We have investigated the color-magnitude diagram of \omega Centauri and find that the blue main sequence ( bMS ) can be reproduced only by models that have a of helium abundance in the range Y = 0.35 – 0.40 .
To explain the faint subgiant branch of the reddest stars ( “ MS-a/RG-a ” sequence ) , isochrones for the observed metallicity ( [ Fe/H ] \approx - 0.7 ) appear to require both a high age ( \sim 13 Gyr ) and enhanced CNO abundances ( [ CNO/Fe ] \approx 0.9 ) . Y \approx 0.35 must also be assumed in order to counteract the effects of high CNO on turnoff colors , and thereby to obtain a good fit to the relatively blue turnoff of this stellar population .
This suggest a short chemical evolution period of time ( < 1 Gyr ) for \omega Cen .
Our intermediate-mass ( super- ) AGB models are able to reproduce the high helium abundances , along with [ N/Fe ] \sim 2 and substantial O depletions if uncertainties in the treatment of convection are fully taken into account .
These abundance features distinguish the bMS stars from the dominant [ Fe/H ] \approx - 1.7 population .
The most massive super-AGB stellar models ( M _ { ZAMS } \geq 6.8 { M } _ { \odot } , M _ { \mathrm { He,core } } \geq 1.245 { M } _ { \odot } ) predict too large N-enhancements , which limits their role in contributing to the extreme populations .
In order to address the observed central concentration of stars with He-rich abundance we show here quantitatively that highly He- and N-enriched AGB ejecta have particularly efficient cooling properties .
Based on these results and on the reconstruction of the orbit of \omega Cen with respect to the Milky Way we propose the galactic plane passage gas purging scenario for the chemical evolution of this cluster .
The bMS population formed shortly after the purging of most of the cluster gas as a result of the passage of \omega Cen through the Galactic disk ( which occurs today every \sim 40 \mathrm { Myrs } for \omega Cen ) when the initial-mass function of the dominant population had “ burned ” through most of the Type II supernova mass range .
AGB stars would eject most of their masses into the gas-depleted cluster through low-velocity winds that sink to the cluster core due to their favorable cooling properties and form the bMS population .
In our discussion we follow our model through four passage events , which could explain not only some key properties of the bMS , but also of the MS-a/RGB-a and the s -enriched stars .