On the Nature of the Unique Hα-emitting T Dwarf 2MASS J12373919+6526148

We explore and discount the hypothesis that the strong, continual Hα-emitting T dwarf 2MASS J12373919+6526148 can be explained as a young, low-gravity, very low mass brown dwarf. The source is already known to have a marginally fainter absolute magnitude than similar T dwarfs with trigonometric parallax measurements and has a tangential velocity consistent with old disk kinematics. Applying the technique of Burgasser et al. on new near-infrared spectroscopy for this source, estimates of its T_eff, logg, and metallicity ([M/H]) are obtained. 2M 1237+6526 has a T_eff~800-850 K. If [M/H] is solar, logg is as high as ~5.5 (cgs) and this source is older than 10 Gyr. We find a more plausible scenario to be a modestly subsolar metallicity ([M/H]=-0.2) and moderate logg~5.0, implying an age older than 2 Gyr and a mass greater than 0.035 M_solar. The alternative explanation of the unique emission of this source, involving an interacting, close, double degenerate system, should be investigated further. Indeed, there is some evidence of a T_eff<500 K companion to 2M 1237+6526 on the basis of a possible Spitzer IRAC [3.6]-[4.5] color excess. This excess may, however, be caused by a subsolar metallicity.