The Dynamical Mass and Three-Dimensional Orbit of Hr7672b: A Benchmark Brown Dwarf With High Eccentricity

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, a = 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.

Individual Dynamical Masses of Ultracool Dwarfs* † ‡

“…To date there are total dynamical masses for more than a dozen ultracool binaries (e.g., Lane et al 2001;Leinert et al 2001;Bouy et al 2004;Liu et al 2008;Dupuy et al 2009aDupuy et al ,b,c, 2010Dupuy et al , 2014Cardoso et al 2009;Konopacky et al 2010). In a few special cases, individual masses have been determined: for two single companions to main-sequence stars (Ireland et al 2008;Crepp et al 2012); for the companion AB Dor C by assuming a mass for AB Dor A Guirado et al 2006); and for three ultracool binary systems where mass ratios are measured using radial velocities or absolute astrometry (Zapatero Osorio et al 2004;Simon et al 2006;Seifahrt et al 2008;Konopacky et al 2010;Köhler et al 2012;. However, most of the objects in this small sample in the literature are likely to be stars and not brown dwarfs.…”

The LEECH Exoplanet Imaging Survey. Further constraints on the planet architecture of the HR 8799 system

“…These estimates do not depend on evolutionary models of giant planets (e.g., Burrows et al 1997;Chabrier et al 2000;Baraffe et al 2003;Marley et al 2007), which rely on unknown initial conditions (Marley et al 2007;Spiegel & Burrows 2012;Marleau & Cumming 2014), and on poorly constrained stellar ages. Thus, the derivation of the dynamical mass of young low-mass companions may help to calibrate the evolutionary models (e.g., Close et al 2005;Boden et al 2005;Crepp et al 2012;Bonnefoy et al 2014b).…”

The Trends High-Contrast Imaging Survey. I. Three Benchmark M Dwarfs Orbiting Solar-Type Stars

“…RV and direct astrometry observations 8 may be used in concert to calculate the companion orbit (all six elements) and dynamical mass (Boden et al 2006). 9 For example, Crepp et al (2012) have measured the mass of the benchmark brown dwarf HR 7672 B with a fractional uncertainty of only 4% by monitoring its motion over ∼33% of an orbit cycle. Masses determined independent of photometry and spectroscopy inform theoretical atmospheric models by helping to break degeneracies between the various input parameters, such as mass, radius, age, effective temperature, and chemical composition (Barman et al 2011;Janson et al 2011).…”

“…Dynamical masses are likewise important for calibrating thermal evolutionary models, providing a measure of substellar objects' luminosity as they fade with time (Stevenson 1991;Burrows et al 1997). Further, if the companion orbits a solar-type star, the metallicity and age may be inferred from the primary (Liu et al 2007;Dupuy et al 2009;Johnson & Apps 2009;Biller et al 2010;Crepp et al 2012).…”