  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.
