional ingredients. Beuther comments: “In our case, the same physics provides a uniform description. Fragmentation from the largest to the smallest scales seems to be governed by the same physical processes.”
Small accretion disks: a new challenge
- Simplicity is always a boon for scientific descriptions. However, the same observations also provided a discovery that will keep astronomers on their collective toes. In addition to studying fragmentation, Beuther et al. had been looking to unravel the structure of nascent stars (“protostars”) within the cloud. Astronomers expect such a protostar to be surrounded by a swirling disk of gas, called the accretion disk. From the inner disk of the rim, gas falls onto the growing star, increasing its mass. In addition, magnetic fields produced by the motion of ionized gas and the gas itself interact to produce tightly focused streams called jets, which shoot out some of the matter into space perpendicular to that disk. Submillimeter light from those regions carries tell-tale signs (“Doppler-broadening of spectral lines”) of the motion of dust, which in turn traces the motion of gas. But where Beuther and his collaborators had hoped for a clear signature from an accretion disk, instead, he found mainly the signature of jets, cutting a comparatively smooth path through the surrounding gas. Evidently, the accretion disks are even smaller than astronomers had expected – a challenge for future observations at even greater spatial resolution.
• January 15, 2019: New research led by an astronomer at the University of Warwick has found the first confirmed example of a double star system that has flipped its surrounding disc to a position that leaps over the orbital plane of those stars. The international team of astronomers used ALMA (Atacama Large Millimeter/sub-millimeter Array) to obtain high-resolution images of the Asteroid belt-sized disc. 86)
- The overall system presents the unusual sight of a thick hoop of gas and dust circling at right angles to the binary star orbit. Until now this setup only existed in theorists’ minds, but the ALMA observation proves that polar discs of this type exist, and may even be relatively common.
- The new research is published 14 January by Royal Society University Research Fellow Dr Grant M. Kennedy of the University of Warwick's Department of Physics and Center for Exoplanets and Habitability in Nature Astronomy in a paper entitled "A circumbinary protoplanetary disc in a polar configuration". 87)
- Dr Grant M. Kennedy of the University of Warwick said: "Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets. Some of these planets end up being misaligned with the spin of the star, so we've been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form."
- Dr Kennedy and his fellow researchers used ALMA to pin down the orientation of the ring of gas and dust in the binary star system HD 98800 . The orbit of the binary was previously known, from observations that quantified how the stars move in relation to each other. By combining these two pieces of information they were able to establish that the dust ring was consistent with a perfectly polar orbit. This means that while the stellar orbits orbit each other in one plane, like two horses going around on a carousel, the disc surrounds these stars at right angles to their orbits, like a giant ferris wheel with the carousel at the center
- Kennedy added: "Perhaps the most exciting thing about this discovery is that the disc shows some of the same signatures that we attribute to dust growth in discs around single stars. We take this to mean planet formation can at least get started in these polar circumbinary discs. If the rest of the planet formation process can happen, there might be a whole population of misaligned circumbinary planets that we have yet to discover, and things like weird seasonal variations to consider."
- If there were a planet or planetoid present at the inner edge of the dust ring, the ring itself would appear from the surface as a broad band rising almost perpendicularly from the horizon. The polar configuration means that the stars would appear to move in and out of the disc plane, giving objects two shadows at times. Seasons on planets in such systems would also be different. On Earth they vary throughout the year as we orbit the Sun. A polar circumbinary planet would have seasons that also vary as different latitudes receive more or less illumination throughout the binary orbit.
- Future studies at different wavelengths will provide complementary information and further observational constraints for this source, which holds the key to a better understanding of black holes, the most exotic objects in the known universe.
Figure 67: An artist’s impression of a view of the HD 98800BaBb binary star system and the surrounding disk (image credit: Mark Garlick, University of Warwick)
- Co-author Dr Daniel Price of Monash University’s Center for Astrophysics (MoCA) and School of Physics and Astronomy added:“We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun. But with the new images we see a swirling disc of gas and dust orbiting around two stars. It was quite surprising to also find that that disc orbits at right angles to the orbit of the two stars. Incredibly, two more stars were seen orbiting that disc. So if planets were born here there would be four suns in the sky! ALMA is just a fantastic telescope, it is teaching us so much about how planets in other solar systems are born.”
• January 10, 2019: Galaxies come in a wide variety of shapes and sizes. Some of the most significant differences among galaxies, however, relate to where and how they form new stars. Compelling research to explain these differences has been elusive, but that is about to change. 88)
- A vast, new research project with the ALMA (Atacama Large Millimeter/submillimeter Array), known as PHANGS-ALMA (Physics at High Angular Resolution in Nearby GalaxieS), delves into this question with far greater power and precision than ever before by measuring the demographics and characteristics of a staggering 100,000 individual stellar nurseries spread throughout 74 galaxies.
- PHANGS-ALMA, an unprecedented and ongoing research campaign, has already amassed a total of 750 hours of observations and given astronomers a much clearer understanding of how the cycle of star formation changes, depending on the size, age, and internal dynamics of each individual galaxy. This campaign is 10 to 100 times more powerful (depending on your parameters) than any prior survey of its kind.
- "Some galaxies are furiously bursting with new stars while others have long ago used up most of their fuel for star formation. The origin of this diversity may very likely lie in the properties of the stellar nurseries themselves," said Erik Rosolowsky, an astronomer at the University of Alberta in Canada and a co-principal investigator of the PHANGS-ALMA research team.
- He presented initial findings of this research at the 233rd meeting of the American Astronomical Society being held this week in Seattle, Washington. Several papers based on this campaign have also been published in the Astrophysical Journal and the Astrophysical Journal Letters. 89) 90) 91)
- "Previous observations with earlier generations of radio telescopes provide some crucial insights about the nature of cold, dense stellar nurseries," Rosolowsky said. - "These observations, however, lacked the sensitivity, fine-scale resolution, and power to study the entire breadth of stellar nurseries across the full population of local galaxies. This severely limited our ability to connect the behavior or properties of individual stellar nurseries to the properties of the galaxies that they live in."
- For decades, astronomers have speculated that there are fundamental differences in the way disk galaxies of various sizes convert hydrogen into new stars. Some astronomers theorize that larger, and generally older galaxies, are not as efficient at stellar production as their smaller cousins.
- The most logical explanation would be that these big galaxies have less efficient stellar nurseries. But testing this idea with observations has been difficult.
Figure 68: The ALMA telescope is conducting an unprecedented survey of nearby disk galaxies to study their stellar nurseries. With it, astronomers are beginning to unravel the complex and as-yet poorly understood relationship between star-forming clouds and their host galaxies [image credit: ALMA (ESO/NAOJ/NRAO); NRAO/AUI/NSF, B. Saxton]
• January 1, 2019: Using observations from the ALMA radio observatory in Chile, researchers have observed, for the first time, a warped disk around an infant protostar that formed just several tens of thousands of years ago. This implies that the misalignment of planetary orbits in many planetary systems —including our own— may be caused by distortions in the planet-forming disk early in their existence. 92)
- The planets in our Solar System orbit the Sun in planes that are at most about seven degrees offset from the equator of the Sun itself. It has been known for some time that many extrasolar systems have planets that are not lined up in a single plane or with the equator of the star. One explanation for this is that some of the planets might have been affected by collisions with other objects in the system or by stars passing by the system, ejecting them from their initial orbital plane.
- However, the possibility remained that the formation of planets out of the normal plane was actually caused by a warping of the star-forming cloud out of which the planets were born. Recently, images of protoplanetary disks—rotating disks where planets form around a star—have in fact showed such warping. But it was still unclear how early this happened.
- In the latest findings, published in Nature, the group from the RIKEN Cluster for Pioneering Research (CPR) and Chiba University in Japan have discovered that L1527; an infant protostar still embedded within a cloud, has a disk that has two parts —an inner one rotating in one plane, and an outer one in a different plane. The disk is very young and still growing. L1527, which is about 450 light-years away in the Taurus Molecular Cloud, is a good object for study as it has a disk that is nearly edge-on to our view. 93)
- According to Nami Sakai, who led the research group, “This observation shows that it is conceivable that the misalignment of planetary orbits can be caused by a warp structure formed in the earliest stages of planetary formation. We will have to investigate more systems to find out if this is a common phenomenon or not.”
- The remaining question is what caused the warping of the disk. Sakai suggests two reasonable explanations. “One possibility,” she says, “is that irregularities in the flow of gas and dust in the protostellar cloud are still preserved and manifest themselves as the warped disk. A second possibility is that the magnetic field of the protostar is in a different plane from the rotational plane of the disk, and that the inner disk is being pulled into a different plane from the rest of the disk by the magnetic field.” She says they plan further work to determine which is responsible for the warping of the disk.
- The ALMA observatory in Chile is managed by an international consortium including the National Astronomical Observatory of Japan (NAOJ).
Figure 69: Artist’s impression of a warped disk around a protostar. ALMA observed the protostar IRAS04368+2557 in the dark cloud L1527 and discovered that the protostar has a disk with two misaligned parts (image credit: RIKEN, Japan)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).