Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology

It has been known for a long time that some red giants in globular clustersexhibit large star-to-star variations in the abundances of light elements thatare not exhibited by field giants. This fact can be taken as evidence that theextra mixing mechanism(s) that operate in globular cluster giants may beconsequences of star-star interactions in the dense stellar environment. Inorder to constrain the extra mixing mechanism(s), we study the influence ofhelium enrichment along the red giant branch (RGB) on the evolution of starsthrough the horizontal branch. Three possible modes of helium enrichment areconsidered, 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 inthe total mass due to mass loss, the color of horizontal branch models aredistributed over almost all range of horizontal branch. The results arediscussed in relation to the scenario for the origin of the abundance anomaliesand for the effects on the morphology of horizontal branch. We argue that thestar-star interactions can explain not only the source of angular momentum ofrapid rotation but also provide a mechanism for the bimodal distribution ofrotation rates in some globular clusters. We also propose the time elapsed fromthe latest core collapse phase during the gravo-thermal oscillations as thesecond parameter to explain the variations in HB morphology among the globularclusters.Comment: 57 pages, 16 figures, to be published on June 2006 in Ap