Chemical Diagnostics for Tracing the Physical Structures in Disk-Forming Regions of Young Low-Mass Protostellar Sources

To understand the origin of the Solar system, the physical/chemical evolution along the star/planet formation is a key issue. With the advent of ALMA, extensive observational studies have revealed that both the physical structure and the chemical composition drastically change during the disk formation around protostars. Furthermore, it has been found that molecular distributions are sensitive to changes in the physical conditions. Some kinds of molecular lines are therefore prospected to work as 'molecular markers' to slectively highlight particular structures of disk forming regions.

Specifically, sulfur-bearing species seem good tracers; in a young low-mass protostellar source IRAS 16293-2422 Source A, the kinematic structures of its circummultiple structure and its circumstellar disk are traced by the OCS and H2CS emission, respectively. The gas in its circummultiple structure was found to keep falling even beyond its centrifugal radius, which is often assumed to be the outer edge of a Keplerian disk. Machine/deep learnings may help us to extract the key information of the gas dynamics from the enormous molecular line data obtained with ALMA.

Angular momentum of the gas is the essential topic to understand the structure formation. The chemical diagnostics with the aid of the molecular markers can be a helpful tool to tackle with the redistribution of the angular momentum among the disk/envelope and outflow structures. Conversely, detailed physical characterization is essential to elucidating the chemical evolution occurring there.