Time-resolved spectroscopic observations of an M-dwarf flare star EV Lacertae during a flare

摘要

We have performed five night spectroscopic observations of the Hα line of EV?Lac with a medium wavelength resolution (R?～?10000) using the 2?m Nayuta telescope at the Nishi-Harima Astronomical Observatory. EV?Lac always possesses the Hα emission line; however, its intensity was stronger on 2015 August?15 than during the other four night periods. On this night, we observed a rapid rise (～20?min) and a subsequent slow decrease (～1.5?hr) of the emission-line intensity of Hα, which was probably caused by a flare. We also found an asymmetrical change in the Hα line on the same night. The enhancement has been observed in the blue wing of the Hα line during each phase of this flare (from the flare start to the flare end), and absorption components were present in its red wing during the early and later phases of the flare. Such blue enhancement (blue asymmetry) of the Hα line is sometimes seen during solar flares, but only during the early phases. Even for solar flares, little is known about the origin of the blue asymmetry. Compared with solar flare models, the presented results can lead to better understanding of the dynamics of stellar flares.