The Dynamical Mass and Three-dimensional Orbit of HR7672B: A Benchmark Brown Dwarf with High Eccentricity

The companion to the G0V star HR7672 directly imaged by Liu et al. has moved measurably along its orbit since the discovery epoch, making it possible to determine its dynamical properties. Originally targeted with adaptive optics because it showed a long-term radial velocity (RV) acceleration (trend), we have monitored this star with precise Doppler measurements and have now established a 24 year time baseline. The RV variations show significant curvature (change in the acceleration) including an inflection point. We have also obtained a recent image of HR7672B with NIRC2 at Keck. The astrometry also shows curvature. In this paper, we use jointly fitted Doppler and astrometric models to calculate the three-dimensional orbit and dynamical mass of the companion. The mass of the host star is determined using a direct radius measurement from CHARA interferometry in combination with high-resolution spectroscopic modeling. We find that HR7672B has a highly eccentric, e = 0.50^+0.01 _-0.01, near edge-on, i = 97.3^+0.4 _-0.5 deg, orbit with semimajor axis, a = 18.3^+0.4 _-0.5 AU. The mass of the companion is m = 68.7^+2.4 _-3.1 M_J . HR7672B thus resides near the substellar boundary, just below the hydrogen-fusing limit. These measurements of the companion mass are independent of its brightness and spectrum and establish HR7672B as a rare and precious "benchmark" brown dwarf with a well-determined mass, age, and metallicity essential for testing theoretical evolutionary models and synthetic spectral models. Indeed, we find that such models under-predict its luminosity by a factor of ≈2. HR 7672B is presently the only L, T, or Y dwarf known to produce an RV trend around a solar-type star.