Gastine and B. Received: 24 August Accepted: 17 December We study the convection-pulsation coupling that occurs in cold Cepheids close to the red edge of the classical instability strip. We are interested in determining the physical conditions needed to lead to a quenching of oscillations by convection. 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.
Depending on the initial physical conditions, two main behaviours are obtained. 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. The cold Cepheids located near the red edge of the classical instability strip have a large surface convective zone that affects their pulsation properties e.
The first calculations, done without any convection-pulsation modelling, predicted much cooler red edges than the observed ones. More recently, these different models have been largely improved by succeeding in reproducing the correct location of the red edge, despite their disagreements with the physical origin of the mode stabilisation e. Bono et al. However, these models rely on many free parameters e.
Despite their own limitations weak density or pressure contrasts, for instance , 2-D and 3-D direct numerical simulations DNS are a good way to investigate the convection-pulsation coupling as the essential nonlinearities are fully taken into account. We recall that the instability of Cepheid variables relies on blocking the emerging radiative flux near the opacity bumps that are due to the ionisation of light elements like H or He.
As the radiative conductivity is proportional to the inverse of opacity, a conductivity hollow therefore mimics such an opacity bump. By starting from the most linearly-unstable setups, we performed in Paper II the corresponding nonlinear study by means of direct numerical simulations. Thanks to a powerful method that involves several projections of the computed fields onto suitable subspaces e. Bogdan et al. Their temporal evolutions have then emphasised the strong nonlinear coupling existing between the unstable fundamental acoustic mode and the stable second overtone.
