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Astron. Astrophys. 335, 605-621 (1998)

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1. Introduction

The pulsation of stars provides information about the structure and stability in the interior. The Luminous Blue Variables (LBVs) offer the unique opportunity to study the pulsation of stars when their radius changes by factors up to 4 or 8 in the course of years to decades. This paper deals with the study of the pulsational periods and the period changes during the variations in radius of six LBVs.

Luminous Blue Variables are the most unstable massive stars, apart from supernovae. They are variable on all timescales:
(a) small photometric variations (or microvariations) with [FORMULA] on timescales of weeks to months,
(b) typical LBV variations (or moderate photometric variations) with [FORMULA] to 2.0 mag. on timescales of years to a decade,
(c) giant eruptions on timescales of about [FORMULA] years.
In this paper we study the microvariations during several phases of the moderate variations.

Van Genderen et al. (1997a) have shown that there are two types of microvariations:
(1) near the visual minimum, when the star is hot and has a small radius, the star is bluer in the bright phase and redder in the faint phase of the microvariations.
(2) near the visual maximum, when the star is cooler and has a larger radius, the star is redder in the bright phase and bluer in the faint phase of the microvariations.
This points to different pulsational mechanisms.

Soukup et al. (1994) suggested that the microvariations of the LBVs could be due to non-radial g-modes of order between 10 and 20 with periods of about 10 days for [FORMULA]. These modes could be due to the frozen-in convection in the region of the Fe opacity peak near [FORMULA] K. However, more recent calculations by Cox et al. (1995) suggest that the microvariations of LBVs are mostly or all radial pulsations due to strange modes. They argue that these occur when the stars have lost sufficient mass to expose He, which could drive the pulsation in the helium ionization zone.

In this paper we study the pulsational characteristics of six LBVs, observed in the LTPV (Long Term Photometry of Variables) project of ESO (Sterken et al., 1993a,b). The purpose of this study is to understand the periods of the pulsations and period changes and to derive information on the nature of the instability. In Sect. 2 we describe the program stars and the observations. In Sect. 3 we describe the lightcurves and in Sect. 4 we determine the pulsational periods during various phases of the light curves. Section 5 deals with the pulsational constant Q and its changes when the radius changes. We also discuss the amplitude-period relations. In Sect. 6 we consider the possibility that the periods are due to strange modes and we compare the observed microvariations with those of Slowly Pulsating B-stars (SPBs). A very preliminary mode identification in terms of a linear theory is described in Sect. 7. Section 8 gives the discussion and conclusions.

The properties of the LBVs have been reviewed by Humphreys & Davidson (1994). The stars have been discussed extensively during the workshop Luminous Blue Variables: stars in transition in October 1996. The reader is referred to the proceedings of this workshop for reviews on the properties and evolutionary stage of LBVs (Nota & Lamers, 1997).

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998