Slowly Pulsating B stars were named from Waelkens (1991) who detected, in mid-B stars, multiperiodic photometric variations (on time-scales of tens of hours) attributed to non-radial g-modes. This is confirmed by the models where the stars are destabilized by a -mechanism originating in the iron opacity bump (see e.g. Dziembowski et al. 1993). But two problems still remain. First, only a few modes are detected in SPB stars, whereas numerous modes are excited in the models. The study of an SPB star observed over a long time range should provide a good test about mode detection but also on the long-term stability of the pulsations. Second, some of the hotter SPB stars seem to present an additional shorter time-scale variability that may be explained in terms of p-modes which are an observational characteristic of Cephei stars. Actually, the hotter SPB stars may also belong to the cooler part of the Cephei stars instability strip.
The SPB star Herculis (HR 6588, V = 3.80, B3 IV) is a good candidate with respect to these two points: it has a long story of both photometric and spectroscopic variability, and is among the hottest SPB stars. Up to now, many detected frequencies have been reported in the literature, in the range 0.76-34 c.d-1. Smith (1978) found profile variations that were fitted with at least 3 different frequencies (1.7, 2.4 and probably 4.8 c.d-1) with amplitude changing on a time scale of months, and thus classified the star in the 53 Per group. Smith (1981) also analysed six weak absorption lines in the range 4127-4132 and found variations by a factor of 3 in frequencies between one and a few cycles per day. As these lines are very sensitive to temperature changes, he associated these variations with nonradial pulsations. Line profiles variations were also reported by Le Contel et al. (1987). While large variations in one night were observed in photometry by Warman (unpublished, cited by Smith 1981), nothing seemed to appear, at least within 1-2 hundredth of a magnitude, in the data set of Seeds (1971). During the preparation of an ultraviolet atlas of Her (Upson & Rogerson 1980) Rogerson (1984) discovered a larger than expected scatter in the radial velocity of this star, identified as a two-frequency variation of 0.66 and 0.62 c.d-1. Recently, a spectroscopic study (Mathias & Waelkens 1995, hereafter Paper I) showed that the usual low frequencies reported for this star in the literature could not explain the variations of the radial velocity curves, and that two main frequencies were detected in the data, around 0.3 and 0.7 c.d-1. Finally, Mulliss (1996) 1 presented, from 58 spectra, radial velocity variations spanning about 400 nights. He detected the frequencies given by Rogerson (1984) and reported a new frequency (1.25 c.d-1). Mulliss (1996) also pointed out that other frequencies should be present.
In addition to these typical SPBs frequencies, higher ones are often detected in Her. First, Gonzalez-Bedolla (1981) measured a 16 c.d-1 frequency in his photometric data. Later, Chapellier et al. (1987) detected Cephei-type frequencies in their data sets (7 or 8 c.d-1 in photometry, and 8.5 c.d-1 in spectroscopy) during a few days in 1981 and 1983. A higher frequency, of the order of 20 c.d-1, was also present in the data of Paper I. This was attributed to p-modes on the basis of the location of Her in the HR diagram. The conclusion of Paper I was that such "hybrid" stars would be of particular interest for the knowledge of the stellar structure, since both upper (via acoustic modes) and inner (via gravity modes) parts of the star could be simultaneously studied.
In addition to this complicated pulsation pattern, Kodaira (1971) and Abt & Levy (1978) deduced from their data analysis that Her was a component of a binary system, with an orbital period around 113 d.
The aim of this paper is to improve our understanding of this star using new and published spectroscopic and photometric data (described in Sect. 2). We first refined the binary elements (Sect. 3) in order to remove the orbital motion before studying the pulsational behaviour of Her. Then the power spectrum of the variations is analysed in Sect. 4, and discussed in Sect. 5. Finally, some conclusions are given in Sect. 6.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100