Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology

TL;DR

This study models how helium enrichment from self-pollution in globular cluster stars affects their evolution and horizontal branch morphology, especially in clusters with strong CNO variations.

Contribution

It introduces stellar evolution models incorporating helium enhancement due to self-pollution, linking chemical variations to horizontal branch features.

Findings

Differences in evolutionary phases are subtle and hard to observe.

Helium enrichment influences the morphology of the horizontal branch.

Helium enrichment may explain blue tails in certain globular clusters.

Abstract

It is becoming clear that `self--pollution' by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Considering that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn--off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752.