Masters research project: angular momentum transport in stars

There are various processes in stellar evolution which tend to cause differential rotation in stars. For instance, a magnetised stellar wind acts to slow the rotation of the surface, and contraction of the core of a star as it fuel runs out tends will tend to make the core rotate more quickly than the rest of the star. However, many types of star appear to be in 'solid body' rotation, i.e. with uniform angular velocity. For instance, the core of the Sun, or the envelopes of high-mass stars. In addition, white dwarfs, which form out of the cores of red giant stars, often have rotation periods of years or even centuries and so cannot have simply spun up during the core-contraction red giant phase like an ice-skater pulling in her arms. Microscopic viscosity of the gas would take longer than the age of the universe to have the desired effect, and this apparent paradox remains one of the most important unsolved problems in stellar evolution. Some mechanism involving magnetic fields is normally invoked. This thesis project will investigate possible mechanisms for the transfer of angular momentum in stars, using both analytic and numerical methods. In particular the student will run numerical simulations of a differentially rotating star, looking at magnetic torques and turbulence. The effect of the turbulence on the mixing of chemical elements (one of the more important effects on the evolution of the star) will be looked at. Some prior knowledge of fluid mechanics is desirable but not essential.