Atmospheric Retrieval of Cool Y Dwarfs

Brown dwarfs' near-infrared spectra contain a wealth of information which can reveal the physical and chemical processes that occur in their atmospheres. Using a recently developed atmospheric retrieval model, we are able to constrain various molecular abundances, along with photometric radius, gravity, cloud optical depths, and temperature profiles for a set of ultral-cool (T8-Y1) dwarfs observed with the Hubble Space Telescope Wide Field Camera 3. From these spectra, we are able to constrain the abundances of water, methane, ammonia, CO, CO_2, H_2S, and Na+K. Using the retrieved abundances of water and methane, we are able to determine the atmospheric carbon-to-oxygen ratio and metallicity for these objects. We also identify a continuing trend of alkali metal depletion towards cooler effective temperatures likely due to the formation of optically thin Na2S and KCl clouds. In addition, we also make predictions for how such atmospheres may appear with JWST.