However, for solar-type stars, optical spectra of superflares have never been obtained.ĮK Draconis (EK Dra) is known to be an active young solar-type star (a G-type, zero-age main-sequence star with an effective temperature of 5,560–5,700 K and age of 50–125 Myr ref. Optical spectroscopic observations are a promising way to detect stellar filament eruptions, which can be indirect evidence of CMEs. Therefore, no observational indication of filament eruptions/CMEs has been reported for solar-type stars. However, superflares on solar-type stars have been mainly detected by optical photometry (for example, the Kepler space telescope) 3. Superflares may produce much larger CMEs than the largest solar flares, which can severely affect the environment, habitability and development of life around young and intermediate-age stars 6.
Recently, it has been discussed that much larger ‘superflares’ that release the energy of more than 10 33 erg (ten times the largest solar flares, ~10 32 erg) can occur-or have occurred relatively recently-even on the Sun 3, 4, 5, 8. Magnetic reconnection is a key energy release mechanism for flares, which are thought to be sometimes triggered by the instability of cool filaments in active regions 1. Solar flares, filament eruptions and coronal mass ejections (CMEs) are thought to be caused by a common magnetohydrodynamic process, though not all of them are necessarily observed in the same event. The massive filament eruption and an associated coronal mass ejection provide the opportunity to evaluate how they affect the environment of young exoplanets/the young Earth 6 and stellar mass/angular momentum evolution 7. The erupted filament mass of 1.1 × 10 18 g is ten times larger than those of the largest solar coronal mass ejections. Comparing this eruption with solar filament eruptions in terms of the length scale and velocity strongly suggests that a stellar coronal mass ejection occurred. The temporal changes in the spectra strongly resemble those of solar filament eruptions. This superflare emitted a radiated energy of 2.0 × 10 33 erg, and a blueshifted hydrogen absorption component with a high velocity of −510 km s −1 was observed shortly afterwards. Here we show that our optical spectroscopic observation of the young solar-type star EK Draconis reveals evidence for a stellar filament eruption associated with a superflare.
‘Superflares’ are found on some active solar-type (G-type main-sequence) stars 3, 4, 5, but the filament eruption–coronal mass ejection association has not been established.
Solar flares are often accompanied by filament/prominence eruptions (~10 4 K and ~10 10−11 cm −3), sometimes leading to coronal mass ejections that directly affect the Earth’s environment 1, 2.
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