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