Simultaneous H alpha and photometric observations of P Cygni

For the first time an extensive set of (quasi-) simultaneous photometric (U BV) and spectroscopic (H alpha line profiles) observations of P Cygni, cove ring a period from May, 1990 to June. 1994 was analyzed in terms of time va riability. It is found that the H alpha equivalent width (EW) exhibits two different patterns of variability: a slower one, called Long-Term (LT) vari ability, with an amplitude of about 30 Angstrom and a characteristic durati on of about 600 days and a faster one, called Short-Term (ST) variability, with an amplitude up to 10 Angstrom and duration of 40 to 60 days. Suggesti ve evidence for EW variation on a longer time scale (about few years) also exists. The variations in the H alpha luminosity are not solely due to chan ges in the underlying continuum but also reflect variations in the physical properties of the wind. We find, in terms of a simplified spherically-symm etric wind model, that the LT variation of the line can be successfully exp lained in terms of a 26% alteration of the mass-loss rate, possibly accompa nied by variations in the velocity field. From the analysis of the photomet ric behaviour of the star we find evidence for a very slow Variation in the stellar brightness with an amplitude of about 0.13 mag and a duration of a bout 2600 days, i.e. about 7 years. During this variation, i.e, when the st ar brightens, the effective temperature decreases (by about 10%) and the ra dius increases (by about 7%). The properties of this Very Long Term (VLT) v ariation suggest that P Cygni has probably experienced a normal S Dot-type variation with a minimum phase around 1988 and a maximum phase in 1992. Som e hints for a positive correlation between mass loss variations and changes in the stellar radius, due to the normal SD variability, do exist implying that the behaviour of P Cygni is more likely similar to that of R71 and S Dor but different from e.g. AG Car, R127 and HD 160529. Superimposed on the VLT component in the photometric variability of P Cygni, we observe ST bri ghtness variations with an amplitude between 0.1 and 0.2 mag which appears to recur on a time scale of three to four months. The colour behaviour of t hese microvariations, at least of those which appear near the maximum phase of the VLT variation, is redder in B - V and bluer in U - B when the star brightens in V. The properties of this ST photometric variability are simil ar to the properties of the so-called "100 d-type micro-variations": recogn ized in other LBVs by van Genderen et al. (1997a,b). Based on time-scale ev idences we suggest that the microvariabilities observed are rather due to " relaxation oscillations" (Stothers & Chin 1995) than to strange-mode oscill ations in the stellar interior. Evidence for a close relationship between S T Variations in Ha and changes in the stellar brightness and temperature is found. From other results about P Cygni's spectral variations (Markova 200 0a), we conclude that the ST variability of the wind is most likely connect ed with processes in the stellar photosphere.