AU Microscopii in the Far-UV: Observations in Quiescence, during Flares, and Implications for AU Mic b and c

High-energy X-ray and ultraviolet (UV) radiation from young stars impacts planetary atmospheric chemistry and mass loss. The active similar to 22 Myr M dwarf AU Mic hosts two exoplanets orbiting interior to its debris disk. Therefore, this system provides a unique opportunity to quantify the effects of stellar X-ray and UV irradiation on planetary atmospheres as a function of both age and orbital separation. In this paper, we present over 5 hr of far-UV (FUV) observations of AU Mic taken with the Cosmic Origins Spectrograph (COS; 1070-1360 angstrom) on the Hubble Space Telescope (HST). We provide an itemization of 120 emission features in the HST/COS FUV spectrum and quantify the flux contributions from formation temperatures ranging from 10(4) to 10(7) K. We detect 13 flares in the FUV white-light curve with energies ranging from 10(29) to 10(31) erg s. The majority of the energy in each of these flares is released from the transition region between the chromosphere and the corona. There is a 100x increase in flux at continuum wavelengths lambda < 1100 angstrom in each flare, which may be caused by thermal Bremsstrahlung emission. We calculate that the baseline atmospheric mass-loss rate for AU Mic b is similar to 10(8) g s(-1), although this rate can be as high as similar to 10(14) g s(-1) during flares with L-flare similar or equal to 10(33) erg s(-1). Finally, we model the transmission spectra for AU Mic b and c with a new panchromatic spectrum of AU Mic and motivate future JWST observations of these planets.