[P-EM11] 系外惑星

[PEM11-16] XUV Spectra of Active Sun-like Stars: Extension of Solar Scaling Relations with Magnetic Flux

*行方 宏介¹、鳥海 森²、Airapetian Vladimir^3,4、庄田 宗人⁵、渡邉 恭子⁶、野津 湧太⁷ (1.国立天文台、2.宇宙航空研究開発機構、3.NASA Goddard Space Flight Center、4.アメリカン大学、5.東京大学、6.防衛大学校、7.コロラド大学)

キーワード：恒星XUV放射、恒星の磁気活動、惑星の大気流出、恒星進化

Kepler Space Telescope and Transiting Exoplanet Survey Satellite (TESS) unveiled that Sun-like stars frequently host exoplanets. These exoplanets are subject to fluxes of ionizing radiation in the form of X-ray and extreme-ultraviolet (EUV) radiation that may cause changes in their atmospheric dynamics and chemistry. While X-ray fluxes can be observed directly, EUV fluxes cannot be observed because of severe interstellar medium absorption. Here, we present a new empirical method to estimate the whole stellar XUV (X-ray plus EUV) and FUV spectra as a function of the stellar unsigned magnetic flux. The advantage of this method is that we can evaluate the EUV fluxes from the stellar magnetic fluxes available from the ground-based spectroscopic observations, which is more accessible than previous approaches based on space-based observations (e.g., Lyα). We derived the response of the solar XUV and FUV spectrum (0.1-180 nm) to the total unsigned magnetic flux by using the long-term Sun-as-a-star dataset over 10 yrs in the form of the power-law relation obtained for each wavelength with a spectral resolution of 0.1-1 nm. We applied the scaling relations to active young Sun-like stars (G-dwarfs), EK Dra (G1.5V, age of 120 Myr), π1 Uma (G1.5V, age of 500 Myr) and κ1 Ceti (G5V, age of 600 Myr), and found that the observed spectra (except for the unobservable longward EUV wavelength) are roughly consistent with the extension of the derived solar power-law relations with errors of an order of magnitude. In particular, our scaling relations consistently predict the flux around missing EUV range. This suggests that our empirical model is a valuable method to derive the XUV/FUV fluxes of exoplanet-hosting Sun-like stars (and even the young Sun) including the EUV band mostly absorbed at wavelengths longward of 36 nm. We also discuss differences between the solar extensions and stellar observations at the wavelength in the 2-30 nm band and concluded that simultaneous observations of magnetic and XUV/FUV fluxes are necessary for further validations. Finally, we discuss a possible future application to cooler M/K-dwarfs.