2025ApJ...979...93K

M dwarfs show frequent flares and associated coronal mass ejections (CMEs) may significantly impact close-in habitable planets. M dwarf flares sometimes show blue/red asymmetries in the Hα line profile, suggesting prominence eruptions as an early stage of CMEs. However, their high-time-cadence observations are limited. We conducted spectroscopic monitoring observations of the active M dwarf YZ Canis Minoris with an ∼1 minute time cadence using the Seimei telescope, simultaneously with the optical photometric observations by Transiting Exoplanet Survey Satellite. We detected 27 Hα flares with Hα energies ranging from 1.7 × 10²⁹ to 3.8 × 10³² erg and durations from 8 to 319 minutes. Among them, we identified three blue asymmetry and five red asymmetry events based on criteria using the Bayesian information criterion. The maximum velocity of the blueshifted and redshifted components ranges from 200 to 450 km s^–1 and 190 to 400 km s^–1, respectively. The duration and time evolution show variety, and in particular, we discovered rapid, short-duration blue/red asymmetry events with the duration of 6-8 minutes. Among the eight blue/red asymmetry events, two blue and one red asymmetry events are interpreted as prominence eruptions because of their fast velocity and time evolution. Based on this interpretation, the lower limit of occurrence frequency of prominence eruptions can be estimated to be ∼1.1 events per day. Our discovery of short-duration events suggests that previous studies with low time cadence may have missed these events, potentially leading to an underestimation of the occurrence frequency of prominence eruptions/CMEs.