Using Asteroseismology to Locate Differential Rotation in Red Giants

Hannah Klion, University of California, Berkeley

The physics of angular momentum transport in stellar evolution remain uncertain. Stellar evolution timescales are much longer than the hydrodynamic timescale in a star, so stellar evolution codes must treat rotation approximately. One common assumption is that the large diffusion coefficients in convective zones allow efficient angular momentum transport. This would imply that convective regions undergo solid-body rotation. However, three-dimensional simulations of rotating convection find that, instead, rotation falls off as a power law with radius.

Until recently, it has not been possible to resolve this discrepancy observationally. The first probes of the core rotation of stars are coming from Kepler and CoRoT, which have observed rotational frequency splittings of asteroseismic mixed modes in red giant stars. Though these observations have confirmed that the cores of stars rotate more rapidly than their envelopes, few constraints have been placed on the location of the differential rotation.

Our goal is to understand the circumstances under which it is possible to find the differential rotation. We compare rotational frequency splittings for two classes of rotation profiles: one where the differential rotation is concentrated just outside of the helium core and another where the differential rotation follows a power law within the convective envelope. Our analysis includes a representative red giant model and a model similar to Kepler-56. We find that a measurement of the surface rotation rate is critical for the interpretation of the splittings. Additionally, it may be possible to locate the differential rotation in younger red giants, but the larger size of older stars greatly reduces the dependence of the splittings on the rotation profile.

Abstract Author(s): H. Klion, E. Quataert