The ESO SupJup Survey

IX. Isotopic evidence of a recent formation for Luhman 16AB

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
² Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ IPAC, Mail Code 100-22, Caltech, 1200 E. California Boulevard, Pasadena, CA 91125, USA
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ School of Physics, Trinity College Dublin, University of Dublin, Dublin 2, Ireland
⁷ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 November 2025
Accepted: 28 January 2026

Abstract

Context. The distinct formation pathways proposed for directly imaged exoplanets and isolated brown dwarfs might leave imprints in the inherited chemical composition. Elemental and isotopic tracers could help inform the suspected histories, but this requires a careful characterisation of the sub-stellar atmospheres. In particular, objects at the L-T transition exhibit signs of dynamics that can drive their atmospheres out of chemical equilibrium.

Aims. In this work, we studied the nearest L-T brown dwarfs, Luhman 16A and B, to assess the chemical disequilibrium in their atmospheres. We also investigated the elemental and isotopic compositions in the context of their probable formation history within the Oceanus moving group.

Methods. As part of the ESO SupJup Survey, we obtained spatially resolved CRIRES⁺ K-band spectra of the binary. These high-resolution observations were analysed using an atmospheric retrieval framework that couples the radiative transfer code petitRADTRANS to the MultiNest sampling algorithm.

Results. We detect and retrieve the abundances of ¹²CO, H₂O, CH₄, NH₃, H₂S, HF, and the ¹³CO isotopologue. We find that both atmospheres are in chemical disequilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B (K_zz,A ~ 10^8.7 vs K_zz,B ~ 10^8.2 cm² s⁻¹). The tested chemical models, free-equilibrium and disequilibrium chemistry, yield consistent mixing ratios and agree with earlier work at shorter wavelengths. The free-chemistry gaseous C/O ratios show evidence of oxygen trapping in silicate-oxide clouds. While the C/O ratios are consistent with the solar composition, the metallicities are modestly enhanced with [C/H] ~ 0.15. The carbon isotope ratios are measured at ¹²C/¹³C_A = 74₋₂⁺² and ²C/¹³C_B = 74₋₃⁺³.

Conclusions. The coincident constraints of metallicities and isotopes across the binary provide further evidence in favour of a common 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 16A and B as members of the Oceanus moving group.