A 2MASS Ultracool M Dwarf Observed in a Spectacular Flare

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Abstract
2MASSW J0149090+295613 (hereafter 2M0149) is unique among the very late M/L dwarf counterparts to Two Micron All Sky Survey (2MASS) point sources in having shown, in the first set of spectra taken on 1997 December 7, a diverse emission-line spectrum at red wavelengths, which features an Hα equivalent width of 300 Å. On four repeat observations, however, the spectrum was that of a more ordinary dMe object, albeit with a late spectral type of M9.5 V. Our interpretation is that the object underwent an extreme magnetic flare event. The continuum flux at the short-wavelength end of the spectrum was also several times stronger during the apparent flare than in quiescence. Comparison is made with midtype M dwarfs of higher mass that have undergone strong flares, and with another known M9.5 emission-line object PC 0025+0047. The 2MASS object adds to the evidence that magnetic dynamo eruptions may release more energy relative to Lbol as the stellar mass decreases; if the total flare luminosity scales with Hα in the same way as a well-observed event of the middle M dwarf AD Leo, 2M0149 may have had a flare luminosity that approached or exceeded its quiescent Lbol during the brief impulse phase. In contrast, its quiescent state shows an unremarkable level of chromospheric activity, which leads us to question whether it is unique at all; rather, it may be that many of the ultracool M and L dwarfs found by the Deep Near-Infrared Survey (DENIS) and 2MASS undergo similar flares.

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