2006ApJ...643..897S

It has been known for a long time that some red giants in globular clusters exhibit large star-to-star variations in the abundances of light elements that are not exhibited by field giants. This fact can be taken as evidence that the extra mixing mechanism(s) that operate in globular cluster giants may be consequences of star-star interactions in the dense stellar environment. In order to constrain the extra mixing mechanism(s), we study the influence of helium enrichment along the red giant branch on the evolution of stars through the horizontal branch (HB). Three possible modes of helium enrichment are considered, associated with close encounters of stars in the globular clusters. We show that as a consequence of the variations in the core mass, as well as in the total mass due to mass loss, the colors of horizontal branch models are distributed over almost the entire range of the horizontal branch. The results are discussed in relation to a scenario for the origin of the abundance anomalies and for the effects on the morphology of the horizontal branch. We argue that the star-star interactions can not only explain the source of the angular momentum of rapid rotation but also provide a mechanism for the bimodal distribution of rotation rates in some globular clusters. We also propose the time elapsed from the latest core-collapse phase during the gravothermal oscillations as the second parameter to explain the variations in HB morphology among the globular clusters.