Testing Models with Brown Dwarf Binaries
aa r X i v : . [ a s t r o - ph . S R ] D ec Young Stars, Brown Dwarfs, and Protoplanetary DisksXXVIIth IAU General Assembly, August 2009J. Gregorio-Hetem & S. Alencar eds. c (cid:13) Testing Models with Brown Dwarf Binaries
Trent J. Dupuy & Michael C. Liu
Institute for Astronomy, University of Hawai‘i2680 Woodlawn Drive, Honolulu, HI, USA
Abstract.
We have been using Keck laser guide star adaptive optics to monitor the orbits of ultracoolbinaries, providing dynamical masses at lower luminosities and temperatures than previouslyavailable and enabling strong tests of theoretical models. (1) We find that model color–magnitudediagrams cannot reliably be used to infer masses as they do not accurately reproduce the colorsof ultracool dwarfs of known mass. (2) Effective temperatures inferred from evolutionary modelradii can be inconsistent with temperatures derived from fitting observed spectra with atmo-spheric models by at most 100–300 K. (3) For the single pair of field brown dwarfs with a precisemass (3%) and age determination ( ≈ ∼ × higher thanpredicted by model cooling rates (masses inferred from L bol and age are 20–30% larger thanmeasured). Finally, as the sample of binaries with measured orbits grows, novel tests of browndwarf formation theories are made possible (e.g., testing theoretical eccentricity distributions).
400 200 0 −200 ∆ RA (mas)−2000200 ∆ D e c ( m a s ) VLTGeminiKeckHSTVLTGeminiKeckHSTVLTGeminiKeckHSTVLTGeminiKeckHSTVLTGeminiKeckHSTVLTGeminiKeckHSTVLTGeminiKeckHSTVLTGeminiKeckHST
LHS 2397aAB M tot = 0.146 ± Sun N N Figure 1.
Left:
Orbit of the M8+L7 binary LHS 2397aAB (Dupuy et al. 2009b).
Right:
Ec-centricity distribution for all nine published ultracool ( > M6) binary orbits (Bouy et al. 2004;Simon et al. 2006; Seifahrt et al. 2008; Liu et al. 2008; Blake et al. 2008; Cardoso et al. 2009;Dupuy et al. 2009a,b,c), along with six unpublished orbits known to the author.
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