Testing the AGB Scenario as the Origin of the Extreme-Helium Population in omega Centauri

TL;DR

This study tests whether asymptotic giant branch stars can explain the extreme helium enrichment in omega Centauri, finding that the AGB scenario cannot account for the observed high helium levels and chemical abundances.

Contribution

The paper provides a comprehensive simulation showing the failure of the AGB scenario to reproduce the extreme helium and chemical enrichment observed in omega Centauri.

Findings

Maximum-AGB models cannot produce Y ~ 0.4 helium abundance.

Models fail to match observed C and N abundances.

AGB scenario is insufficient to explain the cluster's chemical properties.

Abstract

The most massive Galactic globular cluster, Omega centauri, appears to have multiple populations. Its bluest main sequence and extended horizontal branch stars are suggested to have the common origin, that is, an extremely high helium abundance of Y ~ 0.4. The high helium abundance is most often attributed to asymptotic giant branch (AGB) stars. In this study we test the AGB hypothesis. We simulate the maximum-AGB models where the impact of AGB stars is maximised by assuming that supernova explosions do not affect the chemical evolution of the proto cloud. We compare the enrichment history of helium, metals, carbon and nitrogen to the observed values. Even under the most generous condition, the maximum-AGB models fail to reproduce the large values of helium Y ~ 0.4 and helium enrichment parameter Delta Y / Delta Z ~ 70 which were deduced from the colour-magnitude diagram fits. They also fail to reproduce the C and N contents of the blue population spectroscopically determined. We conclude that the AGB scenario with the canonical stellar evolution theory cannot explain the observational constraints and that the self chemical enrichment does not provide a viable solution. Alternative processes are desperately called for.