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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.