Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?

Abstract

It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low-mass red giant branch (RGB) and asymptotic giant branch (AGB) stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 M_⊙, in both the AGB and RGB phases. The field required for the AGB star in the processing zone is B_0 ~ 5 × 10^6 G; at the base of the convective envelope this yields an intensity B_E ≾ 10^4 G. For the RGB case, B_0 ~ 5 × 10^4 – 4 × 10^5 G, and the corresponding B_E are ~450-3500 G. These results are consistent with existing observations on AGB stars. They also hint at the basis for high field sources in some planetary nebulae, and the very large fields found in some white dwarfs. It is concluded that transport by magnetic buoyancy should be considered as a possible mechanism for extra mixing through the radiative zone, as is required by both stellar observations and the extensive isotopic data on circumstellar condensates found in meteorites.