## 5. Discussion## 5.1. The SPBs behaviourWith its B3 IV spectral type, Her lies in the SPBs instability trip (e.g. Gautschy & Saio 1996). Several other observed SPBs characteristics, as listed by North & Paltani (1994), confirm this statement. First, the presence and stability over a time scale of years of at least three frequencies is pointed out: and in between 1990-1993 (Hipparcos), and in 1985-1987. Second, the amplitude in the Hipparcos filter is larger than that measured in the filter, while no significant phase lag is observed between the two. Finally, the 4 proposed frequencies are in the range of observational and theoretical criteria. Thus, the present study confirms that Her should be classified as an SPB star variable. ## 5.1.1. EphemerisFrom above, Hipparcos data provide a good value of the main
frequency: c.d where is the calculated date of
heliocentric radial velocity maxima after
## 5.1.2. The phase-lagThe phase-lag, defined as the difference between the epoch of maximum luminosity and that of maximum radial velocity, has been computed with two different methods using the period given by the ephemeris: -
comparison of mean luminosity maxima epoch with the ephemeris given above. This method leads, for the Spain and both Hipparcos data sets, to a phase-lag of respectively 0.659, 0.639 and 0.634 P, where P is the main pulsation period -
comparison of the nearly simultaneous spectroscopic and photometric data obtained in 1987. The phase-lag here is 0.845 P
It should be noted that the second method is not as accurate as the first one since we added uncertainties on each maxima determination. Therefore, and because Hipparcos data are the most suitable for SPB stars, the phase-lag value must be around 0.64 P for Her. This value, the first one obtained for an SPB star, differs significantly from reported values concerning pulsating stars on each side of the SPBs instability strip: 0.25 P for the Cephei stars, and 0.5 P for the classical instability strip. However, other observations, involving simultaneous photometric and spectroscopic data, are needed to confirm this result. ## 5.1.3. The amplitudesThe pulsation amplitudes show important variations during the last
10 years. In spectroscopic data, the amplitude associated with the
frequency has dropped from
2.76 km s However, this result should be treated with caution, since both the lines used to derive the velocity and the resolution of the different data sets are not the same. In particular, the larger dispersion noted in the 80's data may be enhanced as a bias in the corresponding amplitude. Because the photometric data concern different filters, nothing can
be said over the 1987-1993 period. However, due to their long time
basis, a crude study of the Hipparcos data was undertaken. The data
corresponding to each filter were binned with 15 points in each
subset. Then, a sine-fit using the
frequency was computed on each subset, providing the evolution of the
photometric amplitude with time. The general observed trend is an
Thus it appears that the amplitude associated with shows opposite variations on time-scale of years. However, observations are neither homogeneous enough nor well spread in time to follow the amplitude evolution over 10 years. A similar phenomenon has also been observed for the SPB star 53 Per. For this latter, the amplitude associated with one of the two main frequencies increases regularly while the other remains constant (Chapellier et al. 1998). This phenomenon could be quite frequent among the SPB stars, since it is detected in the two best monitored stars of this class. Furthermore, the amplitude ratios
() associated with
and
are very different. Using the 1987 data, the values 950 and
430 km s ## 5.2. The short time scale behaviourThe [6-8] c.d Conversely, the high frequencies, in the
[15-25] c.d Another interesting point is the apparent instability of these high
frequencies motions. This variability may be due to a transient
phenomenon. Such an hypothesis, concerning the
-Cephei pulsation type, has already
been invoked for the B2.5 IV star 53 Psc, where a relatively
large frequency (10.4 c.d © European Southern Observatory (ESO) 2000 Online publication: October 30, 19100 |