A Superflare on YZ Canis Minoris Observed by Seimei Telescope and TESS: Red Asymmetry of H$\alpha$ Emission Associated with White-Light Emission

TL;DR

This study presents simultaneous spectroscopic and photometric observations of a superflare on the active M dwarf YZ CMi, revealing red asymmetry in Hα emission linked to white-light emission and providing insights into flare dynamics and chromospheric processes.

Contribution

First detailed simultaneous optical spectroscopic and photometric analysis of a superflare on an M dwarf, linking Hα red asymmetry with white-light emission and flare evolution.

Findings

Hα red asymmetry persisted for 4.6-5.1 hours with velocities of 200-500 km/s.

Hα line broadening and red asymmetry correlated with white-light emission.

Red asymmetry likely originates from dense chromospheric condensation heated by non-thermal electrons.

Abstract

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 $1.3^{+1.6}_{-0.6} \times 10^{34} \,\rm{erg}$ and H$\alpha$ energy was $3.0^{+0.1}_{-0.1} \times 10^{32} \,\rm{erg}$. The H$\alpha$ emission line profile showed red asymmetry throughout the flare with a duration of $4.6-5.1 \,\rm{hrs}$. The velocity of the red asymmetry was $\sim 200-500 \,\rm{km\,s^{-1}}$ and line width of H$\alpha$ was broadened up to $34 \pm 14$ $\r{A}$. 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.