The Gas In Debris Disks Comes From Internal Collisions

Artist's impression of collisions in a debris disk. Higuchi et al/RIKEN

The formation of star systems is still, for the most part, mysterious. However, as our telescopes get better and more sensitive, we are getting a greater glimpse into the beginnings of planets and stars.

One mystery that has now been solved revolved around the presence of gas within debris disks, the donut of leftover material that usually surrounds a baby star system. Debris disks shouldn’t have much gas, but astronomers have found it in a few cases. This puzzled them for awhile, but now they've worked out that the gas is generated by internal collisions in the disk.

The study, published in The Astrophysical Journal Letters, focused on two specific debris disks – one around 49 Ceti and one around Beta Pictoris – using the Atacama Submillimeter Telescope Experiment. The team discovered that, at least around those two young systems, there was very little hydrogen in the debris disks.

Hydrogen is not only the most abundant gas in the universe, but it is also crucial for the formation of carbon monoxide, as it helps carbon react with oxygen. This discovery implies that the gas is not the primordial hydrogen that made the cloud, and the gas is more likely a consequence of collisions between debris.

“We were surprised to find atomic carbon in the disk, the first time this observation has been made at submillimeter wavelengths,” lead author Aya Higuchi of RIKEN said in a statement. “But more so, we were surprised at how much there was. It was about as common as the carbon monoxide.”

Star systems are formed by gas collapsing into stars. The leftover gas is either absorbed by the star, blown away, or it’s captured by giant planets. What remains is a disk of debris, similar to the Oort Cloud – the region at the edge of the Solar System far beyond the orbit of Pluto.

The disk of debris after star formation should have little gas, but the discovery of gas in systems like Beta Pictoris, which is 63 light-years away, didn’t fit well with the theory. These observations finally clarify that puzzle.   

“This work will also help to understand how a protoplanetary disk evolves into a debris disk by distinguishing the origin of the gas in the disks,” Higuchi concluded.



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