Destruction of refractory carbon grains drives the final stage of proto-planetary disk chemistry

TL;DR

This paper investigates how the destruction of refractory carbon grains influences the chemistry of protoplanetary disks, particularly the abundance of C2H, highlighting the role of grain photo-ablation in creating long-lived, oxygen-poor chemical environments.

Contribution

It demonstrates that photo-ablation of carbon-rich grains elevates the C/O ratio, sustaining high C2H levels and explaining observed disk structures, a novel insight into disk chemistry evolution.

Findings

C2H detected in both young and old systems, indicating long-lived chemistry.

Photo-ablation of carbon grains increases the C/O ratio, enabling sustained C2H abundance.

Optimal conditions for high C2H are near disk gaps and just outside pebble disks.

Abstract

Here we aim to explore the origin of the strong C2H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C2H is detected in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C2H is rapidly destroyed, within <1000 yr. These two statements hint that the chemistry responsible for C2H emission must be predominantly in the gas-phase and must be in equilibrium. Combining new and published chemical models we find that elevating the total volatile (gas and ice) C/O ratio is the only natural way to create a long lived, high C2H abundance. Most of the \ce{C2H} resides in gas with a Fuv/n-gas ~ 10^-7 G0 cm^3. To elevate the volatile C/O ratio, additional carbon has to be released into the gas to enable an equilibrium chemistry under oxygen-poor conditions. Photo-ablation of carbon-rich grains seems the most straightforward way to elevate the C/O ratio above 1.5, powering a long-lived equilibrium cycle. The regions at which the conditions are optimal for the presence of high C/O ratio and elevated C2H abundances in the gas disk set by the Fuv/n-gas ~ 10^-7 G0 cm^3 condition lie just outside the pebble disk as well as possibly in disk gaps. This process can thus also explain the (hints of) structure seen in C2H observations.