2025A&A...694A.164M

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 ¹²C/¹³C 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 ¹²C/¹³C 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 ¹²C/¹³C ratios, and investigate whether the oxygen ¹⁶O/¹⁸O 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 ¹²C/¹³C ratios of 89_–11⁺¹¹ and 117_–17⁺²⁰ for J0835 and J0500 with strong ¹³CO significance (>6.5σ) and a tentative (3σ) detection of ¹³CO for J0501, resulting in a carbon isotope ratio of 155_–53⁺⁵⁶. Only a weak detection of the H₂¹⁸O 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 ¹²C/¹³C ratios appear to be higher than that of the interstellar medium.