June 6, 2013
Dust Trap Around Young Star Is A ‘Comet Factory’
Lee Rannals for redOrbit.com — Your Universe Online
The team found a region around a young star where dust particles can grow by clumping together, giving the scientists the first clear observation of a long-standing mystery of how dust particles in disc grow to larger sizes that eventually form comets, planets and other rocky bodies.
Scientists do not fully understand how planets around other stars form, and there are many aspects of the formation of comets, planets and other rocky bodies that have eluded researchers. However, the latest observations give a great opportunity for scientists to help put the pieces of this puzzle together using computer modeling.
"There is a major hurdle in the long chain of events that leads from tiny dust grains to planet-sized objects," said Til Birnstiel, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and co-author on the paper published in Science. "In computer models of planet formation, dust grains must grow from submicron sizes to objects up to ten times the mass of the Earth in just a few million years. But once particles grow larger enough, they begin to pick up speed and either collide, sending them back to square one, or slowly drift inward, thwarting further growth."
Current computer models show that dust grains grow when they collide and stick together, but when these larger grains collide, they are smashed to pieces and sent back to square one. Somehow the dust needs a safe haven where the particles can continue growing until they are big enough to survive on their own.
The astronomers observing the disc Oph-IRS 48 said they found that the star was surrounded by a ring of gas with a central hole that was probably created by an unseen planet or companion star.
“At first the shape of the dust in the image came as a complete surprise to us,” says Nienke van der Marel, a PhD student at Leiden Observatory in the Netherlands. “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.”
They discovered a region where bigger dust grains were trapped and could grow much larger by colliding and sticking together.
“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," Marel said. "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.
“The combination of modeling 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 a 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 cause of the dust trap has a typical life of hundreds of thousands of years. Even when the dust trap stops working, the dust accumulated could take millions of years to disperse, which provides ample time for the dust grains to grow larger.
One comet that will emerge in our skies in November may have originated from a dust trap like this. Comet ISON is expected to put on a spectacular show in the night sky this fall. Scientists even believe Earth will be passing through the dust from this space object around January 12, 2014.