Coronal X-ray Emission from the MUSCLES/MegaMUSCLES Sample of Nearby, Low-Mass, Exoplanet Host Stars

Stellar X-ray and UV emission play important roles in the structure and evolution of exoplanet atmospheres. X-ray and EUV radiation at wavelengths shortward of 912 A plays a major role in thermospheric heating and erosion of planetary atmospheres, while FUV/NUV radiation controls the atmospheric chemistry via molecular formation and photolysis. The HST MUSCLES and Mega MUSCLES projects have observed the UV/X-ray radiation fields for a representative sample of 23 nearby (most within 15 pc), early K to late-M exoplanet host stars. These stars cover the mass range 0.09–0.8 M_⊙ and include many older, slowly-rotating, “inactive” stars, for which few detailed UV and X-ray datasets were available previously. We present results from the Chandra and XMM-Newton data obtained by these projects. More than 750 ksec of Chandra and XMM-Newton observing time has been devoted to these stars in our GO and archival observations. In addition to establishing the high energy spectral energy distribution, the observations show that strong X-ray and UV flares persist on very old and slow rotating stars. These planetary system provide a variety of test cases to model exoplanetary evolution and investigate habitability.

This work has been supported by Chandra grants GO4-15014X, GO5-16155X, and GO8-19017X, NASA XMM-Newton grant NNX16AC09G and HST grants to Treasury programs 13650 and 15071.