Fast spectrophotometry of the flare star EV lacertae

Abstract

We present the results of fast spectrophotometry for two flares on EV Lac with a time resolution of 8 s and a spectroscopic resolution R ∼ 100. The observations were performed in May and August 2010 with a slitless spectrograph on the Zeiss-600 telescope at Peak Terskol. We have estimated the UBVR magnitudes from spectrograms through a mathematical convolution of the spectra with the filter transmission curves. The UBV R amplitudes of the August 10, 2010 flare are \(2\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 83\), \(1\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 94\), \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 82\), and \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 28\), respectively. The UBV R amplitudes of the May 30, 2010 flare are \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 65\), \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 25\), \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 15\), and \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 10\), respectively. A detailed colorimetric analysis has allowed important parameters of the flares on EV Lac to be estimated: the temperatures at maximum light and their sizes. The color-color (U − B)−(B − V ) diagrams confirm that both flares at maximum light radiate as a blackbody. The temperatures at maximum light was 13 400 ± 500 K for the August flare and 5700 ± 100 K for the May flare. During the May flare, an additional hydrogen emission appeared in the Balmer Hα, Hβ, Hγ, Hδ, Hɛ, Hζ lines and the Balmer continuum (3647 A) in the spectrum of EV Lac. The excess of emission in the Balmer lines was approximately from two to thirty percent. Based on our colorimetric analysis, we have estimated the linear sizes of the flares at maximum light. The size of the May 30, 2010 flare is approximately 7% of the stellar radius. The size of the August 10, 2010 flare is 3.9% of the stellar radius.