Keele Research Repository

The treatment of convection remains one of the key uncertainties in stellar evolution. In particular, mixing processes at the boundaries of convective regions are complex and diffcult to define analytically. Therefore, hydrodynamic simulations are used to model fluid flow in convective regions and the neighbouring stable regions, allowing the convective boundary mixing to be characterised. Hydrodynamic simulations currently provide the most accurate modelling of convection and convective boundary mixing, but are limited to time-spans which are a negligible fraction of the stellar lifetime. One way of mitigating this limitation is to transfer the key results of hydrodynamic simulations into 1D evolution models, which are able to model the whole life of a star. This is the aim of this thesis, in which two forms of convective boundary mixing (turbulent entrainment and convective shear) have been implemented into the Geneva stellar evolution code.
The entrainment prescription has been used to compute a grid of models from 1.5M* to 60M* on the main sequence. These were compared both to standard 1D models and to observational limits on the main sequence width. The strength of mixing due to entrainment was found to increase with mass, in line with observational evidence. The convective boundaries in previously calculated hydrodynamic simulations of the carbon shell have been reanalysed and compared to 1D convective shear models. It was found that the boundary shapes seen in the hydrodynamic simulations can be better modelled using an additional layer of mixing above the shear layer. Finally, a more general, multi-layered boundary structure has been discussed and future work outlined.