We report on the results of high dispersion spectroscopy of two ‘ superflare stars ’ , KIC 9766237 , and KIC 9944137 with Subaru/HDS .
Superflare stars are G-type main sequence stars , but show gigantic flares compared to the Sun , which have been recently discovered in the data obtained with the Kepler spacecraft .
Though most of these stars are thought to have a rotation period shorter than 10 days on the basis of photometric variabilities , the two targets of the present 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 parameters similar to those of the Sun in terms of the effective temperature , surface gravity , and metallicity .
The projected rotational velocities derived by us are consistent with the photometric rotation period , indicating a fairy high inclination angle .
The average strength of the magnetic field on the surface of these stars are estimated to be 1-20 G , by using the absorption line of Ca ii 8542 .
We could not detect any hint of binarity in our spectra , although more data are needed to firmly rule out the presence of an unseen low-mass companion .
These results claim that the spectroscopic properties of these superflare stars are very close to those of the Sun , and support the hypothesis that the Sun might cause a superflare .