The ESO SupJup Survey

X. A carbon isotope contrast in the young ROXs 12 system

¹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
² Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Sorbonne Paris Cité, CNRS, 61, Avenue de l’Observatoire, 75014 Paris, France
⁷ Institut d’Astrophysique de Paris, UMR7095, CNRS, Université Paris VI, 98bis Boulevard Arago, 75014 Paris, France

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

Received: 19 January 2026
Accepted: 26 March 2026

Abstract

Context. Emerging research suggests that elemental and isotopic ratios of exoplanet and brown dwarf atmospheres may serve as potential tracers of their formation pathways. The ESO SupJup Survey aims to shed light on this hypothesis, with a focus on the ¹²CO/¹³CO ratio, by investigating the atmospheric composition of substellar companions and isolated brown dwarfs.

Aims. In this work, we aim to characterize the atmospheres and determine the ratios of ¹²CO/¹³CO of the Rho Ophiuchus X-ray source (ROXs) 12 system (∼6 Myr), consisting of an M0 host with an L0 companion, as part of the ESO SupJup survey. This system provides a great opportunity to directly compare the atmospheric compositions of the host star and its companion.

Methods. Using high-resolution CRIRES+ K band spectra of these objects, we perform atmospheric retrieval analyses to derive their atmospheric properties, including the ¹²CO/¹³CO ratio. Our retrieval framework is built on the radiative transfer code petitRADTRANS, with which we generate model spectra based on equilibrium chemistry tables computed with FastChem, coupled with the nested sampling algorithm PyMultiNest.

Results. We report the presence of H₂O, ¹²CO, ¹³CO, and HF in both the star and companion, with a tentative detection of H¹⁸₂O in ROXs 12B. The ¹²CO/¹³CO ratios of the two objects show a measurable, though not strongly significant, difference, namely 77₋₇⁺¹⁰ and 55₋₇⁺¹⁰ for ROXs 12A and B. Both are consistent with the local present interstellar medium. We measure a C/O ratio of 0.54 ± 0.01 and obtain a lower limit of H¹⁶₂O/H¹⁸₂O ⪆ 300 for ROXs 12B, while the C/O ratio of the star is not reliably constrained due to the absence of atomic oxygen lines in the K band. The companion also appears to exhibit a more isothermal temperature structure than expected from models. Furthermore, we retrieve moderate veiling in the host star of r_k = 0.17_−0.03^+0.02.

Conclusions. Systems such as ROXs 12, in which both star and planet can be chemically and isotopically characterized, are crucial for constraining potential formation mechanisms of massive, wide-orbit super-Jupiters. The differing ¹²CO/¹³CO ratios in the ROXs 12 system highlight the need for a broader sample to assess the frequency of isotopic variations and whether they may be linked to formation history.