The chemical evolution of Omega Centauri's progenitor system

Abstract

Chemical evolution models are presented for the anomalous globular cluster Omega Centauri. After demonstrating that the chemical features of Omega Cen can not be reproduced in the framework of the closed-box self-enrichment scenario, we discuss a model in which this cluster is the remnant of a dwarf spheroidal galaxy evolved in isolation and then swallowed by the Milky Way. Both infall of primordial matter and metal-enriched gas outflows have to be considered in order to reproduce the stellar metallicity distribution function, the age-metallicity relation and several abundance ratios. Yet, as long as an ordinary stellar mass function and standard stellar yields are assumed, we fail by far to get the enormous helium enhancement required to explain the blue main sequence (and, perhaps, the extreme horizontal branch) stellar data. Rotating models of massive stars producing stellar winds with large helium excesses at low metallicities have been put forward as promising candidates to solve the `helium enigma' of Omega Cen (Maeder & Meynet, 2006, A&A, 448, L37). However, we show that for any reasonable choice of the initial mass function the helium-to-metal enrichment of the integrated stellar population is unavoidably much lower than 70 and conclude that the issue of the helium enhancement in Omega Cen still waits for a satisfactory explanation. We briefly speculate upon possible solutions.