2025A&A...699A.194A

Context. Planet-forming disks around brown dwarfs and very low-mass stars (VLMS) are, on average, less massive and are expected to undergo faster radial solid transport than their higher-mass counterparts. Spitzer had detected C₂H₂, CO₂, and HCN around these objects but did not provide a firm detection of water. With a better sensitivity and spectral resolving power than that of Spitzer, the James Webb Space Telescope (JWST) has recently revealed incredibly carbon-rich spectra and only one water-rich spectrum from such disks. A study of a larger sample of objects is necessary to understand how common such carbon-rich inner disk regions are and to put constraints on their evolution.
Aims. We present and analyze JWST MIRI/MRS observations of ten disks around VLMS from the MIRI guaranteed time observations program. This sample is diverse, with the central object ranging in mass from 0.02 to 0.14 M_⊙. They are located in three star-forming regions and a moving group (1 to 10 Myr).
Methods. We identified molecular emission in all sources based on recent literature and spectral inspection, and reported detection rates. We compared the molecular flux ratios between different species and to dust emission strengths. We also compared the flux ratios with the stellar and disk properties.
Results. The spectra of these VLMS disks are extremely rich in molecular emission, and we detect the 10 μm silicate dust emission feature in 70% of the sample. We detect C₂H₂ and HCN in all the sources and find larger hydrocarbons, such as C₄H₂ and C₆H₆, in nearly all sources. Among oxygen-bearing molecules, we find firm detections of CO₂, H₂O, and CO in 90,50, and 20% of the sample, respectively. We find that the detection rates of organic molecules correlate with other organic molecules and anticorrelate with the detection rates of inorganic molecules. Hydrocarbon-rich sources show weaker 10 μm dust strengths, as well as lower disk dust masses (measured from millimeter fluxes) than the oxygen-rich sources. We find evidence for a C/O ratio enhancement with disk age. The observed trends are consistent with models that suggest rapid inward solid material transport and grain growth.