An Atmospheric Retrieval of the Brown Dwarf Gliese 229B

Abstract

We present results from an atmospheric retrieval analysis of Gl 229B using the BREWSTER retrieval code. We find the best fit model to be cloud-free, consistent with the T dwarf retrieval work of Line et al. 2017, Zalesky et al. 2022 and Gonzales et al. 2020. Fundamental parameters (mass, radius, log(L_{Bol}/L_{Sun}), log(g)) determined from our model agree within 1\sigma to SED-derived values except for T_{eff} where our retrieved T_{eff} is approximately 100 K cooler than the evolutionary model-based SED value. We find a retrieved mass of 50^{+12}_{-9} M_{Jup}, however, we also find that the observables of Gl 229B can be explained by a cloud-free model with a prior on mass at the dynamical value, 70 M_{Jup}. We are able to constrain abundances for H_2O, CO, CH_4, NH_3, Na and K and find a supersolar C/O ratio as compared to its primary, Gl 229A. We report an overall subsolar metallicity due to atmospheric oxygen depletion but find a solar [C/H], which matches that of the primary. We find that this work contributes to a growing trend in retrieval-based studies, particularly for brown dwarfs, toward supersolar C/O ratios and discuss the implications of this result on formation mechanisms, internal physical processes as well as model biases.