Dynamical Mass of the Young Brown Dwarf Companion PZ Tel B

Abstract

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 Jup , semimajor axis of 27 − 4 + 14 au , eccentricity of 0.52 − 0.10 + 0.08 , and inclination of 91.73 − 0.32 + 0.36 ° . 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.

VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab Formed Like a “Failed Star”

“…HD 136164 Ab now joins a very select group of young substellar companions with dynamical mass measurements (see Figure 4 in Franson & Bowler 2023). It is currently the youngest substellar companion with a dynamical mass estimate.…”

“…It is currently the youngest substellar companion with a dynamical mass estimate. Like PZ Tel B (Biller et al 2010;Mugrauer et al 2010;Schmidt et al 2014;Maire et al 2016;Stolker et al 2020;Franson & Bowler 2023), this object will be crucial for benchmarking evolutionary models in the coming decade. HD 136164 Ab does not need to stand alone, however.…”

“…Additionally, evolutionary and spectral models of substellar objects (brown dwarfs and massive giant planets), like their higher-mass stellar counterparts (e.g., Rodet et al 2018;Dieterich et al 2021), need to be tested and refined based on model-independent measurements of their masses derived from orbital characterization (Dupuy & Liu 2017;Brandt et al 2019;Fontanive et al 2019;Rickman et al 2020;Vrijmoet et al 2020;Brandt et al 2021;Bonavita et al 2022;Franson et al 2022;Rickman et al 2022;Vrijmoet et al 2022;Balmer et al 2023;Franson & Bowler 2023). The difficulty is that substellar objects amenable to these measurements, unlike stellar binaries, appear to be intrinsically rare (e.g., Dieterich et al 2012;Fontanive et al 2018;Nielsen et al 2019;Duchêne et al 2023), and generally substellar companions are orders of magnitude fainter than their hosts, necessitating a greater observational expense than stellar binary orbits.…”

“…The difficulty is that substellar objects amenable to these measurements, unlike stellar binaries, appear to be intrinsically rare (e.g., Dieterich et al 2012;Fontanive et al 2018;Nielsen et al 2019;Duchêne et al 2023), and generally substellar companions are orders of magnitude fainter than their hosts, necessitating a greater observational expense than stellar binary orbits. So far, the amount of substellar objects with dynamically derived masses is low, about 20 in total, and the number of these with accurate age estimates younger than a few hundred megayears is of order unity (see Figure 4 in Franson & Bowler 2023). The youngest brown dwarf with a dynamical mass, PZ Tel b ( -+ M 27 9 25 J , 24 ± 3 Myr; Franson & Bowler 2023), has been predicted by evolutionary models to have a higher mass on average than its dynamical mass.…”

“…So far, the amount of substellar objects with dynamically derived masses is low, about 20 in total, and the number of these with accurate age estimates younger than a few hundred megayears is of order unity (see Figure 4 in Franson & Bowler 2023). The youngest brown dwarf with a dynamical mass, PZ Tel b ( -+ M 27 9 25 J , 24 ± 3 Myr; Franson & Bowler 2023), has been predicted by evolutionary models to have a higher mass on average than its dynamical mass. Older, cold brown dwarfs like HD 19467 B have been found to be underluminous compared to evolutionary models (Brandt et al 2021;Greenbaum et al 2023).…”