FROM THE COLOR-MAGNITUDE DIAGRAM OF ω CENTAURI AND (SUPER-)ASYMPTOTIC GIANT BRANCH STELLAR MODELS TO A GALACTIC PLANE PASSAGE GAS PURGING CHEMICAL EVOLUTION SCENARIO

摘要

We have investigated the color-magnitude diagram of ω Centauri and find that the blue main sequence (bMS) can be reproduced only by models that have a 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]?≈–0.7) appear to require both a high age (~13 Gyr) and enhanced CNO abundances ([CNO/Fe] ≈0.9). Y ≈ 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 suggests a short chemical evolution period of time (1 Gyr) for ω Cen. Our intermediate-mass (super-)asymptotic giant branch (AGB) models are able to reproduce the high helium abundances, along with [N/Fe] ~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] ≈–1.7 population. The most massive super-AGB stellar models (M ZAMS ≥ 6.8 M ☉, M He, core ≥ 1.245 M ☉) predict too large N enhancements, which limit 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 ω 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 ω Cen through the Galactic disk (which occurs today every ~40 Myr for ω 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 some key properties not only of the bMS but also of the MS-a/RGB-a and the s-enriched stars.