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

¹ Department of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Department of Astronomy & Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
⁵ Center for Exoplanets and Habitable Worlds, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
⁶ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁷ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁸ FACom, Instituto de Física – FCEN, Universidad de Antioquia, Calle 70 No. 52-21, Medellín 050010, Colombia
⁹ Centre for Astrophysics Research, Department of Physics, Astronomy, and Mathematics, University of Hertfordshire, Hatfield, AL10 9UW, UK
¹⁰ Universitäts-Sternwarte München, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
¹¹ Alma Mater Studiorum, Università di Bologna, Dipartimento di Fisica e Astronomia (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy
¹² INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
¹⁴ AURA for the European Space Agency, Space Telescope Science Institute, Baltimore, MD, USA

^★ Corresponding author.

Received: 28 May 2025
Accepted: 17 July 2025

Abstract

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.