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

We present {\it Spitzer}/IRAC observations of the L5 dwarf, 2MASSIJ1315309-264951 (2M1315). This ultracool dwarf is known to display strongemission in the H$\alpha$ line. The SED for this object does not show any IRexcess, that would indicate the presence of an accretion disk. Although theIRAC colors for 2M1315 are consistent with other L dwarfs, they seem to beredder by $\sim$0.1 mag compared to the other L5 dwarfs, and more like thelate-type L dwarfs. The existing six epochs of spectroscopy suggest that theemission in H$\alpha$ is not persistent, but shows long-term variabilitybetween a flare value of $\sim$100 $\AA$ and a quiescent value of $\sim$25$\AA$. Chromospheric activity seems to be the most likely cause, which is alsoindicated by the detection of Na I D lines in emission (Fuhrmeister et al.). Wehave measured a proper motion of 0.79$\arcsec\pm$ 0.06$\arcsec$/yr, thatcorresponds to a tangential velocity of $\sim$81 km/s, at a distance of$\sim$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 1.4Gyr (Burrows et al.) for this magnetically active L dwarf to be a stablehydrogen-burning star. There seem to be no observational differences betweenthis old L dwarf that has H$\alpha$ emission and the other field L dwarfswithout it.Comment: 14 pages, accepted in Ap