A Superflare on YZ Canis Minoris Observed by the Seimei Telescope and TESS: Red Asymmetry of Hα Emission Associated with White-light Emission

Abstract

Active M-type stars are known to often produce superflares on the surface. Radiation from stellar (super)flares is important for exoplanet habitability, but the mechanisms are not well understood. In this paper, we report simultaneous optical spectroscopic and photometric observations of a stellar superflare on an active M dwarf, YZ Canis Minoris, with the 3.8 m Seimei telescope and the Transiting Exoplanet Survey Satellite. The flare bolometric energy is ${1.3}_{-0.6}^{+1.6}times {10}^{34}, mathrm{erg}$ and the Hα energy is ${3.0}_{-0.1}^{+0.1}times {10}^{32}, mathrm{erg}$. The Hα emission line profile shows red asymmetry throughout the flare, with a duration of 4.6–5.1 hr. The velocity of the red asymmetry is ∼200–500 km s–1 and the line width of Hα broadens up to 34 ± 14 Å. The redshifted velocity and line width of Hα line decay more rapidly than the equivalent width, and their time evolutions are correlated with that of the white-light emission. This indicates the possibility of the white light, the Hα red asymmetry, and the Hα line broadening originating from nearly the same site, i.e., the dense chromospheric condensation region, heated by nonthermal electrons. On the other hand, the flux ratio of the redshifted excess components to the central components is enhanced one hr after the flare's onset. This may be due to the main source of the red asymmetry changing to post-flare loops in the later phase of the flare.