Simultaneous H$\mathsf{\alpha}$ and photometric observations of P Cygni

Abstract

For the first time an extensive set of (quasi-) simultaneous photometric ( UBV ) and spectroscopic (H α line profiles) observations of P Cygni, covering a period from May, 1990 to June, 1994 was analyzed in terms of time variability. It is found that the H α equivalent width ( EW ) exhibits two different patterns of variability: a slower one, called Long-Term (LT) variability, with an amplitude of about 30 Aand a characteristic duration of about 600 days and a faster one, called Short-Term (ST) variability, with an amplitude up to 10 Aand duration of 40 to 60 days. Suggestive evidence for EW variation on a longer time scale (about few years) also exists. The variations in the H α luminosity are not solely due to changes in the underlying continuum but also reflect variations in the physical properties of the wind. We find, in terms of a simplified spherically-symmetric wind model, that the LT variation of the line can be successfully explained in terms of a 26% alteration of the mass-loss rate, possibly accompanied by variations in the velocity field. From the analysis of the photometric 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 about 2600 days, i.e. about 7 years. During this variation, i.e. when the star brightens, the effective temperature decreases (by about 10% ) and the radius increases (by about 7% ). The properties of this Very Long Term (VLT) variation suggest that P Cygni has probably experienced a normal S Dor-type variation with a minimum phase around 1988 and a maximum phase in 1992. Some 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 brightness 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 these 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 similar to the properties of the so-called 100 d-type micro-variations, recognized in other LBVs by van Genderen et al. ([CITE],b). Based on time-scale evidences we suggest that the microvariabilities observed are rather due to relaxation oscillations(Stothers & Chin [CITE]) than to strange-mode oscillations in the stellar interior. Evidence for a close relationship between ST variations in H α and changes in the stellar brightness and temperature is found. From other results about P Cygni's spectral variations (Markova [CITE]), we conclude that the ST variability of the wind is most likely connected with processes in the stellar photosphere.