High Spatial Resolution Observations of Molecular Lines toward the Protoplanetary Disk around TW Hya with ALMA

We present molecular line observations of (CO)-C-13 and (CO)-O-18 J = 3 - 2, CN N = 3 - 2, and CS J = 7 - 6 lines in the protoplanetary disk around TW Hya at a high spatial resolution of similar to 9 au (angular resolution of 0.'' 15), using the Atacama Large Millimeter/Submillimeter Array. A possible gas gap is found in the deprojected radial intensity profile of the integrated (CO)-O-18 line around a disk radius of similar to 58 au, slightly beyond the location of the au-scale dust clump at similar to 52 au, which resembles predictions from hydrodynamic simulations of planet-disk interaction. In addition, we construct models for the physical and chemical structure of the TW Hya disk, taking account of the dust surface density profile obtained from high spatial resolution dust continuum observations. As a result, the observed flat radial profile of the CN line intensities is reproduced due to a high dust-to-gas surface density ratio inside similar to 20 au. Meanwhile, the CO isotopologue line intensities trace high temperature gas and increase rapidly inside a disk radius of similar to 30 au. A model with either CO gas depletion or depletion of gas-phase oxygen elemental abundance is required to reproduce the relatively weak CO isotopologue line intensities observed in the outer disk, consistent with previous atomic and molecular line observations toward the TW Hya disk. Further observations of line emission of carbon-bearing species, such as atomic carbon and HCN, with high spatial resolution would help to better constrain the distribution of elemental carbon abundance in the disk gas.

Automated summary

The study used high spatial resolution dust continuum observations and molecular line observations to confirm the physical and chemical structure models of the TW Hya disk, revealing specific gas and dust distributions in different parts of the disk. Significant differences in gas composition and intensity were observed at different radii within the disk. The research is important for understanding the disk-star system interaction in the process of planetary formation.