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Astron. Astrophys. 356, 913-928 (2000)

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1. Introduction

1.1. Importance of systematic studies of individual Be stars

In spite of the huge effort of several generations of astronomers, the nature of the Be phenomenon remains unexplained. Also the character of amazing variations of virtually all Be stars has been only partly understood. It is clear that the observed changes of Be stars consist of overlapping variations on several different times scales and contain both, stochastic and periodic components. This, together with irregular observational coverage, makes the problem so complicated. In the period of photographic spectroscopy, observers were mainly attracted by the large, irregular long-term changes of the emission and shell lines of Be stars. More recently, there is a marked tendency to decompose the observed variations, especially those on time scales of days or shorter, into periodic terms. Interpretations in terms of non-radial pulsations, rotating surface structures and corotating structures above the stellar photosphere (i.e. in the inner parts of the Be-star envelopes) are the main, still competing hypotheses. Some types of medium-term variations were found to be connected with the binary nature of particular Be stars and the role of the duplicity for the whole phenomenon still deserves more dedicated studies. It is clear that one way how to deal with this complicated situation is to carry out systematic studies of individual Be stars and investigate as carefully as possible their variability patterns and their possible mutual relations. Only accumulation of such data for a statistically significant number of objects will allow some general conclusions to be made. With this in mind, we undertook a study of one bright Be star for which such a study has been missing, and report our findings here.

1.2. Observational history of 60 Cyg

For a long time, 60 Cyg (V1931 Cyg, HD 200310, HR 8053, MWC 360; B1 Ve, V=5[FORMULA]37 (var.), v sin i = 320 km s-1, according to the Bright Star Catalogue , Hoffleit & Jaschek, 1982) has been known as an emission-line star. Variable radial velocity (RV hereafter) was announced by Plaskett & Pearce (1931), who reported it as a spectroscopic binary with a velocity range of 57 km s-1 but without any information about the period. Harmanec et al. (1986) analysed their and other published RVs measurements (c.f., e.g., Abt & Biggs 1972) together with their own UBV photometry and tentatively reported the presence of a period of 2[FORMULA]48 in both data sets. The RV and light curves with this period led them to suspect that 60 Cyg is an eclipsing binary. Later, Percy et al. (1988, 1997) confirmed the presence of rapid light variations of 60 Cyg on a time scale of days. However, they were unable to confirm the 2[FORMULA]48 period. Recently, Hubert & Floquet (1998) analysed the Hipparcos photometry of 60 Cyg and concluded that the star is a rapid light variable, with a possible period of 0[FORMULA]3.

The available records of spectroscopic observations of 60 Cyg (Copeland & Heard 1963, Andrillat & Houziaux 1967, Hubert-Delplace & Hubert 1979 (Atlas hereafter), Fontaine et al. 1982, Slettebak 1982, Andrillat 1983 and Andrillat & Houziaux 1991) clearly indicate the presence of pronounced long-term variations of the Balmer emission lines.

60 Cyg was included among the targets of an international photometric campaign on bright Be stars (Harmanec et al. 1980, 1982) during which it was rather intensively observed at Hvar (Harmanec et al. 1986, Pavlovski et al. 1997, Harmanec et al. 1997), Toronto (Percy et al. 1988, 1997) and Xinglong Observatories. New series of UBV observations were secured in a close succession from San Pedro Mártir and Hvar during summer 1998. It was also put on spectroscopic observing programs at the Haute Provence Observatory (OHP), Ondejov Observatory and more recently also Dominion Astrophysical Observatory (DAO). A few high-quality blue spectra taken in 1992, 1993 and 1994 showed mainly changing asymmetries in the helium lines, thus giving a hint of line-profile variations. Rapid line-profile variations (lpv hereafter) of the He I 4471 Å line of 60 Cyg were discovered by Koubský et al. (1995).

Clearly, 60 Cyg is a good representative of Be stars, showing variability of its line and continuum spectrum on several time scales. It was, therefore, deemed useful to carry out a more systematic investigation of its spectral and light variability on various timescales and to search for possible periodic components of these changes. As already mentioned, a reliable identification of truly periodic terms and their possible mutual interaction could help to restrict the possible class of the model(s) of Be stars.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000