Detection of mass ejection from a superflare on a solar-type star

Solar and stellar flares are caused by the sudden release of magnetic energy on the surfaces. In the case of the Sun, mass ejections often accompany solar flares and affect the Earth’s environment. Active solar-type stars (G-type main-sequence stars) sometimes show larger `superflares' (Maehara et al. 2012) that may cause more huge mass ejections than those of solar flares. The stellar mass ejections can greatly affect the exoplanet habitability and the stellar mass evolution (e.g. Airapetian et al. 2020). However, no observational indication of mass ejection has been reported especially for solar-type stars.

We conducted spectroscopic monitoring observations of the active young solar analog EK Dra (a famous zero-age main-sequence G-dwarf) by our new 3.8-m Seimei telescope, simultaneously with TESS photometry. Our time-resolved optical spectroscopic observation shows clear evidence for a stellar mass ejection associated with a superflare on the solar-type star (Namekata et al. submitted). After the superflare brightening with the radiated energy of 2.0×10³³ erg observed by TESS, a blue-shifted H-alpha absorption component with a velocity of -510 km s^-1 appeared. The velocity gradually decayed in 2 hours and the deceleration 0.34 km s^-2 was consistent with the surface gravity on EK Dra (0.30 ± 0.05 km s^-2). The temporal changes in the spectra greatly resemble that of a solar mass ejection observed by the SMART telescope at Hida observatory. Moreover, the ejected mass of 1.1×10¹⁸ g roughly corresponds to those predicted from solar flare-energy/ejected-mass relation. These discoveries imply that a huge stellar mass ejection occurs possibly in the same way as solar ones. Our high-quality dataset can be helpful for future studies to estimate its impacts on the young-planet atmosphere around young solar-type stars as well as stellar mass/angular momentum evolution.