A rich hydrocarbon chemistry and high C to O ratio in the inner disk around a very low-mass star

TL;DR

This study reports JWST detection of rich hydrocarbon chemistry and a high carbon-to-oxygen ratio in the inner disk of a very low-mass star, revealing implications for planet composition and habitability.

Contribution

First detection of abundant hydrocarbons and high C/O ratio in a very low-mass star's disk using JWST, highlighting active warm hydrocarbon chemistry.

Findings

Detection of C$_2$H$_2$, C$_4$H$_2$, benzene, and possibly CH$_4$ in the disk.

Weak or absent water, PAH, and silicate features indicating deep disk view.

High C$_2$H$_2$/CO$_2$ and C$_2$H$_2$/H$_2$O ratios suggest oxygen sequestration outside the water ice line.

Abstract

Carbon is an essential element for life but how much can be delivered to young planets is still an open question. The chemical characterization of planet-forming disks is a crucial step in our understanding of the diversity and habitability of exoplanets. Very low-mass stars ($<0.2~M_{\odot}$) are interesting targets because they host a rich population of terrestrial planets. Here we present the JWST detection of abundant hydrocarbons in the disk of a very low-mass star obtained as part of the MIRI mid-INfrared Disk Survey (MINDS). In addition to very strong and broad emission from C$_2$H$_2$ and its $^{13}$C$^{12}$CH$_2$ isotopologue, C$_4$H$_2$, benzene, and possibly CH$_4$ are identified, but water, PAH and silicate features are weak or absent. The lack of small silicate grains implies that we can look deep down into this disk. These detections testify to an active warm hydrocarbon chemistry with a high C/O ratio in the inner 0.1 au of this disk, perhaps due to destruction of carbonaceous grains. The exceptionally high C$_2$H$_2$/CO$_2$ and C$_2$H$_2$/H$_2$O column density ratios suggest that oxygen is locked up in icy pebbles and planetesimals outside the water iceline. This, in turn, will have significant consequences for the composition of forming exoplanets.