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The mass and age of substellar objects are degenerate parameters leaving theevolutionary state of brown dwarfs ambiguous without additional information.Theoretical models are normally used to help distinguish between old, massivebrown dwarfs and young, low mass brown dwarfs but these models have yet to beproperly calibrated. We have carried out an infrared high-contrast imagingprogram with the goal of detecting substellar objects as companions to nearbystars to help break degeneracies in inferred physical properties such as mass,age, and composition. Rather than using imaging observations alone, our targetsare pre-selected based on the existence of dynamical accelerations informedfrom years of stellar radial velocity (RV) measurements. In this paper, wepresent the discovery of a rare benchmark brown dwarf orbiting the nearby($d=18.69\pm0.19$ pc), solar-type (G9V) star HD 4747 ([Fe/H]=$-0.22\pm0.04$)with a projected separation of only $\rho=11.3\pm0.2$ AU ($\theta \approx$0.6"). Precise Doppler measurements taken over 18 years reveal the companion'sorbit and allow us to place strong constraints on its mass using dynamics ($m\sin(i) = 55.3\pm1.9M_J$). Relative photometry ($\Delta K_s=9.05\pm0.14$,$M_{K_s}=13.00\pm0.14$, $K_s - L' = 1.34\pm0.46$) indicates that HD 4747 B ismost-likely a late-type L-dwarf and, if near the L/T transition, an intriguingsource for studying cloud physics, variability, and polarization. We estimate amodel-dependent mass of $m=72^{+3}_{-13}M_J$ for an age of $3.3^{+2.3}_{-1.9}$Gyr based on gyrochronology. Combining astrometric measurements with RV data,we calculate the companion dynamical mass ($m=60.2\pm3.3M_J$) and orbit($e=0.740\pm0.002$) directly. As a new mass, age, and metallicity benchmark, HD4747 B will serve as a laboratory for precision astrophysics to testtheoretical models that describe the emergent radiation of brown dwarfs.

The TRENDS High-Contrast Imaging Survey. VI. Discovery of a Mass, Age, and Metallicity Benchmark Brown Dwarf