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

Abstract: We have performed 5 night spectroscopic observation of the Halpha line of EV Lac with a medium wavelength resolution (R~ 10,000) using the 2m Nayuta telescope at the Nishi-Harima Astronomical Observatory. EV Lac always possesses the Halpha emission line; however, its intensity was stronger on August 15, 2015 than during other four-night periods. On this night, we observed a rapid rise (~ 20min) and a subsequent slow decrease (~ 1.5h) of the emission-line intensity of Halpha, which was probably caused by a flare. We also found an asymmetrical change in the Halpha line on the same night. The enhancement has been observed in the blue wing of the Ha 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 Halpha 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 the understanding of the dynamics of stellar flares.