Simulating the X-ray and EUV Emission of Cool Exoplanet Host Stars

By analogy with the Earth-Sun system, the search for potentially habitable exoplanets focuses mainly on terrestrial exoplanets orbiting cool stars. Cool stars include the range of partially-convective stars of late-F, G, K, and early-M types, in addition to fully-convective late-M stars. In previous work we have employed a surface flux transport (SFT) model (Schrijver 2001, Schrijver et al. 2003) to examine the emergence and dynamics of magnetic flux on the surfaces of cool stars like the Sun and other exoplanet host stars of interest. Terrestrial exoplanets orbiting cool stars are influenced by their host stars in a variety of ways, including via interaction with the stellar magnetic field (Garaffo et al. 2016, Farrish et al. 2019) and by stellar coronal X-ray emission (e.g., Farrish et al. 2021) which may ionize planetary atmospheric gases. Exoplanet atmospheres are also influenced by stellar emission in the extreme ultraviolet (EUV) wavelength regime (~100-900 Å) through photochemical reactions and escape processes. An understanding of the high-energy emission of the central host star through its X-ray and EUV (collectively, XUV) output is therefore integral to the study of atmospheric and ionospheric evolution at the associated exoplanets. However, stellar EUV observations are historically extremely sparse (Youngblood et al. 2019). Thus, detailed modeling of the dependence of host star XUV emission on stellar magnetic activity can fill many gaps in our current understanding of exoplanet atmospheric processes. We present a study integrating our previous simulations of exoplanet host star magnetic activity with models of coronal heating and the associated XUV emission for a range of cool stars. Particular attention is paid to the relevance of this high-energy emission to atmospheric processes at the associated planets.