Molecules with ALMA at Planet-forming Scales (MAPS) VIII: CO Gap in AS 209--Gas Depletion or Chemical Processing?

TL;DR

This study investigates the cause of CO gas depletion in the AS 209 protoplanetary disk, finding that chemical processing likely explains the observed gas substructure better than a massive planet, challenging previous assumptions.

Contribution

The paper demonstrates that chemical processing, rather than a planet, is the main cause of CO gas depletion in the disk, using thermochemical modeling and high-resolution ALMA data.

Findings

CO depletion can be explained by chemical processing rather than planet-induced gaps.

A massive planet (>0.2 M_Jup) is unlikely to be responsible for the observed CO depression.

C$_2$H traces changes in C/O ratio, not H$_2$ surface density.

Abstract

Emission substructures in gas and dust are common in protoplanetary disks. Such substructures can be linked to planet formation or planets themselves. We explore the observed gas substructures in AS 209 using thermochemical modeling with RAC2D and high-spatial resolution data from the Molecules with ALMA at Planet-forming Scales(MAPS) program. The observations of C$^{18}$O J=2-1 emission exhibit a strong depression at 88 au overlapping with the positions of multiple gaps in millimeter dust continuum emission. We find that the observed CO column density is consistent with either gas surface-density perturbations or chemical processing, while C$_2$H column density traces changes in the C/O ratio rather than the H$_2$ gas surface density. However, the presence of a massive planet (> 0.2 M$_{Jup}$) would be required to account for this level of gas depression, which conflicts with constraints set by the dust emission and the pressure profile measured by gas kinematics. Based on our models, we infer that a local decrease of CO abundance is required to explain the observed structure in CO, dominating over a possible gap-carving planet present and its effect on the H$_2$ surface density. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement Series.