X-SHYNE: X-shooter spectra of young exoplanet analogs

Petrus, S.; Chauvin, G.; Bonnefoy, M.; Tremblin, P.; Charnay, B.; Delorme, P.; Marleau, G.-D.; Bayo, A.; Manjavacas, E.; Lagrange, A.-M.; Mollière, P.; Palma-Bifani, P.; Biller, B.; Jenkins, J.-S.; Goyal, J.-M.; Hoch, K. (2023). X-SHYNE: X-shooter spectra of young exoplanet analogs. Astronomy and astrophysics, 670, L9. EDP Sciences 10.1051/0004-6361/202244494

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We present simultaneous 0.65–2.5 μm medium resolution (3300 ≤ Rλ ≤ 8100) VLT/X-shooter spectra of the relatively young (150–300 Myr)
low-mass (19 ± 5MJup) 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 Teff = 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 Teff 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.

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