Investigation of coronal properties of X-ray bright G-dwarf stars based on the solar surface magnetic field -- corona relation

Abstract: We investigated the coronal properties of G-dwarf stars including the Sun over a wide range of X-ray luminosity $L_{\rm X}$ ($3\times 10^{26}$ to $2\times 10^{30}~{\rm erg~s^{-1}}$). We analyzed the archival data of ten X-ray bright ($L_{\it X}>10^{28}~{\rm erg~s^{-1}}$) G-dwarf stars to derive their emission measure (EM) and the coronal temperature ($T$) during the periods when no prominent stellar flares were observed. We attempted to explain the relation on the basis of our understanding of the present Sun: a steady corona model based on the so-called RTV scaling laws and the observed power-law distribution function of surface magnetic features. We derived a theoretical scaling law of the EM--$T$ relation for a star with multiple active regions, and applied it to the observations combined with data in literature. We found that with the solar parameters, our scaling law seems to be consistent with the data of slowly-rotating stars. However, more X-ray bright stars are located well above the scaling law based on the solar parameter. The scaling law may explain the observations if those stars show a power-law distribution function of active regions with the same power-law index but a 10-100 times larger coefficient. This suggests that X-ray bright stars show more active regions for a given size than the Sun. Since our samples include rapidly-rotating stars, we infer that the offset of the X-ray bright stars from the present-Sun-based scaling law is due to the enhancement of the surface magnetic field generation by their rapid rotation.