High-velocity blueshifts of Hα and Fe XXV Heα lines during superflares of the RS CVn-type stars

In December 2020, we conducted simultaneous optical spectroscopic and photometric observations of the RS CVn-type star V1355 Ori with Seimei and TESS. In July 2023, we performed X-ray observations of the RS CVn-type star IM Peg with NICER. Then, we found a blueshifted excess component of Hα extending its velocity up to 760−1690 km s −1 during a superflare of V1355 Ori and a blueshift of the line center of the Fe XXV Heα line with its maximum Doppler velocity reaching 2200±600 km s −1 during a superflare of IM Peg. These results suggest an upward-moving plasma during the flares, such as a prominence eruption, coronal mass ejection (CME) and chromospheric evaporation. The high-velocity blueshifts, which overwhelmingly exceed the escape velocity of the stars, will help elucidate whether such events follow existing theories and scaling laws on solar flares and CMEs even when the energy scale far exceeds solar cases.