COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3308 Konkoly Observatory Budapest 17 March 1989 HU ISSN 0374 - 0676 ON THE VARIABILITY OF VEGA At present the question on the photometric stability of the radiation of Vega remains open. Investigators give contradictory results. For example, Johnson (1980), analysing the series of photometric observations, came to the conclusion that the V brightness changes with time in the range of +-0.03m. However, Kozyrev et al. (1981) on the basis of measurements in B and V filters claim that the brightness of Vega is constant. The most numerous spectrophotometric results are obtained in the visible spectral region and particularly at the wavelength of 5556 A. In this spectral region the allowance for the atmospheric radiation attenuation is made the most correctly, and the estimated errors sigma of the value of the spectral irradiance density E_lambda are of the order of 1-2%. Figure 1 shows the values of the spectral irradiance density E_lambda produced by the emission of Vega, at the wavelength lambda=5556 A at the outer limit of the Earth's atmosphere, published from 1960 to 1987 by Code (1) (1963); Wilstrop (2) (1965); Kharitonov et al. (3) (1967); Oke et al. (4) (1970); Hayes et al. (5) (1975); Terez et al. (6) (1976); Terez et al. (7) (1979); Mal'zev et al. (8) (1979); Tüg et al. (9) (1977); Boiko et al. (10) (1979); Arkharov (11) (1985); Vasil'yev et al. (12) (1988), respectively. A number near each point is a reference to the paper from which the value of E_5556 is taken. The errors sigma are assumed to be +-1.5%. The difference between the highest and lowest values of E_5556 is 0.34*10^-9 erg*cm^-2*s^-1*A^-1 or 0.10m on the magnitude scale that makes an interval of 5-6 sigma width which cannot be accounted for by pure photometric measurement errors. In the first approximation the data can be fitted by a cosine curve with a period of about 23 years (see Figure 1). The maximum deviation is 5% of an average 3.52*10^-9 erg*cm^-2*s^-1*A^-1 value. There is little information after 1983 but the available data show a tendency of reducing the brightness of the star. In our opinion, the resulting curve is of interest. The continuum variability of Vega either characterizes the intrinsic stellar variability or is due to the effect of systematic measurement errors not allowed for in using reference radiators. It is worthwhile to notice that there are some other data showing instability in Vega's radiation (Johnson and Wisniewski, 1979). [FIGURE 1] The interpretation of the curve shown in Figure 1 is given in the paper by Vasil'yev et al.(1988). I.A. VASIL'YEV, V.P. MEREZHIN, V.N. NALIMOV, V.A. NOVOSYOLOV Department of Astronomy, Kazan State University Kazan 8, Lenin Street 18, USSR References: Arkharov, A. 1985, Thesis, Leningrad. Boiko, P.I., Knyazeva, L.N., Mal'zev, V.V., Samoilov, L.N., Saprizkii, V.I., Tereshchenko, V.M., Kharitonov, A.V. 1979, Photometry and its Metrological Provision', No. 3, 227. Code, A. 1963, in "Stellar Atmospheres", ed. J.L. Greenstein, Moscow, IIL, p.127. Hayes, D., Latham, D. 1975, Astrophys. J., 197, 593. [BIBCODE 1975ApJ...197..593H ] Johnson, H.L. 1980, Rev. Mexicana Astron. Astrophys., 5, 25. [BIBCODE 1973BAAS....5...25J ] Johnson, H.L., Wisniewski, W.Z. 1979, Sky and Telescope, 57, 4. [BIBCODE 1979S&T....57....4W ] Kharitonov, A.V., Nelyubin, N.F. 1967, Proc. Astrophys. Inst. Acad. Sci. Kazakh SSR, 8, 73. Kozyrev, V.S., Moshkalyov, V.G., Khaliullin, Kh.V. 1981, Sov. Astron.J., 58, 1242. Mal'zev, V.V., Novopashennyi, V.B., Samoilov, L.N., Terez, E.I. 1979, "Metrological Provision of Energy Photometry of Non-coherent Radiation", Proc. VNIIFTRI, p.36. Oke, J.B., Shild, R.E. 1970, Astrophys. J., 161, 1015. [BIBCODE 1970ApJ...161.1015O ] Terez, G.A., Terez, E.I. 1976, Astron. Circ. No. 918. [BIBCODE 1976ATsir.918....2T ] Terez, G.A., Terez, E.I. 1979, Sov. Astron. J., 56, 632. Tug, H., White, N.M., Lockwood, J.W. 1977, Astron. Astrophys., 61, 679. [BIBCODE 1977A&A....61..679T ] Vasil'yev, I.A., Merezhin, V.P., Nalimov, V.N., Novosyolov, V.A. 1988, Tr. astron. obs. Kazan State Univ., 52, in the press. Wilstrop, R.V., 1965, Mem. Royal Astron. Soc., 69, 83. [BIBCODE 1965MmRAS..69...83W ]