Keele Research Repository

In this thesis, I study the evolution of low-mass (around 2 M?) solar-metallicity stars including the effect of rotation and magnetic fields. These stars produce a significant amount of elements heavier than iron via the so-called s process and thus have a large impact on galactic chemical evolution. In the last decade, researchers have been able to obtain rotational properties from asteroseismic observations of stars. These observations cannot be reproduced by current stellar evolution models. It is now generally accepted that a process of transport of angular momentum is missing from the current implementations of rotation in stellar evolution models. The aim of the thesis is to explore the impact of rotation on the evolution and nucleosynthesis of low-mass stars, and to use the asteroseismic and s-process nucleosynthesis observations as constraints. To do so, I calculated rotating and non rotating models, with and without the Tayler-Spruit dynamo. To constrain the missing process of angular momentum, I included an additional, artificial viscosity to models. The main findings are the following. I determined the amount of additional viscosity needed for the cores within my stellar evolution models to rotate within the asteroseismically constrained rotation rates of core helium burning stars and white dwarfs. The value I had to use for such viscosity is ?add =106-107 cm2 s-1, several orders of magnitude higher than the value found to match observations for lower mass stars. I then calculated for the first time the s-process nucleosynthesis of stellar evolution models that match these constraints on rotation rates. I concluded that the effect of rotation on the s-process production of low mass AGB stars is negligible, which is in agreement with s process observations. I also placed constraints on the mixing of chemical elements by the missing process of angular momentum, and I have listed future work involving magnetic dynamos.