A Dynamical Mass of $70 \pm 5$ Jupiter Masses for Gliese 229B, the First T Dwarf

TL;DR

This paper measures a dynamical mass of 70 ± 5 Jupiter masses for Gliese 229B, the first T dwarf with such a measurement, challenging existing cooling models and suggesting an older age than previously estimated.

Contribution

It provides the first dynamical mass measurement for Gliese 229B, using combined astrometric and radial velocity data, and discusses implications for brown dwarf models and age estimates.

Findings

Dynamical mass of 70 ± 5 Jupiter masses for Gliese 229B.

Cooling models underestimate the mass compared to dynamical measurements.

No evidence of additional companions or binarity affecting the mass estimate.

Abstract

We combine Keck/HIRES radial velocities, imaging with HiCIAO/Subaru and the Hubble Space Telescope, and absolute astrometry from Hipparcos and Gaia to measure a dynamical mass of $70 \pm 5$ Jupiter masses for the brown dwarf companion to Gl 229. Gl 229B was the first imaged brown dwarf to show clear signs of methane in its atmosphere. Cooling models have been used to estimate a mass in the range of 20 - 55 Jupiter masses, much lower than our measured value. We argue that our high dynamical mass is unlikely to be due to perturbations from additional unseen companions or to Gl 229B being itself a binary, and we find no evidence of a previously claimed radial velocity planet around Gl 229A. Future Gaia data releases will confirm the reliability of the absolute astrometry, though the data pass all quality checks in both Hipparcos and Gaia. Our dynamical mass implies a very old age for Gl 229, in some tension with kinematic and activity age indicators, and/or shortcomings in brown dwarf cooling models. Gl 229B joins a small but growing list of T dwarfs with masses approaching the minimum mass for core hydrogen ignition.