Ca II H&K stellar activity parameter: a proxy for stellar Extreme Ultraviolet Fluxes

TL;DR

This paper develops empirical relations to estimate stellar EUV fluxes from the Ca II H&K activity index, aiding studies of exoplanet atmospheres where direct EUV observations are unavailable.

Contribution

The study introduces new analytic functions to predict stellar EUV fluxes from the widely observed Ca II H&K activity parameter, expanding tools for exoplanet atmospheric research.

Findings

EUV fluxes can be predicted with about threefold accuracy from Ca II H&K measurements.

The relations are based on a sample of ~100 nearby stars with known EUV and activity data.

Predictions are slightly less accurate than those based on FUV or X-ray fluxes.

Abstract

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 atmosphere. Our understanding of atmospheric escape and chemistry, therefore, depends on our knowledge of the stellar UV fluxes. While the far-ultraviolet 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, to interstellar medium absorption. Thus, it becomes essential to have 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, where the latter are either direct measurements or inferences from high-quality far-ultraviolet (FUV) spectra. The scaling relations presented here return EUV flux values with an accuracy of about three, which is slightly lower than that of other similar methods based on FUV or X-ray measurements.