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

Abstract

We report results from a five-year (132-night) dedicated observational campaign targeting two nearby young solar-type stars, EK Draconis ($\sim$50-125 Myr age) and V889 Hercules ($\sim$30 Myr age), using the 3.8m Seimei Telescope and Transiting Exoplanet Survey Satellite. The aim is to observationally constrain statistical properties of flaring radiation/heating as well as coronal mass ejections (CMEs), through high time-cadence H$\alpha$ spectroscopy. We obtained an unprecedented sample of 15 H$\alpha$ 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 H$\alpha$ line widths, up to 14.1$_{\pm 2.4}$ {\AA}, indicating higher chromospheric heating than solar flares. (2) The long-lasting redshifted event at $\sim$100 km s$^{-1}$ may indicate dense post-flare loops. (3) H$\alpha$ 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$_{\pm0.12}$ and $<$0.32$^{+0.46}_{-0.32}$ events per day for EK Draconis and V889 Hercules, respectively. (5) We derived the first direct estimate of the lower limit of the mass-loss rate driven by super-CMEs ($\gtrsim10^{33}$ erg) for EK Dra as $4 \times (10^{-13}$-$10^{-12})$ $M_{\odot}$ 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.