The Dynamical Mass and Three-dimensional Orbit of HR7672B: A Benchmark Brown Dwarf with High Eccentricity

Creators: Crepp, Justin R.; Johnson, John Asher; Fischer, Debra A.; Howard, Andrew W.; Marcy, Geoffrey W.; Wright, Jason T.; Isaacson, Howard; Boyajian, Tabetha; von Braun, Kaspar; Hillenbrand, Lynne A.; Hinkley, Sasha; Carpenter, John M.; Brewer, John M.

Abstract

The companion to the G0V star HR7672 directly imaged by Liu et al. has moved measurably along its orbit since the discovery epoch, making it possible to determine its dynamical properties. Originally targeted with adaptive optics because it showed a long-term radial velocity (RV) acceleration (trend), we have monitored this star with precise Doppler measurements and have now established a 24 year time baseline. The RV variations show significant curvature (change in the acceleration) including an inflection point. We have also obtained a recent image of HR7672B with NIRC2 at Keck. The astrometry also shows curvature. In this paper, we use jointly fitted Doppler and astrometric models to calculate the three-dimensional orbit and dynamical mass of the companion. The mass of the host star is determined using a direct radius measurement from CHARA interferometry in combination with high-resolution spectroscopic modeling. We find that HR7672B has a highly eccentric, e = 0.50^(+0.01)_(–0.01), near edge-on, i = 97.3^(+0.4)_(–0.5) deg, orbit with semimajor axis, ɑ = 18.3^(+0.4)_(–0.5) AU. The mass of the companion is m = 68.7^(+2.4)_(–3.1) M_J . HR7672B thus resides near the substellar boundary, just below the hydrogen-fusing limit. These measurements of the companion mass are independent of its brightness and spectrum and establish HR7672B as a rare and precious "benchmark" brown dwarf with a well-determined mass, age, and metallicity essential for testing theoretical evolutionary models and synthetic spectral models. Indeed, we find that such models under-predict its luminosity by a factor of ≈2. HR 7672B is presently the only L, T, or Y dwarf known to produce an RV trend around a solar-type star.

Additional Information

© 2012 American Astronomical Society. Received 2011 September 25; accepted 2012 March 19; published 2012 May 10. We thank Brendan Bowler for drawing our attention to the VLT archival images of HR7672 taken with NACO in 2007 September, Dimitri Mawet for help interpreting the archival data, and Randy Campbell for assisting with observations during our 2011 May astrometric measurement with NIRC2. Trent Dupuy, Brendan Bowler, and the anonymous referee provided helpful suggestions that improved the quality of the paper. We also thank R. Paul Butler, Chris McCarthy, and Steve Vogt for making many of the early RV observations. We thank the University of California for the use of Lick Observatory, including both the Coude Auxiliary Telescope and the 3 m. A large portion of the data presented herein were obtained at the W. M. Keck Observatory, which is 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. We acknowledge usage of the ESO scientific archive for providing complementary astrometric data from the VLT. This research made use of the SIMBAD literature database, operated at CDS, Strasbourg, France, and of NASA's Astrophysics Data System. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research made use of the NASA/IPAC/NExScI Star and Exoplanet Database, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. T.S.B. acknowledges support provided by NASA through Hubble Fellowship grant HST-HF-51252.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. The CHARA Array is funded by the National Science Foundation through NSF grants AST-0606958 and AST-0908253 and by Georgia State University through the College of Arts and Sciences, the W. M. Keck Foundation, the Packard Foundation, and the NASA Exoplanet Science Institute.

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