exoALMA shows intricate structures in planet-forming disks

In a major project milestone, the first 17 papers with scientific results from the exoALMA project have been published. The project has studied 15 protoplanetary disks around young stars in unprecedented detail, revealing the complex dynamics within these planetary birthplaces. The results are based on observations with the ALMA observatory, and were obtained using newly developed techniques for producing particularly detailed images with ALMA.
 

A group of astronomers hunting for planets as they form within disks of gas and dust, which includes researchers from the Max Planck Institute for Astronomy (MPIA), has observed such protoplanetary disks in unprecedented detail. The observations are part of the collaborative project exoALMA at the ALMA (Atacama Large Millimeter/submillimeter Array) observatory in Chile. The results, which are now being presented in 17 research articles that comprise a special issue of the Astrophysical Journal Letters, constitute a major milestone for a project that has included the development new observational techniques, more detailed submillimeter observations of planetary birthplaces than ever before, and thorough analyses linking the observations to the physics of planet formation.

exoALMA is what is known as a “large program” – a large-scale project requiring considerable observation time and promising major advances or even breakthroughs in the chosen subfield of astronomy. The international exoALMA team combines the expertise of fourty astronomers from different institutions. Reaching the required accuracy in their observations forced the astronomers to push the boundaries on observational techniques: "The new approaches we’ve developed to gather this data and images are like switching from reading glasses to high-powered binoculars—they reveal a whole new level of detail in these planet-forming systems," says Richard Teague, overall coordinator (PI) of the exoALMA project (and formerly a PhD student at the Max Planck Institute for Astronomy).

Unprecedented insight into the 3D structure of protoplanetary disks

Planets are born in protoplanetary disks of gas and dust around young stars. exoALMA looked for the early stages of that process by studying disturbances in 15 of those disks. "It's like trying to spot a fish by looking for ripples in a pond,” says Christophe Pinte, an astrophysicist at the Institute of Astrophysics and Planetology of Grenoble and at Monash University, who is part of the exoALMA leadership team as a Co-PI (“Co-Principal Investigator”).

But exoALMA can do much more than merely spot the presence of young planets. “Our program allowed for a systematic study of the 3-dimensional structure of many of these disks, providing key insights into the physical properties of the planet formation environment,” says Myriam Benisty, a director at the Max Planck Institute for Astronomy in Heidelberg and an exo-ALMA Co-PI. Mario Flock, a research group leader at MPIA, adds: “With the exoALMA project we enter a new area of planet formation research. For the first time, we can directly observe the gas kinematics within protoplanetary disks.”

Stefano Facchini of the University of Milan, another exoALMA Co-PI, says: “One of the main results of the programs is the evidence that subtle pressure differences within disks ‘shepherd’ dust grains into concentric rings. Such rings have been observed for more than a decade now, and in a large number of protoplanetary disks. This mechanism is something we theoretically expected, but it is incredible to finally observe it in a large sample of disks."

The observations also allow for an estimate of the mass distribution within the disk, by measuring how fast different portions of the disk rotate. (Incidentally, this is the same method by which astronomers have deduced the presence of dark matter in disk galaxies.) The results give an indication of how much mass is available for forming planets within the disk.

A complex collaborative project

ALMA is an interferometric observatory, with up to 66 radio antennas that can be connected to perform as if they were a single kilometer-size radio telescope. Reconstructing clean, detailed images with this kind of set-up requires careful analysis. “We developed new techniques to precisely align observations taken at different times and remove unwanted noise and distortions,” says Ryan Loomis, a scientist with the U.S. National Science Foundation National Radio Astronomy Observatory. These techniques resulted in improved resolution and sensitivity of the observations. The exoALMA team took great care to test and validate their newly-developed methods, and the improved techniques not only helped to obtain the exoALMA results, but will also benefit future observations of this kind by other researchers.

One of the benefits of a large collaborative project is the stimulating environment it provides for researchers who have just started their scientific careers. “Most of this work was done by researchers early in their career, who wrote 12 out of 17 of our papers,” adds Misato Fukagawa, of the National Astronomical Observatory of Japan, an exoALMA co-PI. The 17 papers with scientific results that have been published now are the first of what are planned to be two waves of exoALMA publications, with the second set of papers addressing the complex two-dimensional patterns that exoALMA has observed in protoplanetary disks. Together with the papers published now, the complete calibrated exoALMA data set has been made public.

Background information

The 17 exoALMA articles are being published as a special issue of the Astrophysical Journal Letters. The MPIA researchers involved are Myriam Benisty (also Université Côte d’Azur; Co-PI), Daniele Fasano (also Université Côte d’Azur), Mario Flock, Marcelo Barraza (now at MIT), and Andrew Winter (also Université Côte d’Azur).

Part of MPIA’s contributions to the exoALMA project were made possible by the ERC Consolidator Grant awarded to Myriam Benisty by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (PROTOPLANETS, grant agreement No. 101002188), as well as the ERC Consolidator Grant for Mario Flock (RAPTOR, Grant Agreement Nr. 757957).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (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 National Science and Technology Council (NSTC) in Taiwan 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 by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.