Infrared Observations of a Mid-L Dwarf with Strong Hα Emission

We present Spitzer/IRAC observations of the L5 dwarf, 2MASSI J1315309-264951 (2M 1315). This ultracool dwarf is known to display strong emission in the H alpha line. The spectral energy distribution (SED) for this object does not show any IR excess that would indicate the presence of an accretion disk. Although the IRAC colors for 2M 1315 are consistent with other L dwarfs, they seem to be redder by similar to 0.1 mag than the other L5 dwarfs, and more like the late-type L dwarfs. The existing six epochs of spectroscopy suggest that the emission in H alpha is not persistent, but shows long-term variability between a flare value of similar to 100 angstrom and a quiescent value of similar to 25 angstrom. Chromospheric activity seems to be the most likely cause, which is also indicated by the detection of Na I D lines in emission (Fuhrmeister et al.). We have measured a proper motion of 0.79 '' +/- 0.06 '' yr(-1), which corresponds to a tangential velocity of similar to 81 km s(-1) at a distance of similar to 22 pc. The high V-tan for this object suggests an old age. Evolutionary models indicate lower limits of 3.3 Gyr (Chabrier et al.) or 1.4 Gyr (Burrows et al.) for this magnetically active L dwarf to be a stable hydrogen burning star. There seem to be no observational differences between this old L dwarf that has H alpha emission and the other field L dwarfs without