2023ApJ...948....9I

Stellar coronal mass ejections (CMEs) have recently received much attention for their impacts on exoplanets and stellar evolution. Detecting prominence eruptions, the initial phase of CMEs, as the blueshifted excess component of Balmer lines is a technique to capture stellar CMEs. However, most of prominence eruptions identified thus far have been slow and less than the surface escape velocity. Therefore, whether these eruptions were developing into CMEs remained unknown. In this study, we conducted simultaneous optical photometric observations with Transiting Exoplanet Survey Satellite and optical spectroscopic observations with the 3.8 m Seimei Telescope for the RS CVn-type star V1355 Orionis that frequently produces large-scale superflares. We detected a superflare releasing 7.0 × 10³⁵ erg. In the early stage of this flare, a blueshifted excess component of Hα extending its velocity up to 760-1690 km s^–1 was observed and thought to originate from prominence eruptions. The velocity greatly exceeds the escape velocity (i.e., ∼350 km s^–1), which provides important evidence that stellar prominence eruptions can develop into CMEs. Furthermore, we found that the prominence is very massive (9.5 × 10¹⁸ g < M < 1.4 × 10²¹ g). These data will clarify whether such events follow existing theories and scaling laws on solar flares and CMEs even when the energy scale far exceeds solar cases.