MINDS: Strong oxygen depletion in the inner regions of a very low-mass star disk?

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.