CHARACTERIZING A DRAMATIC Δ <i>V</i> ∼ –9 FLARE ON AN ULTRACOOL DWARF FOUND BY THE ASAS-SN SURVEY

Abstract

We analyze a $\\Delta V\\sim-9$ magnitude flare on the newly identified M8\ndwarf SDSS J022116.84+194020.4 (hereafter SDSSJ0221) detected as part of the\nAll-Sky Automated Survey for Supernovae (ASAS-SN). Using infrared and optical\nspectra, we confirm that SDSSJ0221 is a relatively nearby (d$\\sim$76 pc) M8\ndwarf with strong quiescent H$\\alpha$ emission. Based on kinematics and the\nabsence of features consistent with low-gravity (young) ultracool dwarfs, we\nplace a lower limit of 200 Myr on the age of SDSSJ0221. When modeled with a\nsimple, classical flare light-curve, this flare is consistent with a total\n$U$-band flare energy $E_U\\sim$ 10$^{34}$ erg, confirming that the most\ndramatic flares are not limited to warmer, more massive stars. Scaled to\ninclude a rough estimate of the emission line contribution to the $V$ band, we\nestimate a blackbody filling factor of $\\sim$$10-30\\%$ during the flare peak\nand $\\sim$$0.5-1.6\\%$ during the flare decay phase. These filling factors\ncorrespond to flare areas that are an order of magnitude larger than those\nmeasured for most mid-M dwarf flares.\n