EDP Sciences
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Volume 366, Number 3, February II 2001
Page(s) 935 - 944
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20000332

A&A 366, 935-944 (2001)
DOI: 10.1051/0004-6361:20000332

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

N. Markova1, S. Scuderi2, M. de Groot3, H. Markov1 and N. Panagia4, 5

1  Institute of Astronomy and Isaac Newton Institute of Chile Bulgarian Branch, Bulgarian National Astronomical Observatory, PO Box 136, 4700 Smoljan, Bulgaria
2  Osservatorio Astrofisico di Catania, Viale A. Doria 6, 95125, Catania, Italy
3  Armagh Observatory College Hill, Armagh, BT61 9DG, Northern Ireland
4  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
5  On assignment from the Space Science Department of ESA

(Received 3 August 2000 / Accepted 23 November 2000)

For the first time an extensive set of (quasi-) simultaneous photometric (UBV) and spectroscopic (H$\alpha$ 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$\alpha$ equivalent width (EW) exhibits two different patterns of variability: a slower one, called Long-Term (LT) variability, with an amplitude of about 30 Åand a characteristic duration of about 600 days and a faster one, called Short-Term (ST) variability, with an amplitude up to 10 Åand 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$\alpha$ 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. (1997a,b). Based on time-scale evidences we suggest that the microvariabilities observed are rather due to "relaxation oscillations"(Stothers & Chin 1995) than to strange-mode oscillations in the stellar interior. Evidence for a close relationship between ST variations in H$\alpha$ and changes in the stellar brightness and temperature is found. From other results about P Cygni's spectral variations (Markova 2000a), we conclude that the ST variability of the wind is most likely connected with processes in the stellar photosphere.

Key words: stars: early type -- stars: atmosphere -- stars: mass loss -- stars: individual: P Cyg

Offprint request: N. Markova

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