The ESO SupJup Survey II. The 12C/13C isotope ratios of three young brown dwarfs with CRIRES+

Context . Young brown dwarfs exhibit atmospheric characteristics similar to those of super-Jupiters, providing a unique opportunity to study planetary atmospheres. Atmospheric retrievals of high-resolution spectra reveal detailed properties of these objects, with elemental and isotopic ratios offering insights into their formation history. The ESO SupJup Survey, utilising CRIRES + on the Very Large Telescope, aims to assess the role of 12 C/ 13 C as a formation tracer. Aims . We present observations of three young brown dwarfs: 2MASS J12003792-7845082, TWA 28, and 2MASS J08561384-1342242. Our goal is to constrain their chemical compositions, thermal profiles, surface gravities, spin rotations, and 12 C/ 13 C. Methods . We conducted atmospheric retrievals of CRIRES + K -band spectra, coupling the radiative transfer code petitRADTRANS with the Bayesian inference algorithm MultiNest . Results . The retrievals provide a detailed characterisation of the atmospheres of the three objects. We report the volume mixing ratios of the main molecular and atomic species: H 2 16 O, 12 CO, HF, Na, Ca, and Ti, including the novel detection of hydrogen fluoride (HF) in the atmosphere of a brown dwarf. We determine 12 C/ 13 C values of 81 −19 +28 and 79 −14 +20 in the atmospheres of TWA 28 and J0856, respectively, with strong significance (>3 σ ). We also report tentative evidence (~2 σ ) of 13 CO in J1200, at 12 C/ 13 C = 114 −33 +69 . Additionally, we detect H 2 18 O at moderate significance in J0856 (3.3 σ ) and TWA 28 (2.1 σ ). The retrieved thermal profiles are consistent with hot atmospheres (2300–2600 K) with low surface gravities and slow spins, as expected for young objects. Conclusions . The measured carbon isotope ratios are consistent among the three objects and show no significant deviation from that of the local interstellar medium, suggesting a fragmentation-based formation mechanism similar to star formation. The tentative detection of H 2 18 O in two objects of our sample highlights the potential of high-resolution spectroscopy to probe additional isotope ratios, such as 16 O/ 18 O, in the atmospheres of brown dwarfs and super-Jupiters.