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The physical properties of brown dwarf companions found to orbit nearby,solar-type stars can be benchmarked against independent measures of their mass,age, chemical composition, and other parameters, offering insights into theevolution of substellar objects. The TRENDS high-contrast imaging survey hasrecently discovered a (mass/age/metallicity) benchmark brown dwarf orbiting thenearby (d=18.69+/-0.19 pc), G8V/K0V star HD 4747. We have acquired follow-upspectroscopic measurements of HD 4747 B using the Gemini Planet Imager to studyits spectral type, effective temperature, surface gravity, and cloudproperties. Observations obtained in the H-band and K1-band recover thecompanion and reveal that it is near the L/T transition (T1+/-2). Fittingatmospheric models to the companion spectrum, we find strong evidence for thepresence of clouds. However, spectral models cannot satisfactorily fit thecomplete data set: while the shape of the spectrum can be well-matched inindividual filters, a joint fit across the full passband results indiscrepancies that are a consequence of the inherent color of the brown dwarf.We also find a $2\sigma$ tension in the companion mass, age, and surfacegravity when comparing to evolutionary models. These results highlight theimportance of using benchmark objects to study "secondary effects" such asmetallicity, non-equilibrium chemistry, cloud parameters, electron conduction,non-adiabatic cooling, and other subtleties affecting emergent spectra. As anew L/T transition benchmark, HD 4747 B warrants further investigation into themodeling of cloud physics using higher resolution spectroscopy across a broaderrange of wavelengths, polarimetric observations, and continued Doppler radialvelocity and astrometric monitoring.

GPI Spectroscopy of the Mass, Age, and Metallicity Benchmark Brown Dwarf HD 4747 B