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 \sim 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 \sim 0.35 at most , while the observed blue-shift requires the presence of Y \sim 0.4 matter .
Here , we show that extra mixing , which operates in the red giant phase of stars less massive than \sim 2 M _ { \odot } , could be a mechanism that enhances He content in their envelopes up to Y \sim 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 \sim 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 .