Three-dimensional hydrodynamic simulations of deep convection in massive stars

Abstract

Convection plays a key role in the evolution of massive stars. Despite many decades of work on this topic, the treatment of convection (convective boundary mixing in particular) is still one of the major uncertainties in stellar evolution modelling. Fortunately computing power has reached a level that enables detailed three dimensional hydrodynamic simulations, these can provide valuable insights into the processes and phenomena that occur during stellar convection. The aim of this thesis is to explore the physics responsible for convective boundary mixing within the deep interiors of massive stars, through the calculation of stellar evolution and three-dimensional hydrodynamic models. The latter focus on carbon shell burning and are the first of their kind within the stellar hydrodynamic community.
To prepare the input models as well as study the evolution of convective boundaries, a 15M? stellar model was computed and a parameter study was undertaken on the convective regions of this model. The carbon shell was chosen as an input model for three-dimensional simulations. Two sets of simulations were calculated with the aim of conducting a resolution study and luminosity study. The simulations were analysed using the Reynolds averaged Navier-Stokes (RANS) framework and within the context of the entrainment law.
The following is a summary of the key findings. The lower convective boundary was found to be ‘stiffer’ (according to the bulk Richardson number) than the upper boundary. The boundaries are shown to have a significant width which is likely formed through Kelvin-Helmholtz instabilities, the width of the lower boundary is much narrower than the upper. The shape of the boundaries (interpreted through the composition) is smooth and sigmoid-like, whereas in the one-dimensional models it is sharp and discontinuous. Finally, these simulations confirm the scaling of the bulk Richardson number with both the entrainment rate and the driving luminosity.