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

Creators: Thompson, William¹; Blakely, Dori^{1, 2}; Xuan, Jerry W.³; Bouchard-Côté, Alexandre⁴; Bourdarot, Guillaume⁵; Biron-Lattes, Miguel⁴; Campbell, Trevor⁴; Eisenhauer, Frank⁵; Henning, Thomas⁶; Janson, Markus⁷; Johnstone, Doug^{1, 2}; Kammerer, Jens⁸; Konopacky, Quinn⁹; Lacour, Sylvestre¹⁰; Marois, Christian^{1, 2}; Mawet, Dimitri³; Mérand, Antoine⁸; Nguyen, Jayke Samson⁹; Nielsen, Eric¹¹; Rickman, Emily¹²; Ruffio, Jean-Baptiste⁹; Surjanovic, Nikola⁴; Wang 王, Jason J. 劲飞¹³; Winterhalder, Thomas⁸

1. Herzberg Institute of Astrophysics
2. University of Victoria
3. California Institute of Technology
4. University of British Columbia
5. Max Planck Institute for Extraterrestrial Physics
6. Max Planck Institute for Astronomy
7. Stockholm University
8. European Southern Observatory
9. University of California, San Diego
10. Laboratory of Space Studies and Instrumentation in Astrophysics
11. New Mexico State University
12. Space Telescope Science Institute
13. Northwestern University

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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 ± 1.0M_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.°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.

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Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

The authors thank Lindy Blackburn for helpful suggestions surrounding KPs.

This paper is based on observations collected at the European Southern Observatory under ESO programs 0112.C-2369(A), 0112.C-2369(B), and 2112.D-5036(A).

Some of the data presented herein were obtained at the Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The Pigeons package's development and collaboration are supported by a Canadian Statistical Sciences Institute Collaborative Research Teams Grant.

This research was enabled in part by support provided by Cedar, hosted at Simon Fraser University (www.sfu.ca), and the Digital Research Alliance of Canada (www.alliancecan.ca/en).

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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

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