MINDS. JWST-MIRI reveals a peculiar CO2-rich chemistry in the drift-dominated disk CX Tau
Authors: Vlasblom Marissa , Temmink Milou , Grant Sierra L. , Kurtovic Nicolas , Sellek Andrew D. , van Dishoeck Ewine F. , Güdel Manuel , Henning Thomas , Lagage Pierre-Olivier , Barrado David , Caratti o Garatti Alessio , Glauser Adrian M. , Kamp Inga , Lahuis Fred , Olofsson Göran , Arabhavi Aditya M. , Christiaens Valentin , Gasman Danny , Jang Hyerin , Morales-Calderón Maria , Perotti Giulia , Schwarz Kamber , Tabone Benoît 
Published: January 2025

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.