The ESO SupJup Survey IX: Isotopic evidence of a recent formation for Luhman 16AB
Abstract
The distinct formation pathways of directly-imaged exoplanets and isolated brown dwarfs might leave imprints in the inherited elemental and isotopic abundances, but such measurements require careful characterisation of the atmospheres. In particular, L-T transition objects exhibit signs of dynamics that drive their atmospheres out of chemical equilibrium. In this work, we studied the nearest L-T brown dwarfs, Luhman 16A and B, to assess the chemical dis-equilibrium in their atmospheres and to investigate their elemental and isotopic composition. As part of the ESO SupJup Survey, we obtained high-resolution CRIRES+ K-band spectra of the binary, which were analysed using an atmospheric retrieval framework. We detect and retrieve the abundances of 12CO, H2O, CH4, NH3, H2S, HF, and the 13CO isotopologue. Both atmospheres are in chemical dis-equilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B (Kzz,A108.7, Kzz,B108.2\ cm2\ s-1). The tested chemical models, free- and dis-equilibrium chemistry, yield consistent mixing ratios and agree with earlier work at shorter wavelengths. The gaseous C/O ratios show evidence of oxygen trapping in silicate-oxide clouds. While the C/O ratios are consistent with solar, the metallicities are modestly enhanced with [C/H]0.15. The carbon isotope ratios are measured at 12C/13CA=74+2-2 and 12C/13CB=74+3-3. The coincident constraints of metallicities and isotopes across the binary reinforce their likely shared formation. The 12C/13C ratios are aligned with the present-day interstellar medium, but lower than the solar-system value. This suggests a recent inheritance and corroborates the relatively young age (500 Myr) of Luhman 16AB as members of the Oceanus moving group.