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OHP Preprint No. 107 : Turbulence in pulsating stars

Turbulence amplification in the atmosphere of pulsating stars: a first approach

D.Gillet1, A.B.Debiève2, A.B.Fokin3 and S.Mazauric4

1 Observatoire de Haute-Provence - CNRS
  F-04870 Saint-Michel l'Observatoire, France
  E-mail: gillet@obs-hp.fr
 
2 IRPHE, UM CNRS Université d'Aix-Marseille I et II,
  12 av. Général Leclerc 
  13003 Marseille, France 
 
3 Institute for Astronomy of the Russian Academy of Sciences
  48 Pjatnitskaja, 
  109017 Moscow, Russia 
 
4 von Karman Institute for Fluid Dynamics,
  72 Chaussée de Waterloo, 
  1640 Rhode-Saint-Genèse, Belgium
Astronomy & Astrophysics (in press)

Abstract
This paper is devoted to the understanding of the "missing temperature", called microturbulence by the astrophysicists, which appears when we want to modelling the width of stellar line profiles. In the framework of the two limiting turbulent regimes called "incompressible" and "pressure released", and expecting that the dissipation is negligible ("rapid distortion" or RDT case), it is shown that the turbulence amplification in the atmosphere of a radially pulsating star is not only due to the global compression of the atmosphere during the pulsation. Strong shock waves propagating from the deep atmosphere to the very low density layers also play a role in the turbulence variation, especially when they become very strong i.e., hypersonic. The predicted turbulence amplification induced by the global atmospheric compression is well consistent with the solenodial RDT. For shocks, the predicted turbulence amplification in the "pressure released" regime is overestimated with respect to stellar observations when the compression rate becomes larger than 2 which corresponds to a limit Mach number near 2. Thus, when radiative effects take place, the present turbulence amplification theory breaks down. A new approach is required.

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

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