3D hydrodynamic analysis of Cepheid star pulsations

Abstract

Asteroseismology is a rapidly growing field dedicated to the study of stellar pulsations. Among the various classes of pulsating stars, Cepheids occupy a particularly important place owing to their key role as distance indicators. At the same time, one of the major challenges in stellar physics remains the modelling of turbulent convection. Consequently, understanding the interaction between convection and pulsations is an important and still largely unresolved problem.

This thesis presents a comprehensive analysis of this interaction through the study of a high-resolution three-dimensional numerical simulation of a Cepheid convective envelope exhibiting self-excited pulsations. Particular emphasis is placed on identifying and understanding the physical mechanisms responsible for sustaining these pulsations.

The analysis begins with an examination of the radial profiles of the main thermodynamic quantities, together with a study of the horizontal structure of the flow. This first investigation reveals a strongly asymmetric convective pattern, characterised by narrow, intense downflows surrounded by broader and smoother upflows. Such an asymmetry limits the relevance of horizontally averaged radial profiles, as they combine regions exhibiting very different behaviours.

To overcome this limitation, the flow is separated into upflow and downflow regions and analysed independently. This decomposition highlights the dominant role of the downflows in the transport of convective energy.

The thermodynamic cycle associated with the pulsations is then investigated through the study of the work integral and the heat exchanges occurring during the pulsation cycle. While the work integral does not allow any firm conclusion to be drawn, the analysis of the heat supply demonstrates the existence of heat gains during the hot phase and heat losses during the cold phase, resulting in a thermodynamic cycle that sustains the pulsations. This behaviour is dominated by the downflows, whose relative abundance increases during the hot phase as a consequence of the stronger convective instability and enhanced turbulence. Concerning the upflows, signatures of the well-known kappa-mechanism can be identified, but its contribution is completely dominated by other processes.