Pulsating motions and turbulence in Delta Cephei*

A.B.Fokin1, D.Gillet2 and M.G.Breitfellner3

1Institute for Astronomy of the Russia Academy of Sciences,
48 Pjatnitskaja, 109017 Moscow, Russia

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 . 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_rotsin i close to 7.5 2 km\s. Moreover, because the FWHM is not affected by non-LTE effects, we have only one basic dependent parameter left: the turbulent velocity. Thus our result is strong in the framework of our semi-quantitative approach.

Keywords
stars: variables: Cepheids -- line: profiles -- turbulence -- shock waves -- stars: individual: Delta Cephei


*Based on observations obtained at the Observatoire de Haute-Provence (France).