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

TL;DR

This study uses high-resolution ALMA observations to analyze molecular lines in the TW Hya protoplanetary disk, revealing potential planet-disk interactions, chemical structure, and gas depletion zones, advancing understanding of disk composition and planet formation.

Contribution

First high-resolution ALMA molecular line observations of TW Hya's disk revealing gas gaps and detailed chemical structure modeling, including dust-gas interactions.

Findings

Detection of a possible gas gap at ~58 au in C18O emission.

Reproduction of flat CN line profile with high dust-to-gas ratio inside 20 au.

Evidence for CO gas depletion or oxygen depletion in the outer disk.

Abstract

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 towards 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.