Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have captured an image of a so-called ‘dust trap’ – a region where dust particles can grow by clumping together – around the young star Oph-IRS 48.
Artist’s impression of the dust trap in the system Oph-IRS 48 (ESO / L. Calçada)
Scientists know that planets around other stars are plentiful, but they do not fully understand how they form and there are many aspects of the formation of comets, planets and other rocky bodies that remain a mystery.
The new observations 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 the dust and gas and fall onto their parent star, leaving no chance that they could 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. Such ‘dust traps’ have been proposed, but there was no observational proof of their existence up to now.
The astronomers have studied the disc in a system called Oph-IRS 48. They found that the star was circled by a ring of gas with a central hole that was probably created by an unseen planet or companion star. Earlier observations had already shown that the small dust particles also formed a similar ring structure. But the new view of where the larger millimeter-sized dust particles were found was very different.
This ALMA image shows the dust trap around Oph-IRS 48 (ALMA/ ESO / NAOJ / NRAO / Nienke van der Marel)
“At first the shape of the dust in the image came as a complete surprise to us. 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. Then we realized what we had found,” said Nienke van der Marel from Leiden Observatory in the Netherlands, lead author of a paper published in the journal Science.
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.
“It’s likely that we are looking at a kind of comet factory as the conditions are right for the particles to grow from millimeter to comet size. 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 regions of higher pressure. Computer modeling 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.
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Bibliographic information: Nienke van der Marel et al. 2013. A Major Asymmetric Dust Trap in a Transition Disk. Science, vol. 340, no. 6137, pp. 1199-1202; doi: 10.1126/science.1236770
