2MASSI J1315309-264951: An L Dwarf with Strong and Variable Hα Emission

The object 2MASSI J1315309-264951 is an L3 dwarf with strong Hα emission discovered in the course of a color-selected survey for active galactic nuclei using the Two Micron All Sky Survey (2MASS). The strength of its Hα emission decreased by about a factor of 2 between two epochs separated by 137 days. This is the first time that variable Hα emission has been reported in an L dwarf, and it is probably the first observation of an Hα flare in an L dwarf. The value of log(L_Hα/L_bol)>-4.17 observed at the discovery epoch is larger than that of any other L dwarf, but it is comparable to that of 2MASSI J1237392+652615, the only reported T dwarf with Hα emission. The observed variability indicates that the Hα emission of 2MASSI J1315309-264951 is powered either by magnetic fields or by accretion in a binary system. Spectroscopic or narrowband Hα monitoring of L and T dwarfs on timescales of hours to days would be the most useful step toward a better understanding of their Hα emission mechanism(s).