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 ∼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 Å) 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 10⁴ to 10⁷ K. We detect 13 flares in the FUV white-light curve with energies ranging from 10²⁹ to 10³¹ 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 100× increase in flux at continuum wavelengths λ < 1100 Å 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 ∼10⁸ g s⁻¹, although this rate can be as high as ∼10¹⁴ g s⁻¹ during flares with 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.