Modelling and seismic probing of core-helium burning stars with mixed modes

Abstract

The study of stellar pulsations known as Asteroseismology provides insights into internal structures and dynamics of stars. In particular, red giant stars exhibit coupled oscillation modes known as mixed mode, which are important for distinctly probing the core and envelope. The recent space missions such as Kepler and CoRoT have led to useful advances in the asteroseismology of red giant stars. In this thesis, we study low-mass red giant stars undergoing helium burning in their convective core during their evolution on the Horizontal Branch. The primary focus is on analysing the core physical parameters throughout the evolution and then some features of mixed modes such as the Brunt–Väisälä frequency and the period spacing are investigated. The effects of envelope mass and core size on these different parameters are explored. The stellar evolution code Clés (Code Liégeois d’Évolution Stellaire), developed within the Department of Astrophysics, Geophysics and Oceanography of the University of Liège, is used to provide the evolution sequences, and then a simplified model based on polytropes is developed in Python code to complement the study. The latter provides a simpler framework to interpret the trend in the central physical parameters over the course of the evolution.