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We report on the results of high dispersion spectroscopy of two `superflarestars', KIC 9766237, and KIC 9944137 with Subaru/HDS. Superflare stars areG-type main sequence stars, but show gigantic flares compared to the Sun, whichhave been recently discovered in the data obtained with the Kepler spacecraft.Though most of these stars are thought to have a rotation period shorter than10 days on the basis of photometric variabilities, the two targets of thepresent paper are estimated to have a rotation period of 21.8 d, and 25.3 d.Our spectroscopic results clarified that these stars have stellar parameterssimilar to those of the Sun in terms of the effective temperature, surfacegravity, and metallicity. The projected rotational velocities derived by us areconsistent with the photometric rotation period, indicating a fairy highinclination angle. The average strength of the magnetic field on the surface ofthese stars are estimated to be 1-20 G, by using the absorption line of Ca II8542. We could not detect any hint of binarity in our spectra, although moredata are needed to firmly rule out the presence of an unseen low-masscompanion. These results claim that the spectroscopic properties of thesesuperflare stars are very close to those of the Sun, and support the hypothesisthat the Sun might cause a superflare.

Two Sun-like Superflare Stars Rotating as Slow as the Sun