SIMBAD references

CCD vby Stroemgren photometry of a statistically complete sample of red giants and stars in the main sequence turn-off region in ω Centauri has been used to analyse the apparently complex star formation history of this cluster. From the location of stars in the (b-y),m₁ diagram metallicities have been determined. These have been used to estimate ages of different sub-populations in the color-magnitude diagram and to investigate their spatial distributions. We can confirm several earlier findings. The dominating metal-poor population around -1.7dex is the oldest population found. More metal-rich stars between [Fe/H]=-1.5 and -1.0dex tend to be 1-3Gyr younger. These stars are more concentrated towards the cluster center than the metal-poor ones. The most-metal rich stars around -0.7dex might be up to 6Gyr younger than the oldest population. They are asymmetrically distributed around the center with an excess of stars towards the South. We argue that the Stroemgren metallicity in terms of element abundances has another meaning than in other globular clusters. From a comparison with spectroscopic element abundances, we find the best correlation with the sum C+N. The high Stroemgren metallicities, if interpreted by strong CN-bands, result from progressively higher N and perhaps C abundances in comparison to iron. The large scatter of Stroemgren abundances may come from a variety of evolutionary effects, including C-depletion and N-enrichment. We see an enrichment already among the metal-poor population, which is difficult to explain by self-enrichment alone. An attractive speculation (done before) is that ω Cen was the nucleus of a dwarf galaxy. We propose a scenario in which ω Cen experienced mass inflow over a long period of time, until the gas content of its host galaxy was so low that star formation in ω Cen stopped, or alternatively the gas was stripped off during its infall in the Milky Way potential. This mass inflow could have occurred in a clumpy and discontinuous manner, explaining the second peak of metallicities, the abundance pattern, and the asymmetric spatial distribution of the most metal-rich population. Moreover, it explains the kinematic differences found between metal-poor and metal-rich stars.