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

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 (less than 0.2 M<SUB>⊙</SUB>) are interesting targets because they host a rich population of terrestrial planets. Here we present the James Webb Space Telescope detection of abundant hydrocarbons in the disk of a very low-mass star obtained as part of the Mid-InfraRed Instrument mid-INfrared Disk Survey (MINDS). In addition to very strong and broad emission from C<SUB>2</SUB>H<SUB>2</SUB> and its <SUP>13</SUP>C<SUP>12</SUP>CH<SUB>2</SUB> isotopologue, C<SUB>4</SUB>H<SUB>2</SUB>, benzene and possibly CH<SUB>4</SUB> are identified, but water, polycyclic aromatic hydrocarbons and silicate features are weak or absent. The lack of small silicate grains indicates that we can look deep down into this disk. These detections testify to an active warm hydrocarbon chemistry with a high C/O ratio larger than unity in the inner 0.1 astronomical units (AU) of this disk, perhaps due to destruction of carbonaceous grains. The exceptionally high C<SUB>2</SUB>H<SUB>2</SUB>/CO<SUB>2</SUB> and C<SUB>2</SUB>H<SUB>2</SUB>/H<SUB>2</SUB>O column density ratios indicate that oxygen is locked up in icy pebbles and planetesimals outside the water iceline. This, in turn, will have important consequences for the composition of forming exoplanets.