  The companion to the G0V star HR7672 directly imaged by Liu et al. (2002) 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 acceleration (trend), we have
monitored this star with precise Doppler measurements and have now established
a 24 year time baseline. The radial velocity 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
$m=68.7^{+2.4}_{-3.1}M_J$ at the 68.2% confidence level. 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. It is presently
the only directly imaged L,T,Y-dwarf known to produce an RV trend around a
solar-type star.
