Rotational evolution of solar-type stars with core-envelope decoupling

Abstract

The anomalously rapid decrease in the surface rotation rates of G dwarfs between the ages of the α Persei and Pleiades clusters has been attributed by some authors to the temporary decoupling of the convective envelope from the radiative interior. We investigate the implications of this suggestion for the evolution of the rotational velocity distributions in open clusters using a parametrized model of standard magnetic braking with varying strength of core-envelope coupling. We find that for solar-type stars only a weak coupling model, in which little of the angular momentum stored in the core of a fast rotator has been transferred to the envelope by the age of the Hyades, can provide a reasonable fit to the observed rotational distributions at the ages of the α Per, the Pleiades and the Hyades clusters. Models with moderate core- envelope coupling, in which solid-body rotation is almost restored by the age of the Hyades, give too high a mean rotation rate at the Hyades age and so can be ruled out. The weak coupling model has no strong observational objections but requires an extremely slow rotation rate for the early Sun.