  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 the
evolution of substellar objects. The TRENDS high-contrast imaging survey has
recently discovered a (mass/age/metallicity) benchmark brown dwarf orbiting the
nearby (d=18.69+/-0.19 pc), G8V/K0V star HD 4747. We have acquired follow-up
spectroscopic measurements of HD 4747 B using the Gemini Planet Imager to study
its spectral type, effective temperature, surface gravity, and cloud
properties. Observations obtained in the H-band and K1-band recover the
companion and reveal that it is near the L/T transition (T1+/-2). Fitting
atmospheric models to the companion spectrum, we find strong evidence for the
presence of clouds. However, spectral models cannot satisfactorily fit the
complete data set: while the shape of the spectrum can be well-matched in
individual filters, a joint fit across the full passband results in
discrepancies 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 surface
gravity when comparing to evolutionary models. These results highlight the
importance of using benchmark objects to study "secondary effects" such as
metallicity, non-equilibrium chemistry, cloud parameters, electron conduction,
non-adiabatic cooling, and other subtleties affecting emergent spectra. As a
new L/T transition benchmark, HD 4747 B warrants further investigation into the
modeling of cloud physics using higher resolution spectroscopy across a broader
range of wavelengths, polarimetric observations, and continued Doppler radial
velocity and astrometric monitoring.
