Dynamical Mass of the Young Brown Dwarf Companion PZ Tel B

Dynamical masses of giant planets and brown dwarfs are critical tools for empirically validating substellar evolutionary models and their underlying assumptions. We present a measurement of the dynamical mass and an updated orbit of PZ Tel B, a young brown dwarf companion orbiting a late-G member of the β Pic moving group. PZ Tel A exhibits an astrometric acceleration between Hipparcos and Gaia EDR3, which enables the direct determination of the companion's mass. We have also acquired new Keck/NIRC2 adaptive optics imaging of the system, which increases the total baseline of relative astrometry to 15 yr. Our joint orbit fit yields a dynamical mass of ${27}_{-9}^{+25}\,{M}_{\mathrm{Jup}}$ , semimajor axis of ${27}_{-4}^{+14}\,\mathrm{au}$ , eccentricity of ${0.52}_{-0.10}^{+0.08}$ , and inclination of ${91.73}_{-0.32}^{{+0.36}^\circ} $ . The companion's mass is consistent within 1.1σ of predictions from four grids of hot-start evolutionary models. The joint orbit fit also indicates a more modest eccentricity of PZ Tel B than previous results. PZ Tel joins a small number of young (<200 Myr) systems with benchmark substellar companions that have dynamical masses and precise ages from moving group membership.