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 \alpha \approx - 1.1 ) , starting at rotation periods \gtrsim 3.5 days .
Using N v or Si iv spectra and a 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 \approx 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 .