Do Kepler superflare stars really include slowly-rotating Sun-like stars ? – Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -. (arXiv:1904.00142v1 [astro-ph.SR])

Do Kepler superflare stars really include slowly-rotating Sun-like stars ? – Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -. (arXiv:1904.00142v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Notsu_Y/0/1/0/all/0/1">Yuta Notsu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maehara_H/0/1/0/all/0/1">Hiroyuki Maehara</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Honda_S/0/1/0/all/0/1">Satoshi Honda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hawley_S/0/1/0/all/0/1">Suzanne L. Hawley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Davenport_J/0/1/0/all/0/1">James R. A. Davenport</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Namekata_K/0/1/0/all/0/1">Kosuke Namekata</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Notsu_S/0/1/0/all/0/1">Shota Notsu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ikuta_K/0/1/0/all/0/1">Kai Ikuta</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nogami_D/0/1/0/all/0/1">Daisaku Nogami</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shibata_K/0/1/0/all/0/1">Kazunari Shibata</a>

We report the latest view of Kepler solar-type (G-type main-sequence)
superflare stars, including recent updates with Apache Point Observatory (APO)
3.5m telescope spectroscopic observations and Gaia-DR2 data. First, we newly
conducted APO3.5m spectroscopic observations of 18 superflare stars found from
Kepler 1-min 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 APO3.5m and our previous
Subaru/HDS observations. The measurements of $vsin i$ (projected rotational
velocity) and chromospheric lines (Ca II H&K and Ca II 8542AA) support the
brightness variation of superflare stars is caused by the rotation of a star
with large starspots. We then investigated the statistical properties of Kepler
solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates.
As a result, the maximum superflare energy continuously decreases as the
rotation period $P_{mathrm{rot}}$ increases. Superflares with energies
$lesssim 5times10^{34}$ erg occur on old, slowly-rotating Sun-like stars
($P_{mathrm{rot}}sim$25 days) approximately once every 2000–3000 years,
while young rapidly-rotating stars with $P_{mathrm{rot}}sim$ a few days have
superflares up to $10^{36}$ 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 $P_{mathrm{rot}}gtrsim$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.

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