Dynamical Mass of the Young Substellar Companion HD 984 B

TL;DR

This study measures the dynamical mass of the young substellar companion HD 984 B using combined astrometry, radial velocities, and proper motion data, providing a critical benchmark for models of brown dwarf evolution.

Contribution

It presents the first dynamical mass measurement of HD 984 B, refining its orbit and confirming its brown dwarf status through a comprehensive multi-method analysis.

Findings

Dynamical mass of HD 984 B is 61 ± 4 Jupiter masses.

The orbit has a high eccentricity of 0.76 ± 0.05.

Mass measurement aligns with evolutionary model predictions.

Abstract

Model-independent masses of substellar companions are critical tools to validate models of planet and brown dwarf cooling, test their input physics, and determine the formation and evolution of these objects. In this work, we measure the dynamical mass and orbit of the young substellar companion HD 984 B. We obtained new high-contrast imaging of the HD 984 system with Keck/NIRC2 which expands the baseline of relative astrometry from 3 to 8 years. We also present new radial velocities of the host star with the Habitable-Zone Planet Finder spectrograph at the Hobby-Eberly Telescope. Furthermore, HD 984 exhibits a significant proper motion difference between Hipparcos and Gaia EDR3. Our joint orbit fit of the relative astrometry, proper motions, and radial velocities yields a dynamical mass of $61 \pm 4$ $\mathrm{M_{Jup}}$ for HD 984 B, placing the companion firmly in the brown dwarf regime. The new fit also reveals a higher eccentricity for the companion ($e = 0.76 \pm 0.05$) compared to previous orbit fits. Given the broad age constraint for HD 984, this mass is consistent with predictions from evolutionary models. HD 984 B's dynamical mass places it among a small but growing list of giant planet and brown dwarf companions with direct mass measurements.