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Brown dwarf spectra offer vital testbeds for our understanding of thechemical and physical processes that sculpt substellar atmospheres. Recently,atmospheric retrieval approaches have been applied to a number oflow-resolution (R~100) spectra of brown dwarfs, yielding constraints on theabundances of chemical species and temperature structures of these atmospheres.Medium-resolution (R~1e3) spectra of brown dwarfs offer significant additionalinsight, as molecular features are more easily disentangled from one anotherand the thermal structure of the upper atmosphere is more readily probed. Wepresent results from a GPU-based retrieval analysis of a high signal-to-noise,medium-resolution (R~6000) FIRE spectrum from 0.85-2.5 microns of a T9 dwarf.At 60x higher spectral resolution than previous brown dwarf retrievals, anumber of novel challenges arise. We examine the strong effect of differentopacity sources on our retrieved constraints, in particular for CH4.Furthermore, we find that flaws in the data such as errors from order stitchingcan greatly bias our results. We compare these results to those obtained for aR~100 spectrum of the same object, revealing how constraints on atmosphericabundances and temperatures improve by an order of magnitude or more (dependingon the species) with increased spectral resolution. In particular, we preciselyconstrain the abundance of H2S, which is undetectable at lower spectralresolution. While these medium-resolution retrievals offer the potential ofprecise, stellar-like constraints on atmospheric abundances (~0.02 dex), ourretrieved radius is unphysically small (R~0.50 R$_{Jup}$), indicating lingeringshortcomings with our modeling framework. This work is an initial investigationinto brown dwarf retrievals at medium spectral resolution, offering guidancefor future ground-based studies and JWST observations of substellar objects.

Brown Dwarf Retrievals on FIRE!: Atmospheric Constraints and Lessons Learned from High Signal-to-Noise Medium Resolution Spectroscopy of a T9 Dwarf