2025MNRAS.539.1088I

We present a high signal-to-noise (SNR $\sim$ 450), high-dispersion ($R \equiv \lambda / \Delta \lambda \sim$ 28 000) $H$- and $K$-band spectroscopic atlas of the L7.5 and T0.5 components of the Luhman 16AB binary (WISE J104915.57$-$531906.1AB): the closest pair of brown dwarfs, and one of the best substellar benchmarks. The spectra were combined from a 70-d spectroscopic monitoring campaign of the binary with IGRINS on Gemini South. We fit model photospheres to the combined high-quality spectra to estimate atmospheric parameters. The model is based on the Sonora model atmosphere further incorporating the effects of clouds and disequilibrium. We detect ammonia (NH₃) lines in both binary components, making Luhman 16A the warmest object where individual NH₃ lines were identified. We discover hydrogen (H₂), hydrogen sulfide (H₂S), and hydrogen fluoride (HF) lines in both components, following recent reports of these species in either cooler (H₂, H₂S in a T6 dwarf) or warmer (HF in young late-M or mid-L dwarfs) objects. Methane (CH₄) shows a small contribution, with lines sensitive to the slight temperature difference spanning the L/T transition. Against model expectations, we do not detect FeH lines, implying more efficient iron rainout than incorporated in the models. We find various unidentified features in water-dominated regions, likely the result of residual inaccuracies in the water line lists. We searched for planetary-mass companions by periodogram analysis of radial velocities over 70 d but detected no significant signal. The upper limits of projected planetary mass are $M\sin {i}=$ 0.2 $M_{\mathrm{J}}$ and 0.3 $M_{\mathrm{J}}$ at P $\sim$ 1 d, and 0.4 $M_{\mathrm{J}}$ and 0.7 $M_{\mathrm{J}}$ at P $\sim$ 10 d for Luhman 16A and B, respectively.