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

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 × 1034 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 Hα 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 Hα 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 Hα and white light.

Sun-as-a-star Analysis of Hα Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening

“…The total radiated energies of the Hα and white-light flare is 2.8 × 10 28 erg and 1.6 × 10 30 erg, respectively. The ratio of Hα to white-light energy ∼1.8% are similar to those of stellar superflares on solar-type stars (Namekata et al 2022a(Namekata et al , 2022b. See Section 3.1.…”

“…Since few studies have measured the energy of white light and Hα in solar flares, such values are worth comparing with those of stellar flares (e.g., Osten & Wolk 2015;Guarcello et al 2019). For example, the ratio of Hα to white-light energy ∼1.8% are comparable to those of stellar superflares on solar-type stars (0.85 ∼ 1.5%; Namekata et al 2022aNamekata et al , 2022b.…”

“…Since a wide variety of Hα spectral changes of stellar flares have been reported in recent years, it is necessary to perform a Sun-as-a-star analysis of the Hα line on a wider variety of solar phenomena in order to connect the solar observations to stellar observations. Also, Hα stellar flares are often observed simultaneously with other wavelengths, such as white-light emission with TESS observations (e.g., Maehara et al 2021;Namekata et al 2022aNamekata et al , 2022b, so simultaneous Sun-as-a-star observations with other wavelengths (e.g., white light, ultraviolet, and X-ray) can provide key information for further solar-stellar connection. In this paper, we report a Sun-as-a-star analysis of Hα spectra of an M4.2-class solar flare that occurred at 01:05 UT on 2017 September 5.…”

“…It has a unique ability to take full disk solar images at every 0.25 Å from the Hα line center −9.0 Å to the Hα line center +9.0 Å with 12-20 seconds time cadence, enabling us to make the Sun-as-a-star Hα spectrum. By using the data of SDDI, our previous study performed the Sun-as-a-star analysis of two C-class solar flares associated with a filament eruption and a surge (Namekata et al 2022a). The Sun-as-a-star spectra shows a blueshifted absorption due to the eruptive filaments with gradually decaying velocity.…”

“…Moreover, it showed strong redshift spectra and white-light emission; therefore it is a very good target to investigate the behavior of flare ribbons in relation with other wavelengths in the Sun-as-a-star view. Our aims of this study are the following three points: (1) to reformulate the Sun-as-a-star analysis method of solar flares using SMART/SDDI data in more detail than Namekata et al (2022a), (2) to investigate the nature of red asymmetry and line broadenings of the Sun-as-a-star Hα spectrum in comparison with other wavelengths, and (3) to investigate the relation of the Hα spectra of a solar flare in the Sun-as-a-star view and those in the spatially-resolved view. We introduce the dataset and Sun-as-a-star analysis method in Section 2, show the results in Section 3, discuss the results in Section 4, and describe summary and future work in Section 5.…”