MINDS. JWST-MIRI reveals a peculiar CO₂-rich chemistry in the drift-dominated disk CX Tau

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
² Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748, Garching, Germany
³ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
⁴ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
⁵ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁷ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
¹⁰ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
¹¹ SRON Netherlands Institure for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
¹² Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹⁴ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹⁵ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁶ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France

^★ Corresponding author; vlasblom@strw.leidenuniv.nl

Received: 24 May 2024
Accepted: 13 December 2024

Abstract

Context. Radial drift of icy pebbles can have a large impact on the chemistry of the inner regions of protoplanetary disks, where most terrestrial planets are thought to form. Disks with compact millimeter dust emission (≲50 au) are suggested to have a higher H₂O flux than more extended disks, as well as show excess cold H₂O emission, likely due to efficient radial drift bringing H₂O-rich material to the inner disk, where it can be observed with IR facilities such as the James Webb Space Telescope (JWST).

Aims. We present JWST MIRI/MRS observations of the disk around the low-mass T Tauri star CX Tau (M2.5, 0.37 M_⊙) taken as a part of the Mid-INfrared Disk Survey (MINDS) GTO program, a prime example of a drift-dominated disk based on ALMA data. In the context of compact disks, this disk seems peculiar: the source possesses a bright CO₂ feature instead of the bright H₂O that could perhaps be expected based on the efficient radial drift. We aim to provide an explanation for this finding in the context of the radial drift of ices and the disk’s physical structure.

Methods. We modeled the molecular features in the spectrum using local thermodynamic equilibrium (LTE) 0D slab models, which allowed us to obtain estimates of the temperature, column density, and emitting area of the emission.

Results. We detect molecular emission from H₂O, ¹²CO₂, ¹³CO₂, C₂H₂, HCN, and OH in this disk, and even demonstrate a potential detection of CO ¹⁸O emission. Analysis of the ¹²CO₂ and ¹³CO₂ emission shows the former to be optically thick and tracing a temperature of ∼450 K at an (equivalent) emitting radius of ∼0.05 au. The optically thinner isotopologue traces significantly colder temperatures (∼200 K) and a larger emitting area. Both the ro-vibrational bands of H₂O at shorter wavelengths and its pure rotational bands at longer wavelengths are securely detected. Both sets of lines are optically thick, tracing a similar temperature of ∼500–600 K and emitting area as the CO₂ emission. We also find evidence for an even colder, ∼200 K H₂O component at longer wavelengths, which is in line with this disk having strong radial drift. We also find evidence of highly excited rotational OH emission at 9–11 µm, known as “prompt emission”, caused by H₂O photodissociation. Additionally, we firmly detect four pure rotational lines of H₂, which show evidence of extended emission. Finally, we also detect several H recombination lines and the [Ne II] line.

Conclusions. The cold temperatures found for both the ¹³CO₂ and H₂O emission at longer wavelengths indicate that the radial drift of ices likely plays an important role in setting the chemistry of the inner disk of CX Tau. The H₂O-rich gas has potentially already advected onto the central star, which is now followed by an enhancement of comparatively CO₂-rich gas reaching the inner disk, explaining the enhancement of CO₂ emission in CX Tau. The comparatively weaker H₂O emission can be explained by the source’s low accretion luminosity. Alternatively, the presence of a small, inner cavity with a size of roughly 2 au in radius, outside the H₂O iceline, could explain the bright CO₂ emission. Higher angular resolution ALMA observations are needed to test this.