Ca II H&K stellar activity parameter: a proxy for extreme ultraviolet stellar fluxes

Atmospheric escape is an important factor shaping the exoplanet population and hence drives our understanding of planet formation. Atmospheric escape from giant planets is driven primarily by the stellar X-ray and extreme ultraviolet (EUV) radiation. Furthermore, EUV and longer wavelength UV radiation power disequilibrium chemistry in the middle and upper atmospheres. Our understanding of atmospheric escape and chemistry, therefore, depends on our knowledge of the stellar UV fluxes. While the far-ultraviolet (FUV) fluxes can be observed for some stars, most of the EUV range is unobservable due to the lack of a space telescope with EUV capabilities and, for the more distant stars, due to interstellar medium absorption. Therefore, it becomes essential to have an indirect means for inferring EUV fluxes from features observable at other wavelengths. We present here analytic functions for predicting the EUV emission of F-, G-, K-, and M-type stars from the log R ′ HK activity parameter that is commonly obtained from ground-based optical observations of the Ca II H&K lines. The scaling relations are based on a collection of about 100 nearby stars with published log R ′ HK and EUV flux values, the latter of which are either direct measurements or inferences from high-quality FUV spectra. The scaling relations presented here return EUV flux values with an accuracy of about a factor of three, which is slightly lower than that of other similar methods based on FUV or X-ray measurements.