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

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