Far-ultraviolet Activity Levels of F, G, K, and M Dwarf Exoplanet Host Stars

We present a survey of far-ultraviolet (FUV; 1150–1450 Å) emission line spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs with the goals of characterizing their range of FUV activity levels, calibrating the FUV activity level to the 90–360 Å extreme-ultraviolet (EUV) stellar flux, and investigating the potential for FUV emission lines to probe star–planet interactions (SPIs). We build this emission line sample from a combination of new and archival observations with the Hubble Space Telescope -COS and -STIS instruments, targeting the chromospheric and transition region emission lines of Si iii , N v , C ii , and Si iv . We find that the exoplanet host stars, on average, display factors of 5–10 lower UV activity levels compared with the non-planet-hosting sample; this is explained by a combination of observational and astrophysical biases in the selection of stars for radial-velocity planet searches. We demonstrate that UV activity-rotation relation in the full F – M star sample is characterized by a power-law decline (with index α ≈ −1.1), starting at rotation periods ≳3.5 days. Using N v or Si iv spectra and knowledge of the star’s bolometric flux, we present a new analytic relationship to estimate the intrinsic stellar EUV irradiance in the 90–360 Å band with an accuracy of roughly a factor of ≈2. Finally, we study the correlation between SPI strength and UV activity in the context of a principal component analysis that controls for the sample biases. We find that SPIs are not a statistically significant contributor to the observed UV activity levels.