  We conducted high dispersion spectroscopic observations of 50 superflare
stars with Subaru/HDS. These 50 stars were selected from the solar-type
superflare stars that we had discovered from the Kepler data. More than half
(34 stars) of these 50 target superflare stars show no evidence of binarity,
and we estimated stellar parameters of these 34 stars in our previous study
(Notsu et al. 2015, hereafter called Paper I). According to our previous
studies using Kepler data, superflare stars show quasi-periodic brightness
variations whose amplitude (0.1-10\%) is much larger than that of the solar
brightness variations (0.01-0.1\%) caused by the existence of sunspots on the
rotating solar surface. In this study, we investigated whether these
quasi-periodic brightness variations of superflare stars are explained by the
rotation of a star with fairly large starspots, by using stellar parameters
derived in Paper I. First, we confirmed that the value of the projected
rotational velocity $v \sin i$ is consistent with the rotational velocity
estimated from the period of the brightness variation. Next, we measured the
intensity of Ca II infrared triplet lines and H$\alpha$ line, good indicators
of the stellar chromospheric activity, and compared them with other stellar
properties. The intensity of Ca II infrared triplet lines indicates that the
mean magnetic field strength ($\langle fB\rangle$) of the target superflare
stars can be higher than that of the Sun. A correlation between the amplitude
of the brightness variation and the intensity of Ca II triplet line was found.
All the targets expected to have large starspots because of their large
amplitude of the brightness variation show high chromospheric activities
compared to the Sun. These results support that the brightness variation of
superflare stars is due to the rotation with large starspots.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics