No Clouds in Sight: Retrieving Substellar Objects to Explore the Impact of Metallicity and Clouds

Atmospheric retrievals are a powerful inverse modeling technique that allow us to get at the chemistry and physics at play in the atmosphere of an object. Understanding the interplay between retrieved parameters and the atmospheric makeup of an object is critical to understand limitations arising from the model or data. By examining substellar subdwarfs, low-metallicity brown dwarfs and low-mass stars which are expected to be cloudless due to reduced condensate opacities, we can explore the impact metallicity and clouds have in atmospheric retrievals. Using the Brewster retrieval code, we examined the subdwarf SDSS J1256-0224 to explore its predicted cloud-free nature as well as the impact low metallicity has on atmospheric retrievals. The models explored in this work have provided unprecedented information about low-metallicity atmospheres. Our results show that SDSS J1256 is best fit by a cloud-free model with a subsolar ion metallicity. We can constrain abundances for H₂O, FeH, and CrH, but are unable to constrain any carbon-bearing species likely due to SDSS J1256–0224’s low metallicity. Lastly, from examining our “rejected” models, those with ΔBIC > 45, we were able to learn a great deal about the behavior of clouds and retrieved gas abundances.