SIMBAD references

Quite common among the βCephei stars, the Van Hoof effect is a phase lag existing between the radial velocity curves of hydrogen and metallic lines. It is interpreted as the time propagation of the pulsation wave through the atmosphere. The spectrosocopic study presented in this paper, around both Hβ and Hγ lines for the star RRLyr, clearly reveals the presence of the Van Hoof effect. It seems to exist only between hydrogen and metallic lines. It is not detected between metallic lines probably because these latter are all formed at a comparable altitude. The differential curves of velocity, radius variation and acceleration are discussed, explaining the different behaviour between the bottom and the top of the photosphere. At the moment of the outward acceleration phase, the velocity correponding to the metallic layers varies before that of the hydrogen ones. During the following ballistic motion, both density and gravity gradients seem to be responsible for most of the differential behaviour, especially at the bump phase, representing the colliding of the infalling upper atmospheric layers with the deeper ones.