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

The GAPS Programme at TNG. XXXII. The revealing non-detection of metastable HeI in the atmosphere of the hot Jupiter WASP-80b

“…We collected the system parameters from the literature giving priority to more recent and/or homogeneous sources. The stellar XUV fluxes listed in Table 3 have been either taken from the literature or, when unavailable, extracted employing the scaling relations of Sreejith et al (2020). The XUV fluxes listed in Table 3 correspond to the amount of highenergy stellar radiation at wavelengths shorter than 912 Å irradiating the planet.…”

“…This is not the most relevant wavelength range 5 in relation to the production of metastable Hei, but it is still representative of the high-energy stellar emission, it is the one typically reported in the literature, and it can be readily estimated on the basis of a variety of measurements obtained at X-ray, ultraviolet, and optical wavelengths (e.g. Sanz-Forcada et al 2011;Chadney et al 2015;King et al 2018;Linsky et al 2013Linsky et al , 2014France et al 2018;Sreejith et al 2020).…”

“…Furthermore, following the typical uncertainties of scaling relations employed to convert X-ray, ultraviolet, or optical measurements into XUV flux (e.g. Linsky et al 2013Linsky et al , 2014France et al 2018;Sreejith et al 2020), we assigned an uncertainty to the stellar XUV flux of a factor of two. Figure 10 does not indicate the presence of any clear correlation with any of the considered parameters (i.e.…”

“…GJ832 is an early M dwarf with an effective temperature of about 3600 K (e.g. Kuznetsov et al 2019) and a measured log R HK value of about −5.1 (Jenkins et al 2006;Boro Saikia et al 2018;Hojjatpanah et al 2019;Sreejith et al 2020), thus slightly cooler and less active than WASP-80, but with a measured UV flux and a modelled XUV flux that can be used as anchor (France et al 2016). We estimated the UV spectral emission of WASP-80 by rescaling the stellar flux of GJ832 until the XUV flux matched that derived for WASP-80.…”

Corona and XUV emission modelling of the Sun and Sun-like stars

“…One thus needs to indirectly estimate or reconstruct the EUV spectrum of a star from other observables. The proposed reconstruction methods include the inversion of the differential emission measure from UV and/or X-ray observations (Sanz-Forcada et al 2011;Duvvuri et al 2021), the empirical correlation between other observable lines and XUV emission (Linsky et al 2014;Youngblood et al 2017;France et al 2018;Sreejith et al 2020), and/or the combination of them (Diamond-Lowe et al 2021). The empirical estimations of the stellar EUV emission are, however, based on observations with large uncertainty and limited number of samples, and thus, need validations from different points of view.…”