The ESO SupJup Survey. X. A carbon isotope contrast in the young ROXs 12 system

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 12 CO/ 13 CO ratio, by investigating the atmospheric composition of substellar companions and isolated brown dwarfs. In this work, we aim to characterize the atmospheres and determine the ratios of 12 CO/ 13 CO of the Rho Ophiuchus X-ray source (ROXs) 12 system (about 6Myrs), consisting of an M0 host with an L0 companion, as part of the ESO SupJup survey. Using high-resolution CRIRES+ K band spectra of these objects, we perform atmospheric retrieval analyses to derive their atmospheric properties, including the 12 CO/ 13 CO 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. We report the presence of H 2 O, 12 CO, 13 CO, and HF in both the star and companion, with a tentative detection of H 2 18 O in ROXs 12B. The 12 CO/ 13 CO ratios of the two objects show a measurable, though not strongly significant, difference, namely 77 and 55 for ROXs 12A and B. We measure a C/O ratio of 0.54 plus or minus 0.01, while the C/O ratio of the star is not reliably constrained due to the absence of atomic oxygen lines in the K band. Furthermore, we retrieve moderate veiling in the host star of r k = 0.17. 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.