Discovery of a Long-duration Superflare on a Young Solar-type Star EK Draconis with Nearly Similar Time Evolution for Hα and White-light Emissions
Kosuke Namekata, Hiroyuki Maehara, Satoshi Honda, Yuta Notsu, Soshi Okamoto, Jun Takahashi, Masaki Takayama, Tomohito Ohshima, Tomoki Saito, Noriyuki Katoh, Miyako Tozuka, Katsuhiro L. Murata, F. Ogawa, Masafumi Niwano, R. Adachi, Motoki Oeda, K. Shiraishi, Keisuke Isogai, Daisaku Nogami, Kazunari Shibata
Abstract
Abstract Young solar-type stars are known to show frequent “superflares,” which may severely influence the habitable worlds on young planets via intense radiation 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. The flare energy 2.6 × 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 α 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. Also, it did not show significant line broadening, indicating that the nonthermal heating at the flare footpoints is not essential or that the footpoints are behind the limb. The time evolution and duration of the H α 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, such as: (1) radiation from (off-limb) flare loops and (2) re-radiation via radiative back-warming, in both of which the cooling timescales of flare loops could determine the timescales of H α and white light.