Molecules with ALMA at Planet-forming Scales (MAPS). VII. Sub-stellar O/H and C/H and super-stellar C/O in planet feeding gas

TL;DR

This study uses ALMA data and thermochemical models to analyze the elemental composition of gas in protoplanetary disks, revealing high C/O ratios and substellar O/H and C/H ratios, which influence planet formation and composition.

Contribution

It provides the first detailed constraints on gas-phase elemental abundances in planet-forming regions using high-resolution ALMA observations and thermochemical modeling.

Findings

Elevated C/O ratios (~2.0) are necessary to match observed C2H column densities.

Gas in these disks has substellar O/H and C/H ratios by factors of 4-10.

Most volatile carbon and oxygen are trapped on grains, affecting planetary atmospheres.

Abstract

The elemental composition of the gas and dust in a protoplanetary disk influences the compositions of the planets that form in it. We use the Molecules with ALMA at Planet-forming Scales (MAPS) data to constrain the elemental composition of the gas at the locations of potentially forming planets. The elemental abundances are inferred by comparing source-specific gas-grain thermochemical models, with variable C/O ratios and small-grain abundances, from the DALI code with CO and C2H column densities derived from the high-resolution observations of the disks of AS 209, HD 163296, and MWC 480. Elevated C/O ratios (~2.0), even within the CO ice line, are necessary to match the inferred C2H column densities, over most of the pebble disk. Combined with constraints on the CO abundances in these systems, this implies that both the O/H and C/H ratios in the gas are substellar by a factor of 4-10, with the O/H depleted by a factor of 20-50, resulting in the high C/O ratios. This necessitates that even within the CO ice line, most of the volatile carbon and oxygen is still trapped on grains in the midplane. Planets accreting gas in the gaps of the AS 209, HD 163296, and MWC 480 disks will thus acquire very little carbon and oxygen after reaching the pebble isolation mass. In the absence of atmosphere-enriching events, these planets would thus have a strongly substellar O/H and C/H and superstellar C/O atmospheric composition.