Do Kepler Superflare Stars Really Include Slowly Rotating Sun-like Stars?—Results Using APO 3.5 m Telescope Spectroscopic Observations and Gaia-DR2 Data

DOI DOI HANDLE Web Site Web Site 被引用文献34件 参考文献128件

書誌事項

公開日
2019-05-01
資源種別
journal article
権利情報
  • https://iopscience.iop.org/page/copyright
  • https://iopscience.iop.org/info/page/text-and-data-mining
DOI
  • 10.3847/1538-4357/ab14e6
  • 10.48550/arxiv.1904.00142
公開者
American Astronomical Society

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説明

<jats:title>Abstract</jats:title> <jats:p>We report the latest view of <jats:italic>Kepler</jats:italic> solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5 m telescope spectroscopic observations and <jats:italic>Gaia</jats:italic>-DR2 data. First, we newly conducted APO 3.5 m spectroscopic observations of 18 superflare stars found from <jats:italic>Kepler</jats:italic> 1-minute time-cadence data. 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. The measurements of <jats:italic>v</jats:italic> sin <jats:italic>i</jats:italic> (projected rotational velocity) and chromospheric lines (Ca <jats:sc>ii</jats:sc> H and K and Ca <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>8542) support that the brightness variation of superflare stars is caused by the rotation of a star with large starspots. We then investigated the statistical properties of <jats:italic>Kepler</jats:italic> solar-type superflare stars by incorporating <jats:italic>Gaia</jats:italic>-DR2 stellar radius estimates. As a result, the maximum superflare energy continuously decreases as the rotation period <jats:italic>P</jats:italic> <jats:sub>rot</jats:sub> increases. Superflares with energies ≲5 × 10<jats:sup>34</jats:sup> erg occur on old, slowly rotating Sun-like stars (<jats:italic>P</jats:italic> <jats:sub>rot</jats:sub> ∼ 25 days) approximately once every 2000–3000 yr, while young, rapidly rotating stars with <jats:italic>P</jats:italic> <jats:sub>rot</jats:sub> ∼ a few days have superflares up to 10<jats:sup>36</jats:sup> erg. 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 <jats:italic>P</jats:italic> <jats:sub>rot</jats:sub> ≳ 12 days for Sun-like stars. 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.</jats:p>

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