A new tool to study the wave propagation: the Van Hoof effect
P. Mathias - D. Gillet
Received April 21; accepted May 7, 1993
Abstract:
Discovered in 1953 in the beta Cephei star beta CMa, the Van Hoof
effect, defined as a phase lag of hydrogen compared to metal lines, has
often been considered as a detail in the study of these stars. However,
thanks to modern detectors, it is now possible to get well-defined velocity
curves, and hence to start a serious study of this phenomenon, interpreted as
the time propagation of the pulsation wave through the atmosphere.
We present a new method for the detection of the Van Hoof effect with
the use of velocity curves.
Thus, and contrary to the other methods, only the phase lag due to the pure
variation differences is taken into account.
We considere the case of a small amplitude star (alpha Lupi) and that of
a large amplitude star (BW Vulpeculae). We observe that, for both stars,
the Van Hoof effect is also present between metal lines.
Thanks to the quasi-sinusoidal velocity curves presented by alpha Lupi,
the time propagation of the waves between several elements has been
determined with an excellent accuracy (about 1s).
Using an atmospheric model and the contribution functions of the considered
absorption profiles, the location of the line forming regions have been
determined.
Hence, the velocity of the outward pulsation wave at each altitude has been
deduced.
By comparison with the sound velocity, it is shown that in the deepest
observed photospheric layers, the wave velocity is quite supersonic while
the shock wave strongly dissipates when it reaches the outermost photosphere.
For BW Vulpeculae, the velocity curves show that the phase lag happens
during each atmospheric acceleration.
But, due to a well-marked line doubling phenomenon, the velocity curves are
not so accurately determined, and the propagation time is only measured within
a factor 2.
The results obtained do not seem to be consistent with the use of a single
layer atmospheric model. This means that a more realistic approach, based on
a pulsating atmosphere taking into account the occurence of shocks, is
required for the largest amplitude beta Cephei star.
Keywords:
Shock waves -- Stars: atmospheres --
Stars: variables: alpha Lupi --
Stars: variables: BW Vulpeculae
Back
Home