2026A&A...705A.222K

Context. Thanks to JWST, a plethora of species in planet-forming disks around very low mass stars such as C₂H₂, C₆H₆, C₄H₂, CH₃ etc. are being discovered. The column densities of these species retrieved from 0D slab models are very large (e.g. of the order of 10²⁰ cm^–2). This indicates a carbon-dominated chemistry in a gas with a high C/O ratio. The disk around 2MASS-J1605321-1993159 (M4.5) is one such source showing a molecular pseudo-continuum of C₂H₂. Notably, two oxygen-bearing molecules, CO and CO₂, are also detected in this source.
Aims. We aim to take the next step beyond 0D slab models to interpret the spectrum. We examine whether 2D thermo-chemical disk models can produce the large inferred column densities of C₂H₂ in the inner regions of the disk and produce a pseudo-continuum in the mid-IR spectrum. We also seek to constrain whether the depletion of oxygen or the enrichment of carbon causes the high C/O ratio triggering a carbon-dominated chemistry.
Methods. We utilised the radiative thermo-chemical disk model PRODIMO to identify a disk structure that is capable of producing the observed molecular emission of species such as CO, CO₂, C₂H₂, and H₂O simultaneously. The spectrum was generated using the fast line tracer FLiTs. We derived the gas temperature ⟨T⟩, column density ⟨log₁₀N⟩, and the emitting area ⟨r₁ - r₂⟩ for these molecules from the 2D disk model and compared them to the parameters retrieved originally from 0D slab models. We used the different effect that changing the O or C abundance has on CO and C₂H₂, respectively to discriminate between O depletion and C enhancement.
Results. We find that a disk structure characterised by the presence of a gap can best explain the observations. The inner disk is strongly depleted in dust, especially small grains (<5 µm), and elemental oxygen, leading to a large C/O ratio. This is required to produce a molecular pseudo-continuum of C₂H₂ and at the same time a relatively weak CO emission. The P- and R-branch of C₂H₂ probe deeper layers of the disk whereas the Q-branch probes mostly the surface layers. The combined emission of CO and CO₂ puts strong constraints on the gap's location (0.1-0.5 au) given a disk gas mass. We also report a new detection of the CO ν= 2→1 transition in the JWST spectrum.
Conclusions. Two-dimensional thermo-chemical disk models are able to produce the observed molecular pseudo-continuum of C₂H₂. We find that the combination of different species emission in the JWST spectra can be used to discriminate between different scenarios such as O-depletion, C-enhancement or both, and offers the potential to extract spatial substructure at scales smaller than ∼1 au.