Spectral Energy Distribution of 100 stars

Our first step is to derive new coronal models for the planet-host stars. We use updated data from the X-exoplanets database, archival X-ray spectra of M-type stars (including AU Mic and Proxima Centauri), and new XMM-Newton X-ray data recently obtained for the CARMENES project. These data are complemented at longer wavelengths with publicly available HST, FUSE, and EUVE spectra.

A total of 75 stars are carefully analyzed to obtain a new calibration between X-ray and extreme ultraviolet (EUV) emission. Two distinct relationships between stellar X-ray emission (5-100{AA}) and EUV H (100-920{AA}) or EUV He (100-504{AA}) radiation are obtained to scale the emission from late-type (F to M) stellar coronae. A total of 48 systems with reported planetary HeI 10830{AA} studies, Including 21 positive detections and 27 upper limits, exhibit a robust relationship between the strength of the planetary He I feature and the ionizing XUV He received by the planet, corrected by stellar and planetary radii, and the planet gravitational potential.

Some outliers could be explained by a different atmospheric composition, or the lack of planetary gaseous atmospheres. This relation may serve as a guide to predict the detectability of the HeI 10830{AA} absorption in exoplanet atmospheres.