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

Highly He-Rich Matter Dredged Up by Extra Mixing through Stellar Encounters in Globular Clusters