2021ApJ...914..113N

We present molecular line observations of ¹³CO and C¹⁸O 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 ∼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 C¹⁸O line around a disk radius of ∼58 au, slightly beyond the location of the au-scale dust clump at ∼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 ∼20 au. Meanwhile, the CO isotopologue line intensities trace high temperature gas and increase rapidly inside a disk radius of ∼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.