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

TL;DR

This paper proposes that stellar encounters in globular clusters induce extra mixing in red giants, producing highly He-enriched matter (Y~0.4) that explains observed main-sequence splitting and extreme horizontal branch features.

Contribution

It introduces a novel mechanism where stellar encounters trigger extra mixing in stars, leading to the production of highly He-enriched matter in globular clusters.

Findings

Extra mixing can increase He content up to Y~0.4 in stars less than 2Msun.

He-rich ejecta can be captured by other stars or escape, influencing cluster evolution.

The model explains observed main-sequence splitting and horizontal branch morphology.

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.