Stellar and Exoplanet Science with A Targeted STAR-X Survey

With sensitive X-ray instrumentation and simultaneous ultraviolet observations, the Survey and Time-domain Astrophysical Research eXplorer mission concept provides the means to resolve outstanding questions fundamental to stellar astrophysics and exoplanet science. Unprecedented X-ray sensitivity enables short time cadence monitoring of nearby stars, which for the first time can separate low-level flaring from persistent emissions to directly measure unbiased quiescent fluxes, and thus the nature of short time scale variability as a possible solution to the ongoing problem of coronal heating. The numerous detection of weak flares will enable the most precise measurement of the high-energy flare frequency distribution of nearby exoplanet hosts across a range of activity levels, and thus the total flaring contribution to the high-energy irradiation history of exoplanets. This history is a defining input to planet atmospheric modeling, and with simultaneous UV measurements, STAR-X will enable studies of exoplanet mass-loss, as well as prebiotic and photochemistry throughout planet evolution — both in quiescence and in response to broadband flare spectral energy distributions. The broader question of determining which planets are amenable to the development of life requires this stellar characterization. We discuss how a STAR-X survey of nearby exoplanet hosts stars can address these outstanding science questions, and make a significant leap in understanding the pathways to habitable worlds.