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Astron. Astrophys. 332, 857-866 (1998)

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1. Introduction

A photometric survey investigating the optical instability of massive stars has led to the publication of a series of papers by the first author during the last 13 years. These studies are largely based on the VBLUW photometric system of Walraven. All evolved massive stars are photometric micro variables also called [FORMULA]  Cygni variables. Amplitudes and time scales amount to [FORMULA] and days to months, respectively. Non-radial (e.g. Lovey et al. 1984; Gautschy 1992; Balona 1992) as well as radial pulsations (Cox et al. 1995) have been suggested as possible causes.

A small subgroup consists of the hot S Dor variables or luminous blue variables (LBVs), which in or near minimum brightness show the same type of microvariations (for a review, see Sterken 1989), called [FORMULA]  Cyg-type variations, usually on a time-scale of 2-6 weeks, depending on the phase of the S Dor-activity (van Genderen et al. 1997a): halfway up to maximum light another type of microvariations emerges rather abruptly: the so-called 100 d-type variations (van Genderen et al. 1997b).

Often the photometric oscillations are not strictly periodic, but differ from cycle to cycle (e.g. Balona 1992). Therefore, one often speaks of quasi- or pseudo-periods. The resulting phase diagrams always display considerable scatter (van Genderen 1991; van Genderen et al. 1992).

The cause of the quasi-regularity can be partly attributed to stochastic processes, such as large-scale atmospheric motion fields (e.g. de Jager et al. 1984; Burki 1987; van Genderen 1991), disturbing a regular pattern of stellar oscillations. The discovery that the dormant LBV [FORMULA]  Sco is a multi-periodic variable (Sterken et al. 1997), and that the LBV R 40 (= HD 6884) is at least multi-cyclic if not multi-periodic (Sterken et al. 1998), can be considered as a breakthrough in understanding the complex variability of super- and hypergiants. Independently, Kaufer et al. (1997) found the same phenomenon in a study based on spectroscopic monitoring of BA-type supergiants like HD 92207 and HD 96919. Perhaps all [FORMULA]  Cyg variables are more or less subject to multi-periodicity with an additional intrinsic noise component (e.g. Sterken et al. 1997).

One of the aims of the photometric survey was to investigate the distribution of the quasi-periods in the upper region of the HR-diagram. Earlier attempts to define a semi-empirical [FORMULA] relationship have been undertaken by Burki (1978) and by Maeder (1980). So far, additional variables, especially LBVs in or near minimum brightness, have been found to fit reasonably well into that relationship (e.g. van Genderen & Sterken 1996). However, since we found that the period changes during the S Dor activity, that individual cases show multi-periodicity, and that the latter could be true for all members of this class, we are facing a dilemma: which period should be taken to plot the object in the grid of `P = constant' lines? Thus, it is likely that the semi-empirical [FORMULA] relationship is influenced by a mix of selection effects so that its application should be considered with more reserve.

In the present paper we discuss VBLUW photometry of four new variables in the LMC, of which three are emission-line objects and two are LBVs. Light and colour curves are analysed and searched for periods.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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