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

Abstract

Direct imaging studies have mainly used low-resolution spectroscopy ($R\sim 20-100$) 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 ($R\sim 35,000$ 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 ($2.29-2.49\hat{I}¼$m) and archival low-resolution spectrum ($1-2.2\hat{I}¼$m) of the benchmark brown dwarf HD 4747 B ($m=67.2\pm 1.8M_{\mathrm{Jup}}$, $a=10.0\pm 0.2$ au, $T_{\mathrm{eff}}\approx 1400$ 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 $1-2\ddot{I}ƒ$. 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${}_4$)=$-4.82\pm 0.23$) 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.

33 pages, 16 figures, Accepted to ApJ