A Dozen FUV Flares of AU Mic during two transits of AU Mic b

The high energy radiation from young stars shapes the atmospheres of close-in transiting exoplanets. AU Mic is a ~25 Myr nearby M dwarf with two transiting exoplanets embedded in a debris disk. We present time series UV spectra during a transit of AU Mic b as observed during two visits with HST/COS. We detected 12 flares with “broadband” (1070-1350 AA) energies ranging from 10²⁹-10³² ergs, with the most massive flare event exhibiting 3 peaks. We compare the measured flare energies and profiles as measured from emission features that originate from different formation temperatures, yielding insight into flare formation mechanisms on young M stars. Additionally, we evaluate line fluxes and find a uniform increase in flux across all wavelengths, and find a dramatic increase in blue continuum flux (< 1100 AA) during flares, which we attribute to thermal bremsstrahlung processes. With these spectra, we estimate how the measured UV flare energies and rate affect the photoevaporation rate of close-in exoplanet atmospheres, like AU Mic b and c and motivate future observational and theoretical work on the impact of FUV flares.