MHD as a driver for mixing in AGB stars

We present analytical exact 2D and 3DMHDcomputations for the layers of an AGB star known to be affected by deep mixing phenomena, in order to verify previous suggestions that magnetic buoyancy may provide a sound explaination for the isotopic changes observed in AGB stars and in presolar grains. The structure of the relevant layers is similar to a polytrope of index 3 (a bubble of radiation), containing little mass. Due to this, the material is close to be unstable for expansion. Addition of any extra engine under the form of a magnetic dynamo generating toroidal structures unstable for buoyancy yields plasma phenomena that closely resemble those of the solar wind, in which almost ideal, non-resistive MHD allows for an easy analytical integration of the model equations. The results show that a further expansion occurs for magnetized domains (flux tubes). These last form close to thermonuclear shells and transport outward nucleosynthesis products with a velocity v ∼ r2, faster than for diffusion but slower than for convection, adequate to give a physical interpretation to extra-mixing processes in evolved stars.