Stability of Rotating Stars and Material Mixing in Accreting Stars

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In the present study, based on the examination of the hydrodynamical stability of stratified rotating body to non-axisymmetric perturbations,it is proposed that in differential rotating stars,turbulence prevails due to the baroclinic instability. On the basis of this framework,the model equations of stellar evolution with turbulent mixing has formulated. By applying them,the evolution of neutron stars and white dwarfs accreting in close binary systems is studied,which shows that the material mixing and heat transport due to turbulence caused by the accreted angular momentum reaches deep enough to the sight of thermonuclear runaways and affect their progress. Main conclusion is summarized as follows;1. The baroclinic instability can provide turbulent viscosity efficient enough to supersede the thermal instabilities such as proposed by Goldreich and Schubert. 2. The efficiency of material mixing is dependent on the assumption of turbulent viscosity,and increases for as smaller value is assumed for the latter. This is because the work necessary for the material mixing is extracted from the kinetic energy of differential rotation, which is larger for smaller viscosity. 3. The effect of the accreted angular momentum is more conspicuous for a compact star since larger rotation velocity enlarges the baroclinity.At the same time, the progress of material mixing was studied for the chemical diffusion and for the helium flashes in the metal deficient star for comparison. In order to separate the effect of mixing, the property of helium shell burning is also studied, which gives an insight for the mechanism of evolution to the red giant stars.