Poster Open Access

A missing piece of the habitability puzzle: the first statistical sample of M-dwarf superflare temperatures observed with multi-facility, multi-wavelength monitoring

Howard, Ward S.; Corbett, Hank T.; Law, Nicholas M.; Ratzloff, Jeffrey K.; Galliher, Nathan; Glazier, Amy L.; Gonzalez, Ramses; Vasquez, Alan

Superflares (flares with energy 10^33 erg or greater) provide the dominant source of biologically-relevant UV radiation to rocky planets in M-dwarf habitable zones, altering planetary atmospheres and conditions for prebiotic chemistry and surface life. However, the temperature and therefore UV emission of these rare events is not well constrained. Dozens of M-dwarf superflares were observed simultaneously by Evryscope and TESS as part of the EvryFlare Survey, comprising the first-ever statistical sample of M-dwarf superflares with optical temperatures measured at 2 minute cadence. I present newly-discovered scaling laws between superflare temperatures, energies, and heating environments. Because the blackbody extends well into the UV, I estimate UV-B and UV-C fluxes from the optical by fitting blackbody temperatures to the multi-wavelength flare observations. While flares are often assumed to emit at 9000 K, my survey finds nearly half of M-dwarf superflares emit at temperatures above 14,000 K. The largest and hottest flare in the sample reached 42,000 K. If these colors truly represent the continuum, they suggest that predictions of the UV emission from the optical using a canonical 9000 K blackbody are under-estimating the true emission by up to an order of magnitude. My survey finds rocky planets orbiting in the habitable zones of <200 Myr M-dwarfs receive top-of-atmosphere UV-C fluxes up to 1000 W m^-2, significantly impacting planetary atmospheres.

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