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

We explore and discount the hypothesis that the strong, continualH$\alpha$-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 amarginally-fainter absolute magnitude than similar T dwarfs with trigonometricparallax measurements, and has a tangential velocity consistent with old diskkinematics. Applying the technique of Burgasser, Burrows & Kirkpatrick on newnear infrared spectroscopy for this source, estimates of its {\teff}, $\log{g}$and metallicity ([M/H]) are obtained. 2M 1237+6526 has a {\teff} $\approx$800-850 K. If [M/H] is solar, $\log{g}$ is as high as $\sim$5.5 (cgs) and thissource is older than 10 Gyr. We find a more plausible scenario to be a modestlysubsolar metallicity ([M/H] = -0.2) and moderate $\log{g}$ $\sim$ 5.0, implyingan age older than 2 Gyr and a mass greater than 0.035 M$_{\sun}$. Thealternative explanation of the unique emission of this source, involving aninteracting, close, double degenerate system, should be investigated further.Indeed, there is some evidence of a {\teff} $<$ 500 K companion to 2M 1237+6526on the basis of a possible $Spitzer IRAC$ [3.6]--[4.5] color excess. Thisexcess may, however, be caused by a subsolar metallicity.Comment: Astrophysical Journal, in press 15 pages, 5 figure