The observational status of the Slowly Pulsating B star Herculis *
E. Chapellier 1,
P. Mathias 1,
J.-M. Le Contel 1,
R. Garrido 2,
D. Le Contel 1 and
J.-C. Valtier 1
Received 25 February 2000 / Accepted 29 June 2000
New spectroscopic and photometric measurements are presented concerning the SPB binary star Her. Our observations are spread over 10 years, and consist of 10 spectroscopic nights in 1985, 7 spectroscopic nights and 22 photometric nights in 1987, and 15 spectroscopic nights in 1995. In addition, we analysed the photometric observations provided by Hipparcos (1990-1993) as well as all published radial velocities. We used all the spectroscopic data to refine the binary ephemeris. It was therefore possible to study the pulsation variability properly.
Despite our large data set, only one frequency has been clearly established: c.d-1, for which an ephemeris is provided. Three other frequencies are detected: c.d-1, c.d-1 and c.d-1. The frequency is present in most data sets, is detected only in Hipparcos data (longtime basis of homogeneous data set), while is detected in only one data set.
The precision on the value of allowed us to estimate the phase-lag between photometric and spectroscopic variations for the first time in an SPB star. The lag of the photometric maxima with respect to those of velocity is around 0.64 period i.e., significantly different from what is measured in the classes of variable stars surrounding the SPB stars.
The pulsation amplitude has varied in a complex way during the last 10 years. It has increased during the 3 years of photometric observations by Hipparcos, while a decrease by a factor 2 was observed in spectroscopic data between 1985 and 1995.
Periodograms of both photometric and spectroscopic variations show faint peaks in the [6;8] c.d-1 and [15;25] c.d-1 frequency regions. However, one of our data sets, providing a good precision on the velocity variations, shows a rather flat periodogram after 5 c.d-1, with no detectable peak having an amplitude above 0.05 km s-1. Therefore, these relatively high frequencies may have a transient nature. Although Her is one of the best observed SPB star, its variability behaviour is still uncertain, and long term monitoring is needed.
Key words: stars: binaries: spectroscopic stars: individual: Her stars: oscillations stars: variables: general
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© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100