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

Active M-type stars are known to often produce superflares on the surface. Radiation from stellar (super-)flares is important for the 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 CMi with the 3.8-m Seimei telescope and the Transiting Exoplanet Survey Satellite. The flare bolometric energy was 1.3(-0.6)(+1.6)x10(34)erg and H alpha energy was 3.0(-0.1)(+0.1)x10(32)erg. The H alpha emission line profile showed red asymmetry throughout the flare with a duration of 4.6-5.1hrs. The velocity of the red asymmetry was similar to 200-500kms(-1) and line width of H alpha was broadened up to 34 +/- 14 & Aring;. The redshifted velocity and line width of H alpha line decayed more rapidly than the equivalent width, and their time evolutions are correlated with that of the white-light emission. This indicates a possibility that the white light, H alpha red asymmetry, and H alpha line broadening originate from nearly the same site, i.e., the dense chromospheric condensation region heated by non-thermal electrons. On the other hand, the flux ratio of the redshifted excess components to the central components is enhanced one hour after the flare onset. This may be due to the change of the main source of the red asymmetry to the post-flare loops in the later phase of the flare.

Automated summary

This paper reports simultaneous optical spectroscopic and photometric observations of a stellar superflare on an active M dwarf YZ CMi. The observations reveal that the energy of the flare originates from the dense chromospheric condensation region heated by non-thermal electrons, and the evolution of the redshifted velocity and line width of H alpha is correlated with that of the white-light emission.