6 June, 2013
Astronomers using the new Atacama Large Millimeter/submillimeter Array (ALMA) have imaged a region around a young star where dust particles can grow by clumping together. This is the first time that such a “dust trap” has been clearly observed and modelled. It solves a long-standing mystery how dust particles in discs grow to larger sizes so that they can eventually form comets, planets, and other rocky bodies. The results are published in the journal Science on 7 June 2013.
Astronomers now know that planets around other stars are plentiful. But they do not fully understand how they form and there are many aspects of planet (and comet and asteroid) formation that remain a mystery. However, new observations exploiting the power of ALMA are now answering one of the biggest questions: how do tiny grains of dust in the disc around a young star grow bigger and bigger — to eventually become rubble, and even boulders well beyond a meter in size?
Computer models suggest that dust grains grow when they collide and stick together. However, when these bigger grains collide again at high speed they are often smashed to pieces and sent back to square one. Even when this does not happen, the models show that the larger grains would quickly move inwards because of friction between dust and gas and fall onto their parent star before they have a chance to grow even further.
Somehow the dust needs a safe haven where the particles can continue growing until they are big enough to survive on their own . To get past this vexing size limit, astronomers have theorized that swirling eddies in the disk could create dust traps, regions that enable dust particles to cling together, eventually setting the stage for the formation of larger and larger objects. But there was no observational proof of their existence up to now.
Nienke van der Marel, a PhD student at Leiden Observatory in the Netherlands, and lead author of the article was using ALMA along with her co-workers, to study a disc called Oph-IRS 48 . They found that this system was circled by a ring of gas with a central hole that was probably created by an unseen planet or companion star. Earlier observations using ESO’s Very Large Telescope had already shown that the small dust particles also form a similar ring structure. But the new ALMA view of where the larger dust particles were found was very different!
“At first the shape of the dust in the image was a complete surprise for us,” says van der Marel. “Instead of the ring we had expected to see, we found a very clear cashew-nut shape! We had to convince ourselves that this feature was real, but the strong signal and sharpness of the ALMA observations left no doubt about the structure. Suddenly we realized what we had found.”
What had been discovered was a region where bigger dust grains were trapped and could grow much larger by colliding and sticking together. This was a dust trap — just what the theorists were looking for.
As van der Marel explains: “It’s likely that we are looking at a kind of comet factory because the particles can grow in this dust trap up to cometary sizes of a few kilometers. The dust is not likely to form full-sized planets at this distance from the star. But in the near future ALMA will be able to observe dust traps closer to their parent stars, where the same mechanisms are at work. Such dust traps really would be the cradles for new-born planets.”
The dust trap forms as bigger dust particles move in the direction of higher pressure. Computer modelling has shown that such a high pressure region can originate from the motions of the gas at the edge of a gas hole — just like the one found in this disc.
“The combination of modelling work and high quality observations of ALMA makes this a unique project”, says Cornelis Dullemond from the Institute for Theoretical Astrophysics in Heidelberg, Germany, who is an expert on dust evolution and disc modeling and member of the team. “Around the time that these observations were obtained, we were working on models predicting exactly these kinds of structures: a very lucky coincidence.”
The observations were made while the ALMA array was still being constructed. They made use of the ALMA Band 9 receivers  — European-made devices that allow ALMA to create its sharpest images.
“These observations show that ALMA is capable of delivering transformational science, even with less than half of the full array in use,” says Ewine van Dishoeck of the Leiden Observatory, who has been a major driver of the ALMA project for more than 20 years. “The incredible jump in both sensitivity and image sharpness in Band 9 gives us the opportunity to study basic aspects of planet formation in ways that were simply not possible before.”
 The cause of the dust trap, in this case a vortex in the disc’s gas’, has typical life spans of hundreds of thousand of years. Even when the dust trap ceases to work, the dust accumulated in the trap would take millions of years to disperse providing ample time for the dust grains to grow larger.
 The name is a combination of the constellation name of the star-forming region where the system is found and the type of source, so Oph stands for the constellation of Ophiuchus (The Serpent Bearer), and the IRS stands for infrared source. The distance from Earth to Oph-IRS 48 is about 400 light-years.
 ALMA can observe in different frequency bands. Band 9, operating at wavelengths of about 0.4–0.5 millimeters, is the mode that so far provides the sharpest images.
This research was presented in a paper “A major asymmetric dust trap in a transition disk”, by van der Marel et al, to appear in the journal Science on 7 June 2013.
The team is composed of Nienke van der Marel (Leiden Observatory, Leiden, the Netherlands), Ewine F. van Dishoeck (Leiden Observatory; Max-Planck-Institut für Extraterrestrische Physik
Garching, Germany [MPE]), Simon Bruderer (MPE), Til Birnstiel (Harvard-Smithsonian Center for Astrophysics, Cambridge, USA [CfA]), Paola Pinilla (Heidelberg University, Heidelberg, Germany), Cornelis P. Dullemond (Heidelberg University), Tim A. van Kempen (Leiden Observatory; Joint ALMA Offices, Santiago, Chile), Markus Schmalzl (Leiden Observatory), Joanna M. Brown (CfA), Gregory J. Herczeg (Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China), Geoffrey S. Mathews (Leiden Observatory) and Vincent Geers (Dublin Institute for Advanced Studies, Dublin, Ireland).
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
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