Learning from the Sun-as-a-Star: New Empirical Model of XUV Emission from Cool Dwarfs

Late-type dwarf stars commonly host extremely hot outer atmospheres, i.e., the coronae, transition regions, and chromospheres, which are heated and brightened in response to the appearance of active regions, the home to flares and coronal mass ejections. Therefore, understanding stellar atmospheric heating is critical in investigating the effects on exoplanetary atmospheres. In this study, with the aim of revealing the universality of solar and stellar atmospheric heating, we examined the scaling relationships between irradiances of various spectral lines and photospheric magnetic flux for the Sun and G-type dwarfs with the ages from 50 Myr to 4.5 Gyr. As a result, we found that the scalings are consistent between the Sun and G-stars for all spectral lines we examined. This striking consistency indicates that the atmospheric heating due to surface magnetic flux is common to the late-type stars throughout a wide temperature range from the corona to the chromosphere, regardless of stellar age or activity level. Furthermore, the common scaling laws between irradiances and magnetic flux allow for empirically modeling the XUV spectra of these stars, which can be utilized for chemical calculations of exoplanetary atmospheres and the assessment of habitability.