2023A&A...670L...9P

We present simultaneous 0.65-2.5 μm medium resolution (3300 ≤ R_λ ≤ 8100) VLT/X-shooter spectra of the relatively young (150-300 Myr) low-mass (19 ± 5M_Jup) L-T transition object VHS 1256-1257 b, a known spectroscopic analog of HR8799d. The companion is a prime target for the JWST Early Release Science (ERS) and one of the highest-amplitude variable brown dwarfs known to date. We compare the spectrum to the custom grids of cloudless ATMO models, exploring the atmospheric composition with the Bayesian inference tool ForMoSA. We also reanalyze low-resolution HST/WFC3 1.10-1.67 μm spectra at minimum and maximum variability to contextualize the X-shooter data interpretation. The models reproduce the slope and most molecular absorption from 1.10 to 2.48 μm self-consistently, but they fail to provide a radius and a surface gravity consistent with evolutionary model predictions. They do not reproduce the optical spectrum and the depth of the K I doublets in the J band consistently. We derived T_eff = 1380±54 K, log(g) = 3.97±0.48 dex, [M/H] = 0.21±0.29, and C/O > 0.63. Our inversion of the HST/WFC3 spectra suggests a relative change of $ 27^{+6}_{-5} $ K of the disk-integrated T_eff correlated with the near-infrared brightness. Our data anchor the characterization of that object in the near-infrared and could be used jointly to the ERS mid-infrared data to provide the most detailed characterization of an ultracool dwarf to date.