  We conducted high dispersion spectroscopic observations of 50 superflare
stars with Subaru/HDS, and measured the stellar parameters of them. These 50
targets were selected from the solar-type (G-type main sequence) superflare
stars that we had discovered from the Kepler photometric data. As a result of
these spectroscopic observations, we found that more than half (34 stars) of
our 50 targets have no evidence of binary system. We then estimated effective
temperature ($T_{\rm{eff}}$), surface gravity ($\log g$), metallicity ([Fe/H]),
and projected rotational velocity ($v\sin i$) of these 34 superflare stars on
the basis of our spectroscopic data. The accuracy of our estimations is higher
than that of Kepler Input Catalog (KIC) values, and the differences between our
values and KIC values ($(\Delta T_{\rm{eff}})_{\rm{rms}} \sim 219$K, $(\Delta
\log g)_{\rm{rms}} \sim 0.37$ dex, and $(\Delta\rm{[Fe/H]})_{\rm{rms}} \sim
0.46$ dex) are comparable to the large uncertainties and systematic differences
of KIC values reported by the previous researches. We confirmed that the
estimated $T_{\rm{eff}}$ and $\log g$ values of the 34 superflare stars are
roughly in the range of solar-type stars. In particular, these parameters and
the brightness variation period ($P_{0}$) of 9 stars are in the range of
"Sun-like" stars ($5600\leq T_{\rm{eff}}\leq 6000$K, $\log g\geq$4.0, and
$P_{0}>$10 days). Five of the 34 target stars are fast rotators ($v \sin i \geq
10$km s$^{-1}$), while 22 stars have relatively low $v \sin i$ values ($v \sin
i<5$km s$^{-1}$). These results suggest that stars whose spectroscopic
properties similar to the Sun can have superflares, and this supports the
hypothesis that the Sun might cause a superflare.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics