The Promise of High-Precision Abundances for Substellar Atmospheres: Medium Resolution Retrieval of a T9 Dwarf

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 successfully applied to a number of low-resolution (R≈100) spectra of L, T, and Y dwarfs, yielding constraints on the abundances of chemical species and temperature structures of these atmospheres. Medium-resolution (R≈10³) spectra of brown dwarfs offer significant additional insight, as molecular features are more easily disentangled from one another, alkali lines are well-resolved, and the thermal structure of the upper atmosphere is more readily probed. Furthermore, kinematic information such as the radial and rotation velocities of these objects can be retrieved directly from the spectra. We present results from applying a GPU-version of the CHIMERA retrieval framework to a high signal-to-noise, medium-resolution (R≈6000) FIRE spectrum of a T9 dwarf from 0.85-2.5 microns. At 60x higher spectral resolution than previous brown dwarf retrievals, a number of novel challenges arise, which we explore. For example, we examine the strong effect of different line lists on our retrieved molecular abundances, in particular for CH₄. We compare these retrieval results to those obtained for a R≈100 spectrum of the same object, revealing how constraints on atmospheric abundances improve by an order of magnitude or more (depending on the species) with increased spectral resolution. While these medium-resolution retrievals offer the promising potential of tight, stellar-like constraints on atmospheric abundances (~0.02 dex) for the first time in brown dwarf atmospheres, unphysical radius and gravity constraints indicate lingering challenges with this method. We discuss lessons learned from this work in preparation for both future ground-based studies and JWST observations of substellar objects at medium spectral resolution.