paper

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

arXiv:0710.5271 · doi:10.1086/524943

Abstract

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.

Accepted for publication in ApJ Letters

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