Do Young Suns Produce Frequent, Massive CMEs? Results from Five-year Dedicated Optical Observations of EK Draconis and V889 Hercules

We report results from a five-year (132-night) dedicated observational campaign targeting two nearby young solar-type stars, EK Draconis (∼50–125 Myr age) and V889 Hercules (∼30 Myr age), using the 3.8 m Seimei Telescope and Transiting Exoplanet Survey Satellite. The aim is to observationally constrain statistical properties of flaring radiation/heating, as well as coronal mass ejections (CMEs), through high time-cadence Hα spectroscopy. We obtained an unprecedented sample of 15 Hα superflares, including two blueshifted absorption, two blueshifted emission, one redshifted emission, and nine line broadening events. We obtain the following results: (1) larger flares exhibit broader Hα line widths, up to 14.1±2.4 Å, indicating higher chromospheric heating than solar flares; (2) the long-lasting redshifted event at ∼100 km s−1 may indicate dense postflare loops; (3) Hα blueshifted absorptions/emissions provide evidence of massive filament/prominence eruptions, the core structures of CMEs. One newly identified event showed an unexpected rapid decrease in velocity; (4) the lower limit of the CME/eruption association rate with superflares is 27 −16+25 %, yielding occurrence rates of 0.21±0.12 and <0.32 −0.32+0.46 events per day for EK Draconis and V889 Hercules, respectively; and (5) we derived the first direct estimate of the lower limit of the mass-loss rate driven by super-CMEs (≳1033 erg) for EK Dra as 4 × (10−13–10−12) M⊙ yr−1, comparable to the stellar wind mass loss at a similar age. This study provides critical observational constraints on the radiation and plasma environment around young solar-type stars and the early Sun, which can drive planetary space weather and stellar mass/angular momentum loss.