  Brown dwarfs with well-determined ages, luminosities, and masses provide rare
but valuable tests of low-temperature atmospheric and evolutionary models. We
present the discovery and dynamical mass measurement of a substellar companion
to HD 47127, an old ($\approx$7-10 Gyr) G5 main sequence star with a mass
similar to the Sun. Radial velocities of the host star with the Harlan J. Smith
Telescope uncovered a low-amplitude acceleration of 1.93 $\pm$ 0.08 m s$^{-1}$
yr$^{-1}$ based on 20 years of monitoring. We subsequently recovered a faint
($\Delta H$=13.14 $\pm$ 0.15 mag) co-moving companion at 1.95$''$ (52 AU) with
follow-up Keck/NIRC2 adaptive optics imaging. The radial acceleration of HD
47127 together with its tangential acceleration from Hipparcos and Gaia EDR3
astrometry provide a direct measurement of the three-dimensional acceleration
vector of the host star, enabling a dynamical mass constraint for HD 47127 B
(67.5-177 $M_\mathrm{Jup}$ at 95% confidence) despite the small fractional
orbital coverage of the observations. The absolute $H$-band magnitude of HD
47127 B is fainter than the benchmark T dwarfs HD 19467 B and Gl 229 B but
brighter than Gl 758 B and HD 4113 C, suggesting a late-T spectral type.
Altogether the mass limits for HD 47127 B from its dynamical mass and the
substellar boundary imply a range of 67-78 $M_\mathrm{Jup}$ assuming it is
single, although a preference for high masses of $\approx$100 $M_\mathrm{Jup}$
from dynamical constraints hints at the possibility that HD 47127 B could
itself be a binary pair of brown dwarfs or that another massive companion
resides closer in. Regardless, HD 47127 B will be an excellent target for more
refined orbital and atmospheric characterization in the future.
