Reconstructing the XUV Spectra of Active Sun-like Stars Using Solar Scaling Relations with Magnetic Flux

Abstract

The Kepler space telescope and Transiting Exoplanet Survey Satellite 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 X-ray plus EUV (XUV) and far-UV (FUV) spectra as a function of total unsigned magnetic fluxes of stars. The response of the solar XUV and FUV spectrum (0.1–180 nm) to the solar total unsigned magnetic flux is investigated by using the long-term Sun-as-a-star data set over 10 yr, and the power-law relation is 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), π 1 Uma (G1.5V), and κ 1 Ceti (G5V) and found that the observed spectra (except for the unobservable longward EUV wavelength) are roughly consistent with the extension of the derived power-law relations with errors of an order of magnitude. This suggests that our model is a valuable method to derive the XUV/FUV fluxes of Sun-like stars, including the EUV band mostly absorbed at wavelengths longward of 36 nm. We also discuss differences between the solar extensions and stellar observations at wavelengths in the 2–30 nm band and conclude that simultaneous observations of magnetic and XUV/FUV fluxes are necessary for further validations.

Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. I. Discovery of Prominence Eruptions Associated with Superflares

“…Its effective temperature of 5560-5700 K, radius of 0.94 R e , and mass of 0.95 M e make this star one of the best proxies for an infant Sun at the time of the Hadean period on Earth (Waite et al 2017;Senavcıet al 2021). It is a rapidly rotating star, with a rotation period of 2.77 days (Audard et al 1999;Ayres 2015;Namekata et al 2022d), and it exhibits a high level of magnetic activity in the form of a hot and dense corona (e.g., Scelsi et al 2005;Linsky et al 2012;Namekata et al 2023), strong surface magnetic field and large starspots (e.g., Berdyugina & Järvinen 2005;Waite et al 2017;Järvinen et al 2018), and frequent superflares (e.g., Audard et al 1999Audard et al , 2000Ayres 2015;Namekata et al 2022c;Namekata et al 2022d). Although a faint low-mass companion is reported ∼20 au away in the projected distance from the G-type primary star (here we call the primary G-dwarf EK Dra), these two stars are not believed to be magnetically connected (Waite et al 2017), and the G-dwarf is the main source of stellar superflares and associated eruptive events (see Section "Low mass companion" in Supplementary Information of Namekata et al 2022d).…”

“…High magnetic activity of young solar-type stars is characterized not only by stellar superflares but also by strong average surface magnetic fields (e.g., Vidotto et al 2014;Kochukhov et al 2020) and giant starspots covering a substantial portion of the hemisphere (up to tens of percent; e.g., Maehara et al 2017;Waite et al 2017;Namekata et al 2019;Notsu et al 2019;Namekata et al 2020a;Herbst et al 2021;Okamoto et al 2021;Yamashita et al 2022). The magnetic energy stored in active regions associated with starspots power X-ray and EUV bright (up to 10 30 erg s −1 ), hot (up to 10 MK), and dense (10 10-11 cm −3 ) coronae and massive winds (e.g., Güdel et al 1995Güdel et al , 1997aPevtsov et al 2003;Güdel 2004;Wright et al 2011;Takasao et al 2020;Airapetian et al 2021;Shoda & Takasao 2021;Toriumi & Airapetian 2022;Namekata et al 2023). Observations of the nearby young solar-type stars in various phases of evolution have provided insights into the time history of our Sun and its heliosphere, which have had a profound influence on a young Earth over the span of millions to billions of years (e.g., Güdel et al 1997b;Güdel 2007;Johnstone et al 2019;Okamoto et al 2021).…”

Over seven decades of solar microwave data obtained with Toyokawa and Nobeyama Radio Polarimeters

“…This may be related to the local/small-scale dynamo with convection near the surface; however, further studies are required. In recent solar-stellar relation studies Namekata et al 2023), the lowest values of the solar indices at the solar minimum are assumed to be the basal values, and the solar indices that are subtracted from the basal values are used for studies. However, we did not use the subtracted values in this study because we believed that the basal values would indicate the nonvariable components of the chromosphere during solar cycles, as mentioned earlier.…”

Multiwavelength Sun-as-a-star Analysis of the M8.7 Flare on 2022 October 2 Using Hα and EUV Spectra Taken by SMART/SDDI and SDO/EVE

“…Identifying what occurs on stellar surfaces, however, is difficult because the surfaces of distant stars cannot be resolved with current observational instruments. Hence, some studies have utilized solar data with spatial resolution to interpret stellar data without spatial resolution by performing Sun-as-astar analyses (e.g., Toriumi et al 2020;Namekata et al 2022aNamekata et al , 2022bNamekata et al , 2023Otsu et al 2022;Xu et al 2022;Lu et al 2023). In Sun-as-a-star analyses, solar data are integrated spatially to be compared with stellar data.…”