[P-EM14] Frontiers in solar physics

[PEM14-03] Radiation Mechanism of a Long-duration Superflare on a Young Solar-type Star Observed by Seimei Telescope and TESS

*行方 宏介¹、前原 裕之¹、本田 敏志²、野津 湧太³、野上 大作⁴、柴田 一成⁴ (1.国立天文台、2.兵庫県立大学、3.東京工業大学、4.京都大学)

キーワード：恒星フレア、スーパーフレア、恒星フィラメント噴出

Young solar-type stars are known to show frequent “superflares” (the flare energy of > 10³³ erg). The superflares on young stars is important not only because it may severely influence the habitable worlds on young planets via intense radiation and coronal mass ejections (CMEs), but also because it can provide a proxy of a possible extreme superflare event on our Sun. Superlfares on solar-type stars have been well studied with optical photometry by Kepler and TESS, but the radiation mechanism and its association of stellar CMEs have been unknown so far because of the lack of the spectroscopic observations. EK Draconis is known as a “young Sun” (an age of 50–125 Myr) having a Sun-like atmosphere with an effective temperature of 5560–5700 K and radius of 0.94 R_Sun, and is also known to produce frequent flares. In this presentation, we report an optical spectroscopic and photometric observation of a long-duration superflare on EK Draconis with the Seimei telescope and TESS (Namekata et al. 2022, ApJL). The detected 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 (see Figure A-2), unlike the only previous detection of a superflare on this star (Namekata et al. 2021, Nature Astronomy; and Namekata et al. 2021, JpGU P-EM Session). The lack of a filament eruption signature can be explained if we assume that the superflare occurred around the stellar limb, but may indicate that mass ejection events do not always happen for every superflare, as not all flares are accompanied by CMEs in the case of the Sun. Also, it did not show significant line broadening (see Figure A-2), 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 (see Figure A-1), 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. Interestingly, the energy of the observed superflare in this work is comparable to the estimated upper limit of flare energy on old Sun-like stars and the present-day Sun (∼4×10³⁴ erg; Okamoto et al. 2021), so the revealed properties in this work could be helpful to model the chromospheric radiations from an extreme event on our Sun.