Internal Rotation of Subdwarf B Stars: Limiting Cases and Asteroseismological Consequences

Observations of the rotation rates of horizontal branch (HB) stars show puzzling systematics. In particular, cooler HB stars often show rapid rotation (with velocities in excess of 10 km s super(-1)), while hotter HB stars (those with T sub(eff) in excess of 11,000 K) typically show much smaller rotation velocities of 8 km s super(-1) or less. Simple models of angular momentum evolution of stars from the main sequence through the red giant branch fail to explain these effects. Assuming solid body rotation throughout produces models that rotate much too slowly. On the other hand, assuming local conservation of angular momentum, but with solid body rotation in the convective regions, produces HB models that also rotate too slowly at all T sub(eff) but preserve a rapidly rotating core. In these cases, the observed angular velocities of HB stars require that some of the angular momentum stored in the core reaches the surface. Models that assume constant specific angular momentum in the surface convection zone have faster rotation in the envelope; while the predictions of these models match the observed rotation rates of the cooler HB stars, hot HB stars rotate more slowly than the models. Thus, while there is not yet a coherent explanation of the trends of rotation on the HB, evolutionary models in all cases preserve a rapidly rotating core. To test the idea that HB stars contain such a core, one can appeal to detailed computations of trace element abundances and rotational mixing. However, a more direct probe is available to test these limiting cases of angular momentum evolution. Some of the hottest horizontal branch stars are members of the pulsating sdB class. They frequently show rich pulsation spectra characteristic of nonradially pulsating stars. Thus, their pulsations probe the internal rotation of these stars and should show the effects of rapid rotation in their cores. Using models of sdB stars that include angular momentum evolution, we explore this possibility and show that some of the sdB pulsators may indeed have rapidly rotating cores.