This study well confirms the presence of two shocks during one
pulsation cycle in BWVul.
Dynamically, the ``basic'' shock (outward-shock) is due to a wave propagating
from the subphotosphere, probably initiated by the iron
-mechanism recently found in these stars.
The velocity jump is between 80-100km.s-1 depending on the night.
Considering a sound speed of about 15km.s-1, this leads to a
Mach number around 5-7. It should be noted that the infalling motion
is almost finished when the outward shock occurs during the first night, while
during the second night, the velocity jump is again around 40km.s-1.
This shock gives an appreciable outward impulsion to the atmosphere,
which is not physically detached from the ``unperturbed'' photosphere,
contrary to the Odgers' scenario ([1955])
The atmosphere then follows a ballistic motion until it falls back.
When the increasing density decelerates the layers, it appears a
positive velocity
gradient between the bottom and the top of the atmosphere, caused by the
increasing density of the gas.
When the velocity gradient is large enough, the wave front breakes into
an infalling shock for the observer. The maximum front velocity is quite large
(between 120-180km.s-1 depending on the night). Thus, during its final
acceleration phase
(
), its Mach number is close to 8-12,
i.e. this shock is hypersonic contrary to the outward shock.
This interpretation is opposite to the one given by Crowe & Gillet
([1989]) who invoked a second
-mechanism.
Finally, due to inertia, the atmosphere bounces on the photosphere
and falls back again until complete damping: this explains the stillstand.
It should also be noted that turbulence may play a non-negligeable
role which may contribute to line broadening.
This scenario is based on a pure mechanical consideration. However, as noticed by a few authors (e.g. Young et al. [1981]), effects of radiative transfer due to changes in the continuous opacity can lead to erroneous interpretations of atmospheric motions. Furthermore, Moskalik & Buchler ([1994]), using a full self-consistent hydrodynamic model of BWVul, show that a unique shock wave, originating at the bottom of the He+ zone, is present during a pulsation cycle and causes the stillstand. This shock produces a sudden jump in the optical depth which contributes to the formation of the apparent discontinuity. However, although the amplitude of their radial velocity curve is correct, the stillstand has a velocity of about -100km.s-1 in the stellar rest frame, which has never been observed for this star.
A future interesting test would be to synthetize line profiles along one cycle using a self-consistent pulsating atmospheric model such as the one developed by Fokin ([1992]) for RRLyrae stars. This would represent the next step of this work.
