Mon. Not. R. Astron. Soc. 000, 1–18 (201X)
Printed 5 June 2017
(MN LATEX style file v2.2)
arXiv:1706.00657v1 [astro-ph.SR] 1 Jun 2017
Hubble Space Telescope astrometry of the closest brown dwarf binary system. I. Overview and improved orbit⋆ L. R. Bedin1†, D. Pourbaix2‡, D. Apai3,4, A. J. Burgasser5, E. Buenzli6, H. M. J. Boffin7, and M. Libralato1,8,9§ 1
INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy d’Astronomie et d’Astrophysique, Universit´ e Libre de Bruxelles (ULB), 1050 Bruxelles, Belgium 3 Department of Astronomy and Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA 4 Lunar and Planetary Laboratory, The University of Arizona, 1640 E. University Blvd., Tucson, AZ 85721, USA 5 Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA 6 Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland 7 European Southern Observatory, Karl Schwarzschild Strasse 2, D-85748 Garching, Germany 8 Space Telescope Science Institute, 3800 San Martin Drive, Baltimore, MD 21218, USA 9 Dipartimento do Fisica e Astronomia, Univ. di Padova, Vicolo dell’Osservatorio 3, I-35122 Padova, Italy 2 Institut
Accepted 2017 May 9. Received 2017 May 9; in original form 2017 April 13
ABSTRACT
Located at 2 pc, the L7.5+T0.5 dwarfs system WISE J104915.57−531906.1 (Luhman 16 AB) is the third closest system known to Earth, making it a key benchmark for detailed investigation of brown dwarf atmospheric properties, thermal evolution, multiplicity, and planet-hosting frequency. In the first study of this series – based on a multi-cycle Hubble Space Telescope (HST ) program – we provide an overview of the project and present improved estimates of positions, proper motions, annual parallax, mass ratio, and the current best assessment of the orbital parameters of the A-B pair. Our HST observations encompass the apparent periastron of the binary at 220.5±0.2 mas at epoch 2016.402. Although our data seem to be inconsistent with recent ground-based astrometric measurements, we also exclude the presence of third bodies down to Neptune masses and periods longer than a year. Key words: astrometry – binaries: visual – brown dwarfs.
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INTRODUCTION
The first completeness-corrected planet occurrence rates emerging from the Kepler mission reveal a larger number of short-period Earth-sized planets around M dwarfs than around earlier-type (FGK) stars (Dressing & Charbonneau 2013, Fressin et al. 2013). Combined with stateof-the-art planet radius–mass relationships, studies indicate that there are about 3.5× as many 1-4 MEarth mass planets around M-type stars than around G-type stars, with a period-dependence on planet occurrence rate that varies monotonically with host spectral types (Mulders et al. 2015). The larger number of small planets around M-dwarfs con⋆
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555, under GO-13748 and GO-14330. † E-mail:
[email protected] ‡ Senior Research Associate, F.R.S.-FNRS, Belgium. § Starting a PostDoc at STScI on July 1st, 2017
tains more mass in solids than the same small planet population around G-type stars (Mulders et al. 2016). Given that discs around higher-mass stars have higher masses (e.g., Pascucci et al. 2016), the higher occurrence rates and higher solid mass of small planets around lower-mass stars may be the result of inward migration of planets or their planetary building blocks. The recent discovery of seven approximately Earth-sized planets in the TRAPPIST-1 system (a star at the stellar/substellar boundary) provides a striking demonstration of the high occurrence rates of small planets around small stars (Gillon et al. 2017). These results motivate studies of the small planet population around even lower-mass hosts, the brown dwarfs (BDs, Kumar 1962, Hayashi & Nakano 1963). These lowmass objects, incapable of hydrogen fusion, also host circumstellar discs (e.g., Luhman et al. 2008) which evolve through the first steps of planet formation (e.g., Apai et al. 2005, Pascucci et al. 2009, Ricci et al. 2013). However, planet detection through radial velocity (RV), transit, and high-contrast imaging are not effective for BD primaries due
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to their low luminosities (
