Thus, it has been thought that slowly rotating Sun-like stars basically do not have high magnetic activity events like superflares. In contrast, the Sun rotates slowly ( P rot ∼ 25 days), and the mean magnetic field is weak (a few gauss). 2000) than the largest solar flares (∼10 32 erg Emslie et al. They frequently have "superflares," which have a total bolometric energy 10–10 6 times more energetic (∼10 33–10 38 erg Schaefer et al. In particular, young, rapidly rotating stars, close binary stars, and dMe stars tend to show high magnetic activity levels, and magnetic fields of a few kG are considered to be distributed in large regions on the stellar surface (e.g., Gershberg 2005 Reid & Hawley 2005 Benz & Güdel 2010 Kowalski et al. Not only the Sun but also many stars are known to show stellar magnetic activity, including flares. These two decreasing trends are consistent since the magnetic energy stored around starspots explains the flare energy, but other factors like spot magnetic structure should also be considered.įlares are energetic explosions in the stellar atmosphere and are thought to occur by intense releases of magnetic energy stored around starspots (e.g., Shibata & Magara 2011). The maximum starspot area does not depend on the rotation period when the star is young, but as the rotation slows down, it starts to steeply decrease at P rot ≳ 12 days for Sun-like stars. Superflares with energies ≲5 × 10 34 erg occur on old, slowly rotating Sun-like stars ( P rot ∼ 25 days) approximately once every 2000–3000 yr, while young, rapidly rotating stars with P rot ∼ a few days have superflares up to 10 36 erg. As a result, the maximum superflare energy continuously decreases as the rotation period P rot increases. We then investigated the statistical properties of Kepler solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates. The measurements of v sin i (projected rotational velocity) and chromospheric lines (Ca ii H and K and Ca ii λ8542) support that the brightness variation of superflare stars is caused by the rotation of a star with large starspots. More than half (43 stars) are confirmed to be "single" stars, among 64 superflare stars in total that have been spectroscopically investigated so far in this APO 3.5 m and our previous Subaru/HDS observations. First, we newly conducted APO 3.5 m spectroscopic observations of 18 superflare stars found from Kepler 1-minute time-cadence data. We report the latest view of Kepler solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5 m telescope spectroscopic observations and Gaia-DR2 data.
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