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 h of observations , ten having energy > 10 ^ { 30 } erg .
These imply that flares on young M stars are 100–1000 \times 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 \times more energetic than any stellar flare previously observed in the FUV with HST ’ s COS or STIS spectrographs .
It was accompanied by 15 , 500 \pm 400 K blackbody emission bright enough to designate it as a superflare ( E > 10 ^ { 33 } erg ) , with an estimated bolometric energy of 10 ^ { 33.6 _ { -0.2 } ^ { +0.1 } } erg .
This blackbody emitted 18 ^ { +2 } _ { -1 } % 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 .