Highly unequal-mass ratio binaries are rare among field brown dwarfs , with the mass ratio distribution of the known census described by q ^ { ( 4.9 \pm 0.7 ) } .
However , such systems enable a unique test of the joint accuracy of evolutionary and atmospheric models , under the constraint of coevality for the individual components ( the “ isochrone test ” ) .
We carry out this test using two of the most extreme field substellar binaries currently known , the T1+T6 \epsilon Ind Bab binary and a newly discovered 0.14″ T2.0+T7.5 binary , 2MASS J12095613 - 1004008AB , identified with Keck laser guide star adaptive optics .
The latter is the most extreme tight binary resolved to date ( q \approx 0.5 ) .
Based on the locations of the binary components on the H-R diagram , current models successfully indicate that these two systems are coeval , with internal age differences of \log ( age ) = -0.8 \pm 1.3 ( -1.0 _ { -1.3 } ^ { +1.2 } ) dex and 0.5 ^ { +0.4 } _ { -0.3 } ( 0.3 ^ { +0.3 } _ { -0.4 } ) dex for 2MASS J1209 - 1004AB and \epsilon Ind Bab , respectively , as inferred from the Lyon ( Tucson ) models .
However , the total mass of \epsilon Ind Bab derived from the H-R diagram ( \approx 80 M _ { Jup } using the Lyon models ) is strongly discrepant with the reported dynamical mass .
This problem , which is independent of the assumed age of the \epsilon Ind Bab system , can be explained by a \approx 50–100 K systematic error in the model atmosphere fitting , indicating slightly warmer temperatures for both components ; bringing the mass determinations from the H-R diagram and the visual orbit into consistency leads to an inferred age of \approx 6 Gyr for \epsilon Ind Bab , older than previously assumed .
Overall , the two T dwarf binaries studied here , along with recent results from T dwarfs in age and mass benchmark systems , yield evidence for small ( \approx 100 K ) errors in the evolutionary models and/or model atmospheres , but not significantly larger .
Future parallax , resolved spectroscopy , and dynamical mass measurements for 2MASS J1209 - 1004AB will enable a more stringent application of the isochrone test .
Finally , the binary nature of this object reduces its utility as the primary T3 near-IR spectral typing standard ; we suggest SDSS J1206+2813 as a replacement .