Revisiting Physical Parameters of the Benchmark Brown Dwarf LHS 6343 C Through an HST/WFC3 Eclipse

The LHS 6343 system consists of a resolved M-dwarf binary with an evolved, negligibly irradiated brown dwarf, LHS 6343 C, orbiting the primary star. Such brown dwarf eclipsing binaries present rare and unique opportunities to calibrate sub-stellar evolutionary and atmosphere models since mass, radius, temperature and luminosity can be directly measured. We update this brown dwarf’s mass (62.6 ± 2.2 MJup) and radius (0.788 ± 0.043 RJup) using empirical stellar relations and a Gaia DR3 distance. We use HST/WFC3 observations of an LHS 6343 C secondary eclipse to obtain a NIR emission spectrum of spectral type of T1.5 ± 1. We combine this spectrum with existing Kepler and Spitzer/IRAC secondary eclipse photometry to perform atmospheric characterization using the ATMO-2020, Sonora-Bobcat and BT-Settl model grids. ATMO-2020 models with strong non-equilibrium chemistry yield the best fit to observations across all modelled bandpasses. Finally, we obtain a semi-empirical measurement of LHS 6343 C’s apparent luminosity by integrating its observed and modelled spectral energy distribution. Applying knowledge of the system’s distance yields a bolometric luminosity of log(Lbol/Lsun) = −4.77 ± 0.03 and, applying the Stefan-Boltzmann law for the known radius, an effective temperature of 1303 ± 29 K. We also use the ATMO-2020 and Sonora-Bobcat evolutionary model grids to infer an age for LHS 6343 C of {2.86}^{+0.40}_{−0.33} Gyr and {3.1}1^{+0.50}_{−0.38} Gyr respectively.