Discovery of the Binarity of Gliese 229B, and Constraints on the System's Properties

TL;DR

This study reveals that Gliese 229B likely has an unseen massive companion, resolving previous mass prediction discrepancies and providing new constraints on its orbital properties through radial velocity measurements.

Contribution

First detection of a potential massive companion to Gliese 229B using radial velocities, constraining its orbit and mass, and addressing prior mass prediction tensions.

Findings

Radial velocities are inconsistent with the host star, indicating a companion.

Orbital period likely between a few days and 60 days.

Companion mass estimated to be at least 15 Jupiter masses.

Abstract

We present two epochs of radial velocities of the first imaged T dwarf Gliese 229B obtained with Keck/NIRSPEC. The two radial velocities are discrepant with one another, and with the radial velocity of the host star, at $\approx$$11\sigma$ significance. This points to the existence of a previously postulated, but as-yet undetected, massive companion to Gl 229B; we denote the two components as Gl 229Ba and Gl 229Bb. We compute the joint likelihood of the radial velocities to constrain the period and mass of the secondary companion. Our radial velocities are consistent with an orbital period between a few days and $\approx$60 days, and a secondary mass of at least $\approx$15\,$M_{\rm Jup}$ and up to nearly half the total system mass of Gl 229B. With a significant fraction of the system mass in a faint companion, the strong tension between Gl 229B's dynamical mass and the predictions of evolutionary models is resolved.