Infrared observations of a mid-L dwarf with strong H$\alpha$ emission

Abstract

We present {\it Spitzer}/IRAC observations of the L5 dwarf, 2MASSI J1315309-264951 (2M1315). This ultracool dwarf is known to display strong emission in the H$\alpha$ line. The SED for this object does not show any IR excess, that would indicate the presence of an accretion disk. Although the IRAC colors for 2M1315 are consistent with other L dwarfs, they seem to be redder by $\sim$0.1 mag compared to the other L5 dwarfs, and more like the late-type L dwarfs. The existing six epochs of spectroscopy suggest that the emission in H$\alpha$ is not persistent, but shows long-term variability between a flare value of $\sim$100 $\AA$ and a quiescent value of $\sim$25 $\AA$. 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$\arcsec\pm$ 0.06$\arcsec$/yr, that corresponds 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 1.4 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 it.