2021ApJ...923...19G

We present the first retrieval analysis of a substellar subdwarf, SDSS J125637.13-022452.4 (SDSS J1256-0224), using the Brewster retrieval code base. We find SDSS J1256-0224 is best fit by a cloud-free model with an ion (neutral H, H^–, and electron) abundance corresponding to [Fe/H]_ion=-1.5. However, this model is indistinguishable from a cloud-free model with [Fe/H]_ion=-2.0 and a cloud-free model with [Fe/H]_ion=-1.5 assuming a subsolar carbon-to-oxygen ratio. We are able to constrain abundances for H₂O, FeH, and CrH, with an inability to constrain any carbon-bearing species likely due to the low metallicity of SDSS J1256-0224. We also present an updated spectral energy distribution (SED) and semiempirical fundamental parameters. Our retrieval- and SED-based fundamental parameters agree with the Baraffe low-metallicity evolutionary models. From examining our "rejected" models (those with ΔBIC > 45), we find that we are able to retrieve gas abundances consistent with those of our best fitting model. We find the cloud in these poorer fitting "cloudy" models is either pushed to the bottom of the atmosphere or made optically thin.