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 (∼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 H α spectroscopy. We obtained an unprecedented sample of 15 H α 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 α 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) H α 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>16</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>25</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> %, yielding occurrence rates of 0.21 ±0.12 and &lt;0.32 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.32</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.46</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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 (≳10 33 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.