An Atmospheric Retrieval of the Brown Dwarf Gliese 229B

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.; Zalesky et al. and Gonzales et al. Fundamental parameters (mass, radius, log(L _Bol /L _Sun), log(g)) determined from our model agree within 1σ 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 ${50}_{-9}^{+12}$ 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 H₂O, CO, CH₄, NH₃, 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 and internal physical processes, as well as model biases.