  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 (RGB) on the evolution of stars
through the horizontal branch. 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 color of horizontal branch models are
distributed over almost all range of horizontal branch. The results are
discussed in relation to the scenario for the origin of the abundance anomalies
and for the effects on the morphology of horizontal branch. We argue that the
star-star interactions can explain not only the source of 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 gravo-thermal oscillations as the
second parameter to explain the variations in HB morphology among the globular
clusters.
