Keck laser guide star adaptive optics monitoring of 2MASS J15344984–2952274AB: first dynamical mass determination of a binary T dwarf

Creators: Liu, Michael C.; Dupuy, Trent J.; Ireland, Michael J.

Abstract

We present multiepoch imaging of the T5.0+T5.5 binary 2MASS J15344984–2952274AB obtained with the Keck laser guide star adaptive optics system. Combined with archival HST imaging, our total data span ~50% of the orbital period. We use a Markov chain Monte Carlo analysis to determine a period of 15.1^(+2.3)_(−1.6) yr and a total mass of 0.056 ± 0.003 M⊙ (59 ± 3 MJ). This is the first field binary for which both components are confirmed to be substellar. This is also the coolest and lowest mass binary with a dynamical mass to date. Using evolutionary models and accounting for the measurement covariances, we derive an age of 0.78 ± 0.09 Gyr and a mass ratio of 0.936^(+0.012)_(−0.008). The relatively youthful age is consistent with the low tangential velocity of this system. For the individual components, we find T_(eff) = 1028 ± 17 and 978 ± 17 K and masses of 0.0287 ± 0.0016 M⊙ (30.1 ± 1.7 MJ) and 0.0269 ± 0.0016 M⊙ (28.2 ± 1.7 MJ). These values generally agree with previous studies of T dwarfs and affirm current theoretical models. However, (1) the temperatures are about 100 K cooler than derived for similar objects and suggest that the representative ages of field brown dwarfs may be overestimated. Similarly, (2) the H-R diagram positions are discrepant with current models and taken at face value would overestimate the masses. While this may arise from errors in the luminosities and/or radii predicted by evolutionary models, the likely cause is a modest (≈100 K) overestimate in temperature determined from model atmospheres. We elucidate future tests of theory as the sample of dynamical masses grows. In particular, we suggest that low-mass field binaries with dynamical masses ("mass benchmarks") can serve as reference points for T_(eff) and log g to constrain atmospheric models, as good as or even better than single brown dwarfs with age estimates ("age benchmarks").

Additional Information

© 2008 The American Astronomical Society. Received 2007 December 20; accepted 2008 July 1. We gratefully acknowledge the Keck LGS AO team for their exceptional efforts in bringing the LGS AO system to fruition. It is a pleasure to thank Antonin Bouchez, David LeMignant, Marcos van Dam, Randy Campbell, Al Conrad, Jim Lyke, Hien Tran, Robert LaFon, Kenny Graves, Cindy Wilburn, Joel Aycock, Terry Stickel, Gary Punawai, and the Keck Observatory staff for assistance with the observations. We thank Alan Stockton for a fortuitous swap of observing nights in spring 2005, Brian Cameron for sharing his NIRC2 instrumental distortion analysis, Hai Fu for IDL plotting assistance, Michael Cushing and Mark Pitts for assistance with the IRTF/SpeX spectroscopy, Adam Burrows and Isabelle Baraffe for finely gridded evolutionary tracks, and Thierry Forveille and Adam Burgasser for careful readings of the manuscript. We have benefitted from enlightening discussions with Michael Cushing, Adam Burrows, and Isabelle Baraffe about substellar models, Thierry Forveille about low-mass binaries, Brian Cameron about astrometry with NIRC2, and Jay Anderson about astrometry with HST. Our research has employed the 2MASS data products; NASA's Astrophysical Data System; the SIMBAD database operated at CDS, Strasbourg, France; the M, L, and T dwarf compendium housed at DwarfArchives.org and maintained by Chris Gelino, Davy Kirkpatrick, and Adam Burgasser (Kirkpatrick 2003; Gelino et al. 2004); and the SpeX Prism Spectral Libraries maintained by Adam Burgasser at http://www .browndwarfs.org/spexprism. M. C. L. and T. J. D. acknowledge support for this work from NSF grant AST 05-07833 and an Alfred P. Sloan Research Fellowship. M. J. I. acknowledges Michelson Fellowship support from the Michelson Science Center and the NASA Navigator Program. The United Kingdom Infrared Telescope (UKIRT) is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK. Finally, the authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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