High-energy photons (X-ray to NUV) from exoplanet host stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the long-term stability of planetary atmospheres and the production of potential "biomarker" gases. However, relatively few observational and theoretical constraints exist on the high-energy irradiance from typical (i.e., weakly active) M and K dwarf exoplanet host stars. In this talk, I will describe results from a panchromatic survey (Chandra/XMM/Hubble/ground) of M and K dwarf exoplanet hosts. The MUSCLES Treasury Survey (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) combines UV, X-ray, and optical observations with reconstructed Lyman-alpha and EUV (10-90 nm) radiation to create 5 Angstrom to 5 micron stellar irradiance spectra that are available as a High-Level Science Product on STScI/MAST.
I will present an overview of the project and focus on three main results – 1) the F(FUV)/F(NUV) flux ratio increases by ~3 orders of magnitude as the star's habitable zone moves inward from 1 to 0.1 AU, and I will describe implications for the possible abiotic production of the suggested biomarkers O2 and O3, 2) we develop scaling relations to infer the high-energy particle fluxes from these stars based on solar UV flare/particle flux measurements and to calibrate visible-wavelength proxies for the high-energy irradiance, and 3) to characterize the UV variability and flare frequency of "optically inactive" M dwarfs.