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 $\beta$ 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 years. Our joint orbit fit yields a dynamical mass of $27^{+25}_{-9} \, M_{\mathrm{Jup}}$, semi-major axis of $27^{+14}_{-4} \, \mathrm{au}$, eccentricity of $0.52^{+0.08}_{-0.10}$, and inclination of $91.73^{+0.36}_{-0.32} {}^\circ$. The companion's mass is consistent within $1.1\sigma$ 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 (${<}200 \, \mathrm{Myr}$) systems with benchmark substellar companions that have dynamical masses and precise ages from moving group membership.