  Brown dwarf spectra offer vital testbeds for our understanding of the
chemical and physical processes that sculpt substellar atmospheres. Recently,
atmospheric retrieval approaches have been applied to a number of
low-resolution (R~100) spectra of brown dwarfs, yielding constraints on the
abundances of chemical species and temperature structures of these atmospheres.
Medium-resolution (R~1e3) spectra of brown dwarfs offer significant additional
insight, as molecular features are more easily disentangled from one another
and the thermal structure of the upper atmosphere is more readily probed. We
present 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, a
number of novel challenges arise. We examine the strong effect of different
opacity sources on our retrieved constraints, in particular for CH4.
Furthermore, we find that flaws in the data such as errors from order stitching
can greatly bias our results. We compare these results to those obtained for a
R~100 spectrum of the same object, revealing how constraints on atmospheric
abundances and temperatures improve by an order of magnitude or more (depending
on the species) with increased spectral resolution. In particular, we precisely
constrain the abundance of H2S, which is undetectable at lower spectral
resolution. While these medium-resolution retrievals offer the potential of
precise, stellar-like constraints on atmospheric abundances (~0.02 dex), our
retrieved radius is unphysically small (R~0.50 R$_{Jup}$), indicating lingering
shortcomings with our modeling framework. This work is an initial investigation
into brown dwarf retrievals at medium spectral resolution, offering guidance
for future ground-based studies and JWST observations of substellar objects.
