The Van Hoof effect in the variable star RR Lyrae
P. Mathias
- D. Gillet
- A.B. Fokin
- M. Chadid
Received 4 October 1994; accepted 28 November 1994
Abstract:
Quite common among the beta Cephei 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 spectroscopic study presented in this paper,
around both Hbeta and Hgamma lines for the star RR Lyr, 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 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 corresponding 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.
Keywords:
hydrodynamics -- shock waves -- stars: pulsation -- stars: variables:
RR Lyrae -- stars: individual: RR Lyr
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