2Observatoire de Haute-Provence - CNRS, F-04870
Saint-Michel l'Observatoire, France
3ESTEC, Space Science Department, ISO Science Team,
P.O. Box 299, NL-2200 AG Noordwijk, The Netherlands
Abstract
A nonlinear nonadiabatic pulsational model of the classical Cepheid
star delta Cephei with an extended atmosphere
was computed to firstly determine the dynamical
structure of the atmosphere and secondly to interpret
the variation of FWHM with phase of the
FeI line 5576.0883 Å.
The presence of three bumps within the turbulence curve
as put into evidence by Breitfellner and Gillet (1993), is examined.
The effect of predicted shock waves crossing the
FeI line formation region is specified.
Our detailed model shows that the true level of turbulence velocity is
roughly two times smaller than the upper limit given by these authors but
is close to the microturbulence measured by van Paradijs (1971). The first
peak observed just after the luminosity maximum (phase phi=0.0) in the
FWHM is mainly the
consequence of large velocity gradients induced by the pulsating motion
in presence of a strong shock wave which have been neglected in all previous
works.
No additional small-scale motion field (microturbulence) is needed at this
phase. The strongest peak of the FWHM curve (around phi=0.77) which
occurs at the
same time as the second large amplitude shock, can only be interpreted
by a strong increase of the turbulence. It seems that a large part of
this turbulence increase is due to the atmospheric compression although
a shock wave amplification effect is also plausible.
Our best fit of observed and theoretical FeI profiles
suggests a rotational velocity
v_
Keywords
stars: variables: Cepheids --
line: profiles --
turbulence --
shock waves --
stars: individual: Delta Cephei
*Based on observations obtained at the Observatoire de Haute-Provence (France).