Magnetic Fields in Low-mass Evolved Stars

The interiors of low-mass, post-main sequence stars contain deep convective zones, which coupled with the presence of shear, can amplify magnetic fields via dynamo action. If large-scale magnetic fields exist and are sufficiently strong, their presence might influence physical processes occurring during post-MS evolution. In this paper, I outline two such effects. For low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, isotopic abundance measurements require that extensive mixing must occur between the bottom of the convective envelope and the hydrogen burning shell (so called Cool Bottom Processing; CBP). Buoyant magnetic flux tubes provide a physical mechanism and can supply sufficient transport rates to explain CBP but must operate throughout the RGB/AGB. For the later stages of post-MS evolution, strong binary interactions are likely responsible for the high-velocity, bipolar outflows seen in the post-AGB and planetary nebula (PNe) phases. In particular, maser measurements from post-AGB outflows suggest magnetic collimation due to strong fields. It is unlikely that single AGB stars can generate the necessary field strengths to power post-AGB outflows. If a low-mass companion is present, immersion in a common envelope can generate strong magnetic fields which, in turn can drive bipolar outflows. A consistent picture is that a steady, weak dynamo is operating during the RGB/AGB phases (driving mixing), but that a binary companion is required to produce the strong fields seen in post-AGB/PNe.