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(Abridged) Highly unequal-mass ratio binaries are rare among field browndwarfs, with the known census described by q^(4.9+/-0.7). However, such systemscan test the joint accuracy of evolutionary and atmospheric models, under theconstraint of coevality (the "isochrone test''). We carry out this test usingtwo of the most extreme field substellar binaries currently known, the T1+T6\eps Ind Bab binary and a newly discovered 0.14" T2.0+T7.5 binary, 2MASS1209-10AB. Based on the locations of the components on the H-R diagram, modelssuccessfully indicate that the systems are coeval, with internal agedifferences of log(age) = 0.5{+0.4}{-0.3} and -0.8+/-1.3 dex, respectively.However, the total mass of \eps Ind Bab derived from the H-R diagram (~80 Mjup)is discrepant with the reported dynamical mass. This problem, which isindependent of the assumed age of the system, can be explained by a ~50-100 Ksystematic error in the model atmosphere fitting; bringing the two massdeterminations into consistency leads to an inferred age of ~6 Gyr for the \epsInd system, older than previously assumed. Overall, the two T dwarf binariesstudied here, along with recent results from T dwarfs in age and mass benchmarksystems, yield evidence for small (~100 K) errors in the evolutionary modelsand/or model atmospheres, but not significantly larger. Finally, the binarynature of 2MASS 1209-10AB reduces its utility as the primary T3 near-IRspectral typing standard; we suggest SDSS 1206+28 as a replacement.

astrophysical journal/the astrophysical journal

Discovery of a Highly Unequal-Mass Binary T Dwarf with Keck Laser Guide  Star Adaptive Optics: A Coevality Test of Substellar Theoretical Models and  Effective Temperatures