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

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
² Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
³ Institute for Theoretical Physics and Computational Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁶ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁷ School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China
⁸ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, The Netherlands
⁹ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹⁰ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹² Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹³ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
¹⁴ Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
¹⁵ Current affiliation: Department of Astronomy, University of Michigan, 1085 S. University Ave, Ann Arbor, MI 48109, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 August 2024
Accepted: 6 August 2025

Abstract

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.