The ultraviolet radiation environment in the habitable zones around low-mass exoplanet host stars

The EUV (200-911 ), FUV (912-1750 ), and NUV (1750-3200 ) spectral energy distribution of exoplanet host stars has a profound influence on the atmospheres of Earth-like planets in the habitable zone. The stellar EUV radiation drives atmospheric heating, while the FUV (in particular, Ly alpha) and NUV radiation fields regulate the atmospheric chemistry: the dissociation of H2O and CO2, the production of O-2 and O-3, and may determine the ultimate habitability of these worlds. Despite the importance of this information for atmospheric modeling of exoplanetary systems, the EUV/FUV/NUV radiation fields of cool (K and M dwarf) exoplanet host stars are almost completely unconstrained by observation or theory. We present observational results from a Hubble Space Telescope survey of M dwarf exoplanet host stars, highlighting the importance of realistic UV radiation fields for the formation of potential biomarker molecules, O-2 and O-3. We conclude by describing preliminary results on the characterization of the UV time variability of these sources.