Convective quenching of stellar pulsations

TL;DR

This study uses nonlinear simulations to explore how convection influences stellar pulsations in Cepheids, identifying conditions under which convection can suppress oscillations near the red edge of the instability strip.

Contribution

It provides the first detailed nonlinear analysis of convection-pulsation coupling in Cepheids, revealing how different stratification levels lead to either sustained pulsations or quenching by convection.

Findings

Convection can either allow large amplitude pulsations or quench oscillations depending on stratification.

Larger density contrast leads to smaller plumes that minimally affect pulsations.

Large-scale vortices in convection zones can suppress radial oscillations.

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 kappa-mechanism. Aims: we study the influence of the convective motions onto the amplitude and the nonlinear saturation of acoustic modes excited by kappa-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 kappa-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 affect significantly the mode amplitude that 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 much the purely radial modes, while large-scale vortices may quench the oscillations.