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 C 2 H 2 , C 6 H 6 , C 4 H 2 , CH 3 etc. are being discovered. The column densities of these species retrieved from 0D slab models are very large (e.g. of the order of 10 20 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 C 2 H 2 . Notably, two oxygen-bearing molecules, CO and CO 2 , 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 C 2 H 2 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, CO 2 , C 2 H 2 , and H 2 O simultaneously. The spectrum was generated using the fast line tracer FLiTs. We derived the gas temperature ⟨ T ⟩, column density ⟨log 10 N ⟩, and the emitting area ⟨ r 1 − r 2 ⟩ 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 C 2 H 2 , 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 C 2 H 2 and at the same time a relatively weak CO emission. The P- and R-branch of C 2 H 2 probe deeper layers of the disk whereas the Q-branch probes mostly the surface layers. The combined emission of CO and CO 2 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 C 2 H 2 . 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.

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