WDS'13 Proceedings of Contributed Papers, Part III, 13–18, 2013. ISBN 978-80-7378-252-8 © MATFYZPRESS Parameters of Flares on Star EV Lac in 2010–2011 K. Rieznik, V. M. Rechetnyk Taras Shevchenko National University of Kyiv, Faculty of Physics, Kyiv, Ukraine. Abstract. We present low-resolution spectroscopic observations of star EV Lac. Data were obtained during May, June, August 2010 and May 2011 using the 60-cm Cassegrain telescope at Terskol Peak (North Caucasus, 3100 m a.s.l.). Algorithms and special software were developed for comfortable and quickly processing a large array data because of the lack of suitable software. We processed and analyzed 14800 images with spectra in range from 330 nm to 900 nm with resolution R = 50–100. The temperature and the size of the flare were estimated. Introduction A large number of M dwarfs have been detected in the last few decades, and our knowledge of these faint stars has kept improving. EV Lac (= BD +43◦ 4305 = Gliese 873) is a well known active star with both high level flare and stellar spot activities. It has spectral type dM4.5e [Jenkins et al. 2009] and d = 5.1 persecs [Perryman et al. 1997]. Many efforts have been made so far to detect short-term variations in the quiet-state luminosity and/or the flare activity of the star. Pettersen [1980] showed a rotational modulation with a period of 4d.378 and an amplitude of 0m.07. Pettersen et al. [1983], based on continuous observations from 1979 to 1981, renewed the ephemerides of the variation as a peroid of 4d.375 and an amplitude of 0m.08. This indicates that the light curves of EV Lac were almost constant for 2.5 years because the spot groups on the star are stable during these 2.5 years. We used an optical observations to obtain the temperature and size of outburst in EV Lac. The optical variability of EV Lac is well known; Abdul-Aziz et al. [1995] detected 25 flares during 28 hr of five-color photometric monitoring. Kodaira et al. [1976] saw the flare on EV Lac with an increase of 5m.9 in U band. Observations Outbreaks of dwarf stars are an example of fast-running processes. Star’s brightness, the spectral composition of the radiation may very significant during fraction of minutes. Simul- taneous recording of light changes and the spectrum during the flare is a complex technical problem, because the flaring red dwarf is sufficiently weak objects, above tenth magnitude. Data were obtained at the Zeiss-600 telescope on peak Terskol (North Caucasus, 3100 m a.s.l.) with a slitless UBVR low-resolution grism spectrograph. The spectrograph has a resolution of R ≈ 100 in the vicinity of 4800 A,˚ the accuracy of the wavelength is ≈ 30 A.˚ The spectrograph provides a moderate signal to noise ratio for stars up to ≈ 16m. [Zhilyaev et al., 2013]. The example of data is shown in Figure 1. These frames are made during the period of May, June, August 2010 and May 2011. Totally been processed 14818 frames which totally include 31 hours of observations. Methods of estimating the physical characteristics The special software was developed for image processing which tested by MaxIm DL and a set of IDL subroutines of DAOPHOT aperture photometry. This software was developed for comfortable and quickly processing a large array of data. As a result of program’s work we receive a distribution intensity from wavelength, that is spectrum of star. In Figure 2 is shown the spectrum of EV Lac in the quiet state and in Figure 3 13 RIEZNIK AND RESHETNYK: OUTBURSTS ON EV LACERTAE Figure 1. An example of images which were obtained at the Zeiss-600 telescope with a slit- less UBVR low-resolution grism spectrograph. Arrow is pointed EV Lac, by the right side is spectrum of star. Figure 2. Spectrum of EV Lac in the quiet state. Figure 3. Spectrum of EV Lac during the flare 2010/08/09. is shown the spectrum during the flare 2010/08/09. As we can see the intensity of radiation in the violet and ultraviolet regions have increased. For finding radiant energy in different filters we used a transmission curves UBVRI filters of standard Johnson system (Figure 4) [Bessell, 1990]. The multiplication them with spectrum gives us an instrumental value of radiant flow. So then we have plotted the light curves of EV Lac with other stars in the field. That is shown in Figure 5, the curve of EV Lac is bottom and also there is an outburst. 14 RIEZNIK AND RESHETNYK: OUTBURSTS ON EV LACERTAE Figure 4. Transmission curves of filters UBVRI of standard Johnson system. Figure 5. Light curves in U filter, EV Lac is shown in bottom. There is an outburst 2010/08/09. Figure 6. Finding the outburst energy. Outburst temperature For finding the outburst temperature, at first, we have calculate the energy of flare Eflare. That is shown in Figure 6. Also we used the blackbody approximation, that means that in the maximum of flare it radiate like a blackbody. As we have radiant energy so we can calculate the color index (B − V ) of flash which allows us to estimate the temperature of outburst from formula [Chalenko, 1999]: 8540K B − V = −0.865 + (1) T 15 RIEZNIK AND RESHETNYK: OUTBURSTS ON EV LACERTAE Figure 7. The black-body radiation curves. Figure 8. Multiplication the blackbody spectrum with transmission function of filters. Size of outburst In order to estimate size of the active region we have plotted the black-body radiation curves with temperatures from 1000K to 30000K with step 100K (Figure 7). Than we calculate the flows of radiation for blackbody in different filters the same as for EV Lac (Figure 8) using the formula: ∞ 4 F filter(T ) = B (T )Φ(λ)dλ = (σT )filter (2) pl Z pl 0 where Bpl(T ) is the spectral radiance, Φ(λ) is the transfer function of filter, σ is the Stefan- Boltzmann constant and T is temperature. The next step is to calculate the luminosity of blackbody using formula: 2 4 4 L = 4πR σT = SσT , (3) where R is the radius of the object, σ is the StefanBoltzmann constant, T is temperature and S is the area of the star. Also to calculate the blackbody luminosity we have used the observational data: 2 L = 4πr Eobs (4) where r is the distance to the object, Eobs is an radiation energy of researched object. Equating two formulas and taking division between luminosity of outburst and luminosity of EV Lac in quite state in the same filters we obtain the formula which we used to calculate 16 RIEZNIK AND RESHETNYK: OUTBURSTS ON EV LACERTAE Figure 9. The light curves for first flare 2010/05/28. Magnitudes in filters U, B, and V from top to bottom. Figure 10. The light curves for second flare 2010/08/09. Magnitudes in filters U, B, and V from top to bottom. the size of flare: filter filter E S F (Tflash) obs,flash = flash pl (5) filter S filter Eobs,EV Lac EV Lac Fpl (TEVLac) Results and conclusions After analyzing all light curves we have found two outbursts in dates 2010/05/28 and 2010/08/09 [Zhilyaev et al., 2012] which are shown in Figures 9 and 10. That light curves are plotted for magnitudes in filters U, B, and V from top to bottom. With the methods described above we have estimated next characteristics: • For flare 2010/05/28: T ≈ 11000 K S flare ≈ 0.04 % SEV Lac • For flare 2010/08/09: T =≈ 13600 K S flare ≈ 0.2 % SEV Lac 17 RIEZNIK AND RESHETNYK: OUTBURSTS ON EV LACERTAE Acknowledgments. The authors thank the Laboratory for Transient Phenomena in Stars work- ing teams for the observational data. References Abdul-Aziz, H. et al., Coordinated observations of the red dwarf flare star EV Lacertae in 1992, A&AS, 114, 509, 1995. Bessell, M. S., UBVRI passbands, Publ. A. S. 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