High-time-cadence spectroscopy of stellar flares on the M-dwarf YZ Canis Minoris

Solar and stellar flares are sudden brightenings across a wide wavelength range, from radio to X-ray. They are thought to be caused by rapid releases of magnetic energy via magnetic reconnection. For stellar flares on M-dwarfs, the optical flare continuum shows a blackbody-like spectral energy distribution with an effective temperature of 8000-10000 K (e.g., Hawley and Fisher 1992). Recent studies suggest that the effective temperature of the flare continuum peaks at the flare peak and then decreases during the decay phase (e.g., Howard et al., 2020; Ichihara et al., 2025). We conducted time-series spectroscopy of the M-dwarf YZ CMi at a time cadence of 2.0-2.5 sec using TriCCS mounted on the 3.8-m Seimei telescope at Okayama, Japan, to investigate detailed temporal changes in the spectra during flares. We detected two flares with different temporal profiles: one (flare A) shows a rapid rise and rapid decay with a time scale of <10 seconds, and the other (flare B) shows an initial rapid rise and decay, followed by a long-lasting (~50 seconds) secondary peak and gradual decay. From the observed time-resolved flare spectra, we found that the color temperatures of the flare continuum at the initial flare peak were ~10000 K for both flares. In flare A, the temperature decreased rapidly as the flare decayed. For flare B, the temperature lasted ~8000 K during the secondary peak, then gradually decreased. We also found that the H-alpha/H-gamma ratio reached a minimum at the initial flare peak, then increased as the flare decayed in the case of flare A. For flare B, the H-alpha/H-gamma ratio reached a minimum, remained roughly constant during the initial peak and the long-lasting secondary peak, and then gradually increased. These results suggest that energy deposition and heating due to magnetic reconnection might persist throughout the initial and secondary peaks in the long-duration flare (flare B).