Convective quenching of stellar pulsations

Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
e-mail: thomas.gastine@ast.obs-mip.fr

Received: 24 August 2010
Accepted: 17 December 2010

Abstract

Context. We study the convection-pulsation coupling that occurs in cold Cepheids close to the red edge of the classical instability strip. In these stars, the surface convective zone is supposed to stabilise the radial oscillations excited by the κ-mechanism.

Aims. We study the influence of the convective motions on the amplitude and the nonlinear saturation of acoustic modes excited by κ-mechanism. We are interested in determining the physical conditions needed to lead to a quenching of oscillations by convection.

Methods. We compute two-dimensional nonlinear simulations (DNS) of the convection-pulsation coupling, in which the oscillations are sustained by a continuous physical process: the κ-mechanism. Thanks to both a frequential analysis and a projection of the physical fields onto an acoustic subspace, we study how the convective motions affect the unstable radial oscillations.

Results. Depending on the initial physical conditions, two main behaviours are obtained. (i) Either the unstable fundamental acoustic mode has a large amplitude, carries the bulk of the kinetic energy, and shows a nonlinear saturation similar to the purely radiative case; (ii) or the convective motions significantly affect the mode amplitude, which remains very weak. In this second case, convection is quenching the acoustic oscillations. We interpret these discrepancies in terms of the difference in density contrast: larger stratification leads to smaller convective plumes that do not affect the purely radial modes much, while large-scale vortices may quench the oscillations.