  The unveiled main-sequence splitting in omega Centauri as well as NGC 2808
suggests that matter highly-enriched in He (in terms of its mass fraction
Y~0.4) was produced and made the color of some main-sequence stars bluer in
these globular clusters (GCs). The potential production site for the He-rich
matter is generally considered to be massive AGB stars that experience the
second dredge-up. However, it is found that massive AGB stars provide the
matter with Y~0.35 at most, while the observed blue-shift requires the presence
of Y~0.4 matter. Here, we show that extra mixing, which operates in the red
giant phase of stars less massive than ~2Msun, could be a mechanism that
enhances He content in their envelopes up to Y~0.4. The extra mixing is
supposed to be induced by red giant encounters with other stars in a
collisional system like GCs. The Y~0.4 matter released in the AGB phase has
alternative fates to (i) escape from a GC or (ii) be captured by kinematically
cool stars through encounters. The AGB ejecta in omega Cen, which follows the
latter case, can supply sufficient He to cause the observed blue-shift.
Simultaneously, this scheme generates the extreme horizontal branch, as
observed in omega Cen in response to the higher mass loss rates, which is also
caused by stellar encounters.
