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

Abstract

The unveiled main-sequence splitting in ω 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 blueshift 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 ~2 M☉, 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 of (1) escaping from a GC or (2) being captured by kinematically cool stars through encounters. The AGB ejecta in ω Cen, which follows the latter case, can supply sufficient He to cause the observed blueshift. Simultaneously, this scheme generates the extreme horizontal branch, as observed in ω Cen, in response to the higher mass-loss rates, which is also caused by stellar encounters.