Rotation of horizontal-branch stars in globular clusters

The rotation of horizontal-branch stars places important constraints on ang ular momentum evolution in evolved stars and therefore on rotational mixing on the giant branch. Prompted by new observations of rotation rates of hor izontal-branch stars, we calculate simple models for the angular momentum e volution of a globular cluster giant star from the base of the giant branch to the star's appearance on the horizontal branch. We include mass loss an d infer the accompanied loss of angular momentum for each of four assumptio ns about the internal angular momentum profile. Mass loss is found to have important implications for angular momentum evolution. These models are com pared to observations of horizontal-branch rotation rates in M13. We find t hat rapid rotation on the horizontal branch can be reconciled with slow sol id body main-sequence rotation if giant-branch stars have differential rota tion in their convective envelopes and a rapidly rotating core, which is th en followed by a redistribution of angular momentum on the horizontal branc h. We discuss the physical reasons that these very different properties rel ative to the solar case may exist in giants. Rapid rotation in the core of the main-sequence precursors of the rapidly rotating horizontal-branch star or an angular momentum source on the giant branch is required for all case s if the rotational velocity of turnoff stars is less than 4 km s(-1). We s uggest that the observed range in rotation rates on the horizontal branch i s caused by internal angular momentum redistribution, which occurs on a tim escale comparable to the evolution of the stars on the horizontal branch. T he apparent lack of rapid horizontal-branch rotators hotter than 12,000 K i n M13 could be a consequence of gravitational settling, which inhibits inte rnal angular momentum transport. Alternative explanations and observational tests are discussed.