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[Abbreviated] We have investigated the color-magnitude diagram of OmegaCentauri and find that the blue main sequence (bMS) can be reproduced only bymodels that have a of helium abundance in the range Y=0.35-$0.40. To explainthe faint subgiant branch of the reddest stars ("MS-a/RG-a" sequence),isochrones for the observed metallicity ([Fe/H]\approx0.7) appear to requireboth a high age (~13Gyr) and enhanced CNO abundances ([CNO/Fe]\approx0.9$).Y~0.35 must also be assumed in order to counteract the effects of high CNO onturnoff colors, and thereby to obtain a good fit to the relatively blue turnoffof this stellar population. This suggest a short chemical evolution period oftime (<1Gyr) for Omega Cen. Our intermediate-mass (super-)AGB models are ableto reproduce the high helium abundances, along with [N/Fe]~2 and substantial Odepletions if uncertainties in the treatment of convection are fully taken intoaccount. These abundance features distinguish the bMS stars from the dominant[Fe/H] $\approx1.7$ population. The most massive super-AGB stellar models(M_zams>=6.8M_sun, M_He,core>=1.245M_sun) predict too large N-enhancements,which limits their role in contributing to the extreme populations. We showquantitatively that highly He- and N-enriched AGB ejecta have particularlyefficient cooling properties. Based on these results and on the reconstructionof the orbit of Omega Cen with respect to the Milky Way we propose the galacticplane passage gas purging scenario for the chemical evolution of this cluster.Our model addresses the formation and properties of the bMS population(including their central location in the cluster). We follow our modeldescriptively through four passage events, which could explain not only somekey properties of the bMS, but also of the MS-a/RGB-a and the s-enriched stars.

From the CMD of Omega Centauri and (super-)AGB stellar models to a Galactic plane passage gas purging chemical evolution scenario