2025A&A...698A.252G

Context. The potentially distinct formation pathways of exoplanets and brown dwarfs may be imprinted in their elemental and isotopic ratios. This work is part of the ESO SupJup Survey, which aims to disentangle the formation pathways of super-Jupiters and brown dwarfs through the analysis of their chemical and isotopic ratios.
Aims. In this study, we aim to characterize the atmospheres of two young L4 dwarfs, 2MASS J03552337+1133437 (2M0355) and 2MASS J14252798-3650229 (2M1425), in the AB Doradus Moving Group. This involved constraining their chemical composition, ¹²CO/¹³CO ratio, pressure-temperature profile, surface gravity, and rotational velocity, among other parameters.
Methods. We have obtained high-resolution CRIRES+ K-band spectra of these brown dwarfs, which we analyzed with an atmospheric retrieval pipeline. Atmospheric models were generated with the radiative transfer code petitRADTRANS, for which we employed a free and equilibrium chemistry approach, which we coupled with the PyMultiNest sampling algorithm to determine the best fit.
Results. We report robust detections of ¹³CO (13.4 and 8.0 σ) and HF (11.6 and 15.8 σ) in 2M0355 and 2M1425, respectively. The objects have similar overall atmospheric properties, including ¹²CO/¹³CO isotope ratios of 95.5_–6.4^+6.8 for 2M0355 and 109.6_–9.6^+10.6 for 2M1425. The most notable difference is the robust evidence of CH₄ (5.5 σ) but no H₂S (<2.3 σ) in 2M1425, in contrast to 2M0355, for which we retrieved H₂S (4.6 σ) but not CH₄ (<2.2 σ). We also find tentative hints of NH₃ in 2M1425 (3.0 σ). In both brown dwarfs, we find only tentative hints of H₂¹⁸O (1.1 and 3.0 σ), with lower limits of H₂¹⁶O/H₂¹⁸O ∼ 1000. Both objects appear to be close to chemical equilibrium, considering the main spectral contributors. We also find that 2M1425 is 50-200 K hotter than 2M0355 and has a higher surface gravity.
Conclusions. Retrievals of high-resolution K-band spectra of young brown dwarfs enable a detailed insight into their atmospheres. As with similar targets, these brown dwarfs also exhibit a depletion of ¹³CO compared to the interstellar medium. Future studies will put them into the context of other objects observed as part of the ESO SupJup Survey.