Stellar Magnetic Fields as Drivers of Mixing and Nucleosynthesis Processes

Maurizio M. Busso (Univ. Perugia)

It was recently speculated that stellar dynamo mechanisms might survive through the red giant stages of solar-like stars. Here, they would provide a source for the generation of toroidal magnetic fields, whose perturbations would be subject to the buoyancy instability, thus driving a circulation of material between the regions above the H-burning shell and the convective envelope. For reasonable values of the magnetic field this circulation would reach down to the zones where the temperature is high enough to maintain partial burning by proton captures of Li, of CNO nuclei and of some isotopes in the Ne-Na-Mg-Al chains. This idea is presently one of the most promising possibilities for explaining the anomalous composition of red giants, showing isotopic admixtures of light and intermediate-mass nuclei that cannot be explained by the pure occurrence of the "first dredge-up". Following the above suggestion, we present here the results of nucleosynthesis computations in magnetically-driven mass transport phenomena in red giants. We show that magnetic buoyancy, like other suggested mixing mechanisms, is capable of accounting for the isotopic abundances of CNO nuclei in evolved stars and in presolar grains that were shown to be of circumstellar origin. In many conditions a circulation is established where 7Li is destroyed. However, in some red giants Li is instead produced; in this respect, a peculiar property of magnetic buoyancy (not shared by other mixing mechanisms) is that it can provide transport phenomena at relatively high velocity, up to a few Km/sec. In this way it can offer a tool also for producing (not only for destroying) Li, as production of this nucleus requires that matter from the burning zones be mixed to the envelope at a rate faster than that of 7Be decay. We conclude that, while other nuclei do not provide a strong constraint on the mixing velocity, Li might be more informative. Its production might offer the proof that stellar dynamos, promoting fast magnetic buoyancy, are actually the physical cause that allows evolved stars to mix beyond the limits foreseen by convective dredge up.

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