Testing the AGB scenario as the origin of the extreme-helium population in ω Centauri

Abstract

The most massive Galactic globular cluster, ω 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 maximized by assuming that supernova explosions do not affect the chemical evolution of the protocloud. 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 ΔV/ΔZ ∼ 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.