Study of the Thermal Pulsation of AGB Stars

Abstract

A systematic investigation on the third dredge up in a 3M$_{\odot}$, solar metallicity AGB star will be presented. The model evolves from the main sequence up to the Asymptotic Giant Branch (AGB). Intermediate mass stars are important because they contribute significantly via the slow neutron capture nucleosynthesis. The aim of this work is to gain insight on the behaviour of the AGB star during thermal pulsation. This investigation is based on an extended numerical simulation of the evolutionary phases and full, consistent AGB model calculations. In particular, the convective structure during pulsation will be studied, giving particular emphasis to the analysis of the stability of the Schwarzschild boundary that will eventually determine the occurrence of Third Dredge Up (hereafter referred to as TDUP). We provide a brief description of our updated evolutionary code and focus primarily on the obtaining the TDUP after 14 thermal pulses. We elaborate on the non-standard treatment of convection known as "overshooting" and its efficiency in achieving the TDUP event associated with the thermal pulsations (helium shell flashes). However, our knowledge of the physics behind the TDUP event, and the theoretical description of the overshooting mechanism suffer from embarrassing gaps and are, therefore, not properly treated in stellar modelling. This is not surprising, particularly with the oversimplified descriptions of important mixing mechanisms like convection in stellar models. However, since such event drives the $^{13}$C nucleosynthesis of interest for the following s-process activation, we present a full assessment of this process and its implications as calculations will further proceed in different directions.