The ESO SupJup Survey IX: Isotopic evidence of a recent formation for Luhman 16AB

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 $^{12}$CO, H$2$O, CH$4$, NH$3$, H$2$S, HF, and the $^{13}$CO isotopologue. Both atmospheres are in chemical dis-equilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B ($K\mathrm{zz,A}\sim10^{8.7}$, $K\mathrm{zz,B}\sim10^{8.2}\ \mathrm{cm^2\ s^{-1}}$). 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 $\mathrm{[C/H]}\sim0.15$. The carbon isotope ratios are measured at $\mathrm{^{12}C/^{13}C_A}=74^{+2}{-2}$ and $\mathrm{^{12}C/^{13}C_B}=74^{+3}{-3}$. The coincident constraints of metallicities and isotopes across the binary reinforce their likely shared formation. The $\mathrm{^{12}C/^{13}C}$ 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 ($\sim500$ Myr) of Luhman 16AB as members of the Oceanus moving group.

💡 Research Summary

This paper presents a high‑resolution spectroscopic study of the nearest L‑T transition brown‑dwarf binary, Luhman 16A and B, using CRIRES⁺ K‑band observations obtained as part of the ESO SupJup Survey. The authors acquired spatially resolved spectra (R ≈ 60 000, S/N ≈ 400 for A and 300 for B) covering the 2.16 µm region, which includes the ¹³CO (ν = 2–0) band head. After careful reduction—including extraction of the blended components, telluric correction with molecfit, and masking of deep atmospheric absorption—the data were modeled with a Bayesian retrieval framework that couples the radiative‑transfer code petitRADTRANS to the nested‑sampling algorithm PyMultiNest.

Two chemical modeling approaches were employed. In the “free‑chemistry” mode, volume‑mixing ratios (VMRs) of selected gases (¹²CO, ¹³CO, H₂O, CH₄, NH₃, H₂S, HF) were treated as independent, vertically constant parameters. In the “disequilibrium” mode, the FastChem‑Cond code computed equilibrium abundances on the fly, allowing the retrieval of elemental abundances (