The ESO SupJup Survey

Context . Recent research suggests that the distinct formation processes of exoplanets and brown dwarfs may have an influence on the chemical and isotopic composition of their atmospheres. Variations in the carbon 12 C/ 13 C isotope ratio have been observed and tentatively linked to the top-down formation of brown dwarfs and the core accretion pathway of super-Jupiters. The ESO SupJup Survey, conducted with CRIRES+ on the Very Large Telescope, aims to characterise the atmospheres of young brown dwarfs and super-Jupiters, specifically by investigating the 12 C/ 13 C ratio as a tracer of their formation pathways. Aims. We present the atmospheric characterisation of three isolated L-type brown dwarfs (2MASS J08354256-0819237, 2MASS J05012406-0010452, and 2MASS J05002100+0330501) included in the ESO SupJup Survey. We aim to constrain the C/O and 12 C/ 13 C ratios, and investigate whether the oxygen 16 O/ 18 O isotope ratio can be probed. Methods. We analysed the CRIRES+ K -band spectra of the three targets using our atmospheric retrieval framework. This framework couples the radiative transfer code petitRADTRANS with the sampling algorithm MultiNest . Results. We report 12 C/ 13 C ratios of 89 −11 +11 and 117 −17 +20 for J0835 and J0500 with strong 13 CO significance (>6.5 σ ) and a tentative (3 σ ) detection of 13 CO for J0501, resulting in a carbon isotope ratio of 155 −53 +56 . Only a weak detection of the H 2 18 O isotope was found in J0835. The C/O ratios are found to be in the range 0.65 to 0.71 for the three targets, and all exhibit strong detections of HF. Conclusions. The 12 C/ 13 C ratios appear to be higher than that of the interstellar medium.