Internal mixing processes in massive stars : uncertainties and impact

Massive stars are key contributors to the evolution of galaxies and many observed phenomena. Therefore, understanding them is crucial to explain the evolution of the Universe and its constituents. The evolution of massive stars is strongly in uenced by internal mixing processes. In stellar evolution theory, these processes are simplified due to the assumption of spherical symmetry. This introduces many uncertainties. In this Thesis, I investigate two groups of internal mixing processes in massive stars: turbulent convection and rotation-induced mixing. The study focusses on convective boundary mixing and angular momentum transport. Concerning convective boundary mixing, I study how the location of the convective boundary and different amounts of convective boundary mixing affects the structure and evolution of massive stars. I find an uncertainty of up to ~ 70% in the prediction of core masses at core helium depletion. Furthermore, the surface evolution of massive stars depends critically on the mixing choices. Comparison between model predictions and observations suggests that models require a larger amount of convective boundary mixing than currently adopted in the literature. Concerning angular momentum transport, I investigate angular momentum transport by rotation-induced instabilities and two different magnetic dynamos and how it is affected by related theoretical and implementation uncertainties. The three sets of models predict distinct ranges of the core rotation rate at core collapse. However, the strength and timing of angular momentum transport depends strongly on the transport mechanism and its uncertainty. Generally, the main transport of angular momentum occurs before core helium ignition and nearly no angular momentum is transported after core oxygen ignition. This Thesis shows that the evolution of massive stars is strongly in uenced by the uncertainties linked to convective boundary mixing and rotation-induced mixing and more work is needed to provide reliable predictions for stellar evolution.