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

TL;DR

This work analyzes high-resolution K-band spectra of the nearest L-T brown-dwarf binary Luhman 16AB to extract atmospheric abundances and isotopic ratios as tracers of formation history. Using two complementary chemistry frameworks—free chemistry and disequilibrium via FastChem Cond—the authors detect multiple species and isotopologues, constrain the carbon-to-oxygen ratio and metallicity, and measure the carbon isotope ratio $^{12}$C/$^{13}$C in both components. The results indicate near-solar bulk C/O, modest metal enrichment, and consistent $^{12}$C/$^{13}$C values (~74), supporting a shared formation and recent inheritance from the local ISM within the Oceanus moving group (~500 Myr). Collectively, the findings establish Luhman 16AB as a benchmark for connecting atmospheric chemistry, isotopes, and formation history in substellar objects, with implications for distinguishing planetary and brown-dwarf formation pathways.

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 $^{12}$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.

Figures (11)

• Figure 1: CRIRES$^+$ K-band spectra of Luhman 16A and B in orange and blue, respectively. Top panel: Seven spectral orders covered in the K2166 wavelength setting. The telluric absorption is shown as transparent lines. Lower panels: Zoom-in of the sixth order. The black observed spectra are overlaid with the best-fitting free-chemistry models in orange and blue. The mean scaled uncertainties are displayed to the right of the residuals in the bottom panel. The fits to the other spectral orders can be found in Appendix \ref{['app:best_fitting_spectra']}.
• Figure 2: Cross-correlation analysis of minor species in the spectra of Luhman 16A (solid) and B (dashed). The detection significance at $\textit{v}=0\ \mathrm{km\ s^{-1}}$ is indicated in the upper-right corner of each panel, with Luhman 16A at the top. The panel rows use different y-axis limits for legibility.
• Figure 3: Chemical and isotopic abundance ratios retrieved for Luhman 16AB. Both the results from the free-chemistry and disequilibrium retrievals are shown. For context, we show the abundance ratios of the local ISM Milam_ea_2005Wilson_1999 and the Sun Asplund_ea_2021Lyons_ea_2018.
• Figure 4: Retrieved correlation between surface gravity and the carbon abundance, relative to hydrogen and the solar value (i.e. $\mathrm{[C/H]}$; Asplund_ea_2021). To a first order, these two parameters follow a linear relation and can be projected onto the expected $\log\textit{g}_\mathrm{A}=4.96$ and $\log\textit{g}_\mathrm{B}=4.88$.
• Figure 5: Abundances of carbon, oxygen, nitrogen, sulphur, and fluorine as inferred from the detected gaseous molecules (see Sect. \ref{['sect:detections']}). The values are shown relative to the solar composition and uncertainties reported by Asplund_ea_2021.