THE VANHOOF EFFECT IN THE VARIABLE-STAR RR-LYRAE

Quite common among the beta Cephei stars, the Van Hoof effect is a pha se lag existing between the radial velocity curves of hydrogen and met allic lines. It is interpreted as the time propagation of the pulsatio n wave through the atmosphere. The spectrosocopic study presented in t his paper, around both H beta and H gamma lines for the star RR Lyr, c learly reveals the presence of the Van Hoof effect. It seems to exist only between hydrogen and metallic lines. It is not detected between m etallic lines probably because these latter are all formed at a compar able altitude. The differential curves of velocity, radius variation a nd acceleration are discussed, explaining the different behaviour betw een the bottom and the top of the photosphere. At the moment of the ou tward acceleration phase, the velocity correponding to the metallic la yers varies before that of the hydrogen ones. During the following bal listic motion, both density and gravity gradients seem to be responsib le for most of the differential behaviour, especially at the bump phas e, representing the colliding of the infalling upper atmospheric layer s with the deeper ones.