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

2026/01/30 by Javiera K. Díaz-Berríos, Catherine Walsh, Ewine F. van Dishoeck · 1 voice
Physics and Astronomy · #astro-ph.EP #astro-ph.GA #astro-ph.SR

paper · pdf · doi:10.3847/1538-4357/ae43eb

arxiv published 2026/01/30 · arxiv updated 2026/03/05

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 (∼0.1~\rmM_\odot) stars (VLMSs) have revealed their hydrocarbon-rich inner regions (e.g., \ceC2H2), 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_X∼10²⁹~\mathrmerg~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 (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_gas>200~K), a range of C/O ratios can reproduce the observed N and ratios relative to \ceCO2. The disk-integrated molecular ratio with respect to \ceCO2 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_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.

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Discussions

• Quantifying the C/O ratio in the planet-forming environments around very-low-mass stars Díaz-Berríos et al. arXiv (ApJ) arxiv.org/abs/2601.23069 [bsky, 0 points, 0 comments]

Related

• MINDS. Strong oxygen depletion in the inner regions of a very low-mass star disk?
• A rich hydrocarbon chemistry and high C to O ratio in the inner disk around a very low-mass star
• The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime
• A transition from H₂O to C₂H₂ dominated spectra with decreasing stellar luminosity
• Chemical transformation of CO in evolving protoplanetary discs across stellar masses: a route to C-rich inner regions