A Clear View of a Cloudy Brown Dwarf Companion from High-Resolution Spectroscopy

Direct imaging studies have mainly used low-resolution spectroscopy (\(R\sim20-100\)) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g. C/O, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC (\(R\sim35,000\) in \(K\) band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze KPIC high-resolution spectrum (\(2.29-2.49~\mu\)m) and archival low-resolution spectrum (\(1-2.2~\mu\)m) of the benchmark brown dwarf HD 4747 B (\(m=67.2\pm1.8~M_{\rm{Jup}}\), \(a=10.0\pm0.2\) au, \(T_{\rm eff}\approx1400\) K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with that of its host star within \(1-2\sigma\). The retrieved parameters from the \(K\) band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H\(_2\)O, and CH\(_4\) (volume mixing ratio of log(CH\(_4\))=\(-4.82\pm0.23\)) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory.