Astron. Astrophys. 362, 189-198 (2000)

6. Conclusions

• The analysis of all the data provided by the literature allowed the computation of new binary elements, with a larger eccentricity compared to previous results.

• By subtracting the orbital motion from the observed velocity we could study the "pure" pulsation motion. The observed maximum night-to-night variation is about 8 km s^-1. However, within a night, the amplitude is much smaller.

• One SPBs frequency is clearly established:  c.d^-1. Another frequency is detected in Hipparcos data around  c.d^-1. A frequency  c.d^-1 is present in most spectroscopic data sets, but is very faint in photometric ones. Finally, a frequency  c.d^-1 is present in only one data set (Rogerson 1984). The frequency around 1.7 c.d^-1 is more doubtful since it can be a one-day alias of or . Obviously, other frequencies are present, but we failed in measuring them.

• Amplitude variations are detected along the 10 years of the observations. The typical time-scale is of the order of years. This phenomenon is similar to the one reported concerning the SPB star 53 Per (Chapellier et al., 1998).

• For the first time, the phase-lag is measured for an SPB star. It is found to be around 0.64 P for  Her. This represents a new constraint for theoretical models.

• We failed in detecting the high frequencies (around 20 c.d^-1) reported earlier (e.g. Paper I). New, long term observations are needed to confirm the reality of this particular behaviour. On the contrary, power is present in the [6;8] c.d^-1 range in the frequency spectrum of some data sets, corresponding to  Cephei type variation. It seems that the short time scale has a transient nature. No convincing explanation could be given. As both  Cephei and SPBs typical time scales are observed, and since  Her is located in the HR diagram within the SPBs instability trip and close to the  Cephei one, we propose that it be classified as a "hybrid" star.

Considering their typical low frequencies, together with the many modes potentially excited, SPB stars are difficult to monitor. Although the star  Her is one of the best observed, its behaviour remains still uncertain. Is  Her a typical SPB star in its large time scale behaviour? If yes, very long term monitoring is needed to understand both the atmospheric motions and the internal structure of these stars.

© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100
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