High-Dispersion Spectroscopy of the Superflare Star KIC 6934317

We conducted high-resolution spectroscopic observation with Subaru/HDS for a G-type star (KIC 6934317). We selected this star from the data of the Kepler spacecraft. It produces many superflares, and the total energy of the largest recorded superflare on this star was similar to 10(3)-times larger (similar to 2.2 x 10(35) erg) than that of the most energetic flare on the Sun (similar to 10(32) erg). The core depth and the emission flux of the Ca II infrared triplet lines and the H alpha line show high chromospheric activity in this star, in spite of its low lithium abundance and small amplitude of the rotational modulation. Using empirical relations between the emission flux of chromospheric lines and the X-ray flux, this star is considered to show much higher coronal activity than that of the Sun. It probably has large starspots that can store a large amount of magnetic energy, sufficient to give rise to superflares. We also estimated the stellar parameters, such as the effective temperature, surface gravity, metallicity, projected rotational velocity (nu sin i), and radial velocity. KIC 6934317 is then confirmed to be an early G-type main-sequence star. The value of nu sin i is estimated to be similar to 1.91 km s(-1). In contrast, the rotational velocity is calculated to be similar to 20 km s(-1) by using the period of the brightness variation as the rotation period. This difference can be explained by its small inclination angle (nearly pole-on). The small inclination angle is also supported by the contrast between the large superflare amplitude and the small stellar brightness variation amplitude. The lithium abundance and isochrones implies that the age of this star is more than about a few Gyr, though a problem why this star with such an age has such strong activity remains unsolved.