On the Orbit of the Binary Brown Dwarf Companion GL229 Ba and Bb

Abstract

The companion GL229B was recently resolved by Xuan et al. as a tight binary of two brown dwarfs (Ba and Bb) through VLTI-GRAVITY interferometry and Very Large Telescope-CRIRES+ radial velocity (RV) measurements. Here, we present Bayesian models of the interferometric and RV data in additional detail, along with an updated outer orbit of the brown dwarf pair about the primary. To create a model of the inner orbit with robust uncertainties, we apply kernel phases to the GRAVITY data, to address baseline redundancy in the raw closure phases. Using parallel tempering, we constrain the binary’s orbit using only VLTI-GRAVITY data, despite each epoch having low visibility-plane coverage and/or signal-to-noise ratio (SNR). We demonstrate very good agreement between the VLTI-GRAVITY and CRIRES+ data sets and find that the inner binary has a period of 12.1346 ± 0.0011 days, an eccentricity of 0.2317 ± 0.0025, and a total mass of 71.0 ± 0.4 M _jup , with Ba and Bb having masses of 37.7 ± 1.1 M _jup and 33.4 ± 1.0 M _jup , respectively. With new Keck/NIRC2 astrometry, we update the outer orbit of GL229B around the primary. We find a semimajor axis of 42.9+3.0–2.4 au, an eccentricity of 0.736 ± 0.014, and a total mass for B of 71.7 ± 0.6 M _jup , consistent with that derived from the inner orbit. We find a mutual inclination of 31° ± 2 $\mathop{.}\limits^{\unicode{x000b0}}$ 5, below the threshold for Kozai–Lidov oscillations. The agreement on the mass of Ba+Bb between the inner and outer orbits is an important test of our ability to model the RV, astrometry, and Hipparcos–Gaia proper-motion anomaly. Our methodological advances in handling interferometric data with low SNR and sparse UV coverage will benefit future observations of rapidly orbiting companions with VLTI-GRAVITY.

Keywords

• Brown dwarfs
• T dwarfs
• Interferometers
• Radial velocity
• Astrometry