Photometric and Spectroscopic Observations of the Highly Magnetically Active Star HD 251108 with Starspot Mapping

The largest X-ray superflare (10^34 erg/s) has been detected on the RS CVn-type star candidate HD 251108. To clarify why this system is so magnetically active, what kind of binary it is, and what starspots it possesses, we conducted high-dispersion spectroscopic observations with 3.8 m SEIMEI Telescope/GAOES-RV. Consequently, the system was found to be tidally locked with a period of 21.01 days, and estimated to be composed of a companion with a lower mass (0.3 M_sun) than typical RS CVn stars (1 M_sun) and a K-giant featuring a giant starspot (maximum filling 53 %) at high latitude (>60°).