HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet*

Abstract

M stars are powerful emitters of far-ultraviolet light. Over long timescales, a significant, possibly dominant, fraction of this emission is produced by stellar flares. Characterizing this emission is critical to understanding the atmospheres of the stars producing it and the atmospheric evolution of the orbiting planets subjected to it. Ultraviolet emission is known to be elevated for several hundred million years after M stars form. Whether or not the same is true of ultraviolet flare activity is a key concern for the evolution of exoplanet atmospheres. Hubble Space Telescope (HST) observations by the HAZMAT program (HAbitable Zones and M dwarf Activity across Time) detected 18 flares on young (40 Myr) early M stars in the Tucana–Horologium association over 10 hr of observations, 10 having energy >10 30 erg. These imply that flares on young M stars are 100–1000× more energetic than those occurring at the same rate on “inactive,” field age M dwarfs. However, when energies are normalized by quiescent emission, there is no statistical difference between the young and field age samples. The most energetic flare observed, dubbed the “Hazflare,” emitted an energy of 10 32.1 erg in the FUV, 30× more energetic than any stellar flare previously observed in the FUV with HST ’ s COS or STIS spectrographs. It was accompanied by 15,500 ± 400 K blackbody emission bright enough to designate it as a superflare ( E  > 10 33 erg), with an estimated bolometric energy of erg. This blackbody emitted % of its flux in the FUV (912–1700 Å), where molecules are generally most sensitive to photolysis. Such hot superflares in young, early M stars could play an important role in the evolution of nascent planetary atmospheres.