XUE: The CO2-rich terrestrial planet-forming region of an externally irradiated Herbig disk

Aims. We investigate the James Webb Space Telescope (JWST) MIRI MRS gas molecular content of an externally irradiated Herbig disk, the F-type XUE 10 source, in the context of the eXtreme UV Environments (XUE) program. XUE 10 belongs to the massive star cluster NGC 6357 (1.69 kpc), where it is exposed to an external far-ultraviolet (FUV) radiation ≈103 times stronger than in the solar neighborhood. Methods. We modeled the molecular features in the mid-infrared spectrum with local thermodynamic equilibrium (LTE) 0D slab models. We derived basic parameters of the stellar host from a VLT FORS2 optical spectrum using PHOENIX stellar templates. Results. We detected bright CO2 gas with the first simultaneous detection (>5σ) of four isotopologues (12CO2, 13CO2, 16O12C18O, 16O12C17O) in a protoplanetary disk. We also detected faint CO emission (2σ) and the HI Pf α line (8σ). We placed strict upper limits on the water content, finding a total column density of ≲1018 cm−2. The CO2 species trace low gas temperatures (300–370 K) with a range of column densities of 7.4 × 1017 cm−2 (16O12C17O)−1.3 × 1020 cm−2 (12CO2) in an equivalent emitting radius of 1.15 au. The emission of 13CO2 is likely affected by line optical depth effects. The 16O12C18O and 16O12C17O abundances may be isotopically anomalous compared to the 16O/18O and 16O/17O ratios measured in the interstellar medium and the Solar System. Conclusions. We propose that the mid-infrared spectrum of XUE 10 is explained by H2O removal either via advection or strong photo-dissociation by stellar UV irradiation and enhanced local CO2 gas phase production. Outer disk truncation supports the observed CO2−H2O dichotomy. A CO2 vapor enrichment in 18O and 17O can be explained by means of external UV irradiation and early (10^(4–5) yr) delivery of isotopically anomalous water ice to the inner disk.