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

Abstract

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 thatasaconsequenceofthevariationsinthecoremass,aswellasinthetotalmassduetomassloss,thecolors of horizontal branch models are distributed over almost the entire range of the horizontal branch. The results are discussedinrelationtoascenariofortheoriginof theabundanceanomaliesandfortheeffectsonthemorphologyof 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. Subject headings: globularclusters:general — stars:abundances — stars:evolution — stars:horizontal-branch — stars: interiors — stars: rotation