Discovery of a Long-Duration Superflare on a Young Solar-Type Star EK Draconis with Nearly Similar Time Evolution for H$\alpha$ and White-Light Emissions

TL;DR

This study reports the detection of the largest optical superflare on a young solar-type star EK Draconis, with nearly identical time evolution for H-alpha and white-light emissions, suggesting different radiation mechanisms from solar flares.

Contribution

First detailed optical spectroscopic and photometric observation of a long-duration superflare on EK Draconis, revealing unique emission characteristics and challenging existing flare models.

Findings

Superflare energy of 2.6×10^34 erg and duration of 2.2 hours.

H-alpha emission shows no significant line asymmetry or broadening.

H-alpha and white-light flare evolutions are nearly identical, unlike typical stellar flares.

Abstract

Young solar-type stars are known to show frequent "superflares", which may severely influence the habitable worlds on young planets via intense radiations and coronal mass ejections. Here we report an optical spectroscopic and photometric observation of a long-duration superflare on the young solar-type star EK Draconis (50-120 Myr age) with the Seimei telescope and $Transiting$ $Exoplanet$ $Survey$ $Satellite$ ($TESS$). The flare energy 2.6$\times$10$^{34}$ erg and white-light flare duration 2.2 hr are much larger than those of the largest solar flares, and this is the largest superflare on a solar-type star ever detected by optical spectroscopy. The H$\alpha$ emission profile shows no significant line asymmetry, meaning no signature of a filament eruption, unlike the only previous detection of a superflare on this star (Namekata et al. 2021, $Nat.Astron$). Also, it did not show significant line broadening, indicating that the non-thermal heating at the flare footpoints are not essential or that the footpoints are behind the limb. The time evolution and duration of the H$\alpha$ flare are surprisingly almost the same as those of the white-light flare, which is different from general M-dwarf (super-)flares and solar flares. This unexpected time evolution may suggest that different radiation mechanisms than general solar flares are predominant, as follows: (1) radiation from (off-limb) flare loops, and (2) re-radiation via radiative backwarming, in both of which the cooling timescales of flare loops could determine the timescales of H$\alpha$ and white light.