Burned to ashes: How the thermal decomposition of refractory organics in the inner protoplanetary disc impacts the gas-phase C/O ratio

TL;DR

This paper models how the thermal decomposition of refractory organics in protoplanetary discs influences the gas-phase C/O ratio, revealing prolonged carbon enrichment beyond the organics line and implications for planet formation.

Contribution

It introduces a 1D evolution model incorporating irreversible organic decomposition, showing extended carbon-rich gas presence and its impact on disc composition and planet formation.

Findings

Carbon-rich gas persists beyond the organics line up to 7 au.

Gas-phase C/O ratio is significantly affected by refractory organics.

Extended carbon enrichment influences planet formation processes.

Abstract

The largest reservoir of carbon in protoplanetary discs is stored in refractory organics, which thermally decompose into the gas-phase at the organics line, well interior to the water iceline. Because this region is so close to the host star, it is often assumed that the released gaseous material is rapidly accreted and plays little role in the evolution of the disc composition. However, laboratory experiments show that the thermal decomposition process is irreversible, breaking macromolecular refractory organics into simpler, volatile carbon-bearing compounds. As a result, unlike the iceline of other volatiles, which traps vapor inwards due to recondensation, the organics line remains permeable, allowing gaseous carbon to diffuse outward without returning to the solid phase. In this paper, we investigate how this process affects the disc composition, particularly the gas-phase C/H and C/O ratios, by incorporating it into a 1D evolution model for gas and solids, and assuming refractory organics dominantly decompose into C$_2$H$_2$. Our results show that this process allows this carbon-rich gas to survive well beyond the organics line (out to $7 \mathrm{~au}$ around a solar-mass star) and for much longer timescales, such that its abundance is increased by an order of magnitude. This has several implications in planet formation, notably by altering how the composition of solids and gas relate, and the fraction of heavy elements available to giant planets. In the framework of our model, refractory organics significantly influence the evolution of the gas-phase C/O ratio, which may help interpreting measurements made with Spitzer and JWST.