If you like C/O variations, you should have put a ring on it

TL;DR

This study investigates how dust ring locations relative to the CO snowline influence C$_2$H emission in protoplanetary disks, revealing a correlation that impacts understanding of volatile distribution and planet formation.

Contribution

It provides observational evidence linking CO-icy dust reservoirs and C$_2$H emission, emphasizing the role of dust transport and pressure bumps in disk chemistry.

Findings

C$_2$H emission correlates with disks having dust rings outside the CO snowline.

Disks without pressure bumps or with rings inside the snowline show no C$_2$H detection.

Results highlight the importance of dust transport in chemical models of disks.

Abstract

The C/O-ratio as traced with C$_2$H emission in protoplanetary disks is fundamental for constraining the formation mechanisms of exoplanets and our understanding of volatile depletion in disks, but current C$_2$H observations show an apparent bimodal distribution which is not well understood, indicating that the C/O distribution is not described by a simple radial dependence. The transport of icy pebbles has been suggested to alter the local elemental abundances in protoplanetary disks, through settling, drift and trapping in pressure bumps resulting in a depletion of volatiles in the surface and an increase of the elemental C/O. We combine all disks with spatially resolved ALMA C$_2$H observations with high-resolution continuum images and constraints on the CO snowline to determine if the C$_2$H emission is indeed related to the location of the icy pebbles. We report a possible correlation between the presence of a significant CO-icy dust reservoir and high C$_2$H emission, which is only found in disks with dust rings outside the CO snowline. In contrast, compact dust disks (without pressure bumps) and warm transition disks (with their dust ring inside the CO snowline) are not detected in C$_2$H, suggesting that such disks may never have contained a significant CO ice reservoir. This correlation provides evidence for the regulation of the C/O profile by the complex interplay of CO snowline and pressure bump locations in the disk. These results demonstrate the importance of including dust transport in chemical disk models, for a proper interpretation of exoplanet atmospheric compositions, and a better understanding of volatile depletion in disks, in particular the use of CO isotopologues to determine gas surface densities.