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

Felipe Alarcón, Arthur D. Bosman+23 more|arXiv (Cornell University)|Sep 13, 2021

Astrophysics and Star Formation StudiesPhysics and Astronomy104 references35 citations

TL;DR

This study investigates the origin of a CO emission gap at 88 au in the protoplanetary disk AS 209 using high-resolution ALMA data and thermochemical modeling. It finds that chemical processing—specifically, localized CO depletion due to freeze-out or photodesorption—better explains the observed CO column density depression than a planet-induced gas gap, as the latter would require an implausibly massive planet (>0.2 M$_{\rm Jup}$) inconsistent with dust and kinematic constraints.

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.

Motivation & Objective

• To determine whether the observed CO emission gap in AS 209 is caused by gas depletion from a planet or by local chemical processing.
• To test the degeneracy between CO abundance variations and H2 surface density perturbations in shaping substructures.
• To assess whether C2H emission can serve as a tracer of active chemistry and distinguish between physical and chemical origins of substructures.
• To evaluate the consistency of a massive planet scenario with dust continuum gaps and gas kinematics in AS 209.
• To model the thermal and chemical structure of the disk using RAC2D to compare CO and C2H column density profiles under different physical assumptions.

Proposed method

• Used high-spatial-resolution ALMA observations of C18O J=2–1 and C2H lines at ~0.′′15 resolution from the MAPS program.
• Applied thermochemical modeling with RAC2D to simulate disk chemistry and thermal structure under two scenarios: CO depletion (Model A) and gas gap with constant CO abundance (Model B).
• Constructed models to match observed CO column densities and compared predicted C2H column densities, emission heights, and kinematic deviations.
• Varied small-grain depletion factors (10%, 1%, 0.1%) to assess their impact on CO and C2H abundance profiles.
• Analyzed Keplerian deviations in gas kinematics to constrain H2 surface density perturbations.
• Used emission layer heights from Teague et al. (2018) and Law et al. (2021b) to validate model kinematics and emission geometry.

Experimental results

Research questions

• RQ1 Is the CO emission gap at 88 au in AS 209 best explained by a planet-induced gas gap or by local chemical processing?
• RQ2 What level of CO abundance depletion is required to reproduce the observed CO column density depression?
• RQ3 Can C2H emission serve as a diagnostic for active chemistry, particularly in regions of low gas density or high C/O ratio?
• RQ4 Is a massive planet (>0.2 M$_{\rm Jup}$) required to explain the CO gap, and is this consistent with dust and kinematic constraints?
• RQ5 How do differences in CO and C2H vertical abundance distributions affect the radial emission profiles of CO isotopologues?

Key findings

• The observed CO column density depression in AS 209 is consistent with a local decrease in CO abundance, requiring at least 47% depletion compared to a smooth surface density profile.
• A massive planet (>0.2 M$_{\rm Jup}$) would be necessary to explain the CO gap via gas depletion, but such a planet is inconsistent with dust continuum gaps and kinematic constraints.
• C2H column density traces changes in the C/O ratio rather than H2 surface density, indicating that C2H emission is more sensitive to chemical conditions than gas density.
• In Model A (CO depletion), CO has a more uniform vertical distribution than in Model B (gas gap), which could lead to observable differences in CO isotopologue emission profiles.
• C2H is produced closer to the midplane in regions of small-grain depletion, and Model B shows higher C2H abundances at lower altitudes due to deeper UV penetration and reduced gas-phase reactions.
• Keplerian deviation analysis shows only ~1% deviation in the inner 100 au, ruling out strong H2 depletion at 59 au and disfavoring a massive planet as the cause of the CO gap.