Magnetorotational instability in the presence of composition gradients


Recent space asteroseismic results have revealed that red giant cores are rotating far slower than theoretically expected. State-of-the art stellar evolution codes cannot reproduce these results even using extreme values for the parameters of known angular momentum transport mechanisms, suggesting that unknown transport mechanisms are operating. Evolved stars have dynamically significant composition gradients in their cores. Understanding how angular momentum transport proceeds in the presence of such gradients is of crucial importance in understanding the rotational evolution of low-mass, post-main sequence stars with radiative cores. We will report on our investigations into the local, axisymmetric, linear stability of differentially rotating, magnetized, stratified fluids in the presence of composition gradients. This is a generalization of the classical magnetorotational instability, adding not only stratification and composition gradients but also the diffusion of momentum, heat, and magnetic field. Previous work has demonstrated that such instabilities can transport angular momentum in solar-like interiors without composition gradients. We will present a survey of the conditions under which magnetorotational instabilities are present when composition gradients are important, and comment on the applicability of this mechanism to evolved, low-mass stars.

American Astronomical Society Meeting Abstracts