Protoplanetary Disks ‘Bulge’ in Complex and Asymmetrical Ways
ALMA/DICYT Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have studied two massive protoplanetary disks and discovered large-scale asymmetries in the way their dust is distributed.
By comparing these results with other early ALMA observations, the researchers speculate that asymmetrical disks may be the norm and an essential step along the path to planet formation.
In these protoplanetary systems, dubbed SR 21 and SAO 206462, ALMA revealed dust-depleted disk cavities near the stars and gaseous disks extending farther into space. It was in these extended gaseous regions that the asymmetries in the dust were discovered.
"These observations revealed large-scale asymmetries in the outer portions of the disks, each containing as much material as two Jupiter-sized planets," said Chilean astronomer Laura Perez with the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico. "The best fit to explain these observations is a vortex structure in the outer disk."
Similar in some respects to other asymmetrical bulges known as "dust traps" seen in earlier ALMA observations, these vortices could help the dust and gas around stars to clump together more readily. Models predict that once dust grains achieve a certain size, they either would drift inward toward the star or collide with other similar-size grains. By concentrating dust and gas, these asymmetrical regions may provide a safe haven where grain growth can continue.
The vortices revealed in the new ALMA observations occupy a much wider swath of the disk than seen in previous observations, however. This may indicate that they formed under different conditions, possibly as the result of turbulence in the disk.
Both SR 21 and SAO 206462 are in the process of transforming from optically thick disks brimming with dust and gas to optically thin debris disks, potentially harboring fully formed planets. Earlier observations with other telescopes failed to detect these large asymmetrical regions. ALMA’s enhanced resolution and sensitivity, however, could discern these features.
Perez and her colleagues speculate the process (or processes) that creates these asymmetries may be taking place in most transitional disks.
ALMA also detected residual structures that don’t quite fit into the vortex model. It’s possible these represent spiral structures in the disks, but future observations with ALMA and the Karl G. Jansky Very Large Array (VLA) are necessary to confirm this. The results are published in Astrophysical Journal Letters.