Coronal Mass Ejections from Superflares on Solar-Type Stars

The growing number of exoplanet discoveries has emphasized the importance of stellar magnetic activity in shaping exoplanetary atmospheres. Kepler and TESS have revealed that some exoplanet-hosting stars produce frequent flares, highlighting flare-related XUV radiation and particles as potential key drivers of atmospheric chemistry and escape. Not only M dwarfs but also solar-type stars are capable of producing superflares, giving an indication of extreme solar activity in the past. However, much less is known about whether coronal mass ejections (CMEs)—a major factor of star–planet interaction—accompany these events, especially for solar-type stars. Motivated by this gap, over the last six years we have pursued spectroscopic searches for CMEs, primarily using time-resolved Hα spectroscopy with the 3.8-m Seimei telescope. From extensive monitoring, we discovered the first evidence of gigantic filament eruptions, interpreted as the initial phase of CMEs, associated with superflares on young solar-type stars, supported by comparisons with Sun-as-a-star observations and solar models. These observations demonstrate that superflare-related eruptions can be far more massive and energetic than the largest solar events. By accumulating multiple events, we have constrained the characteristic frequency, mass, and velocity scales of CMEs, which are critical for evaluating their impact on young exoplanetary systems. Recent studies on M dwarfs and close binaries have also reported CME-related signatures using different wavelength diagnostics, and together these efforts are opening a broader stellar-mass perspective. We have recently extended these to multi-wavelength observations and identified simultaneous signatures in Hα, X-ray, and far-UV, marking a transition from detection toward physical characterization of stellar eruptions. I will also discuss future prospects for advancing stellar CME studies through coordinated observations.