Quantifying the C/O Ratio in the Planet-forming Environments around Very Low Mass stars

Abstract

The material in planet-forming disks determines the composition of planets; hence, it is crucial to understand the physical and chemical processes that set the abundance and distribution of key volatiles. James Webb Space Telescope observations of disks around very low mass ($\sim0.1~\rm{M}_\odot$) stars (VLMSs) have revealed their hydrocarbon-rich inner regions (e.g., \ce{C2H2}), with column densities significantly higher than predicted. We employ chemical kinetics models using the physical structure of the inner disk around an M~dwarf star with an X-ray luminosity of $L_\mathrm{X}\sim10^{29}~\mathrm{erg~s^{-1}}$. We adopt initial abundances that mimic the effects of carbon enhancement and oxygen depletion (C/O from 0.44 to 87.47) and quantify how the abundances and distributions of key volatiles respond. The column density and number of molecules ($\mathcal{N}$) of hydrocarbons and oxygen-bearing species are highly sensitive to the C/O ratio, with the largest increases in hydrocarbons occurring when carbon increases by a factor of 2, and/or oxygen decreases by a factor of 10, relative to solar. In the IR-emitting region ($T_\mathrm{gas}>200~\mathrm{K}$), a range of C/O ratios can reproduce the observed $\mathcal{N}$ and ratios relative to \ce{CO2}. The disk-integrated molecular ratio with respect to \ce{CO2} is highly sensitive to the underlying C/O ratio. However, our results apply only to a source with a single X-ray luminosity value at the middle of that observed for VLMSs; hence, a degeneracy between the stellar $L_\mathrm{X}$ and the C/O ratio cannot be discarded. Nonetheless, our findings support that an enhanced C/O is required to drive the hydrocarbon-rich chemistry observed in the inner disks around VLMSs.