Abstract

CCD vby Stromgren photometry of a statistically complete sample of red giants and stars in the main sequence turn-off region in omega Centauri has been used to analyse the apparently complex star formation history of this cluster. From the location of stars in the (b - y),m(1) 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 -1.7 dex is the oldest population found. More metal-rich stars between [Fe/H]=-1.5 and -1.0 dex tend to be 1-3 Gyr younger. These stars are more concentrated towards the cluster center than the metal-poor ones. The most-metal rich stars around -0.7 dex might be up to 6 Gyr younger than the oldest population. They are asymmetrically distributed around the center with an excess of stars towards the South. We argue that the Stromgren 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 Stromgren metallicities, if interpreted by strong CN-bands, result from progressively higher N and perhaps C abundances in comparison to iron. The large scatter of Stromgren 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 omega Cen was the nucleus of a dwarf galaxy. We propose a scenario in which omega 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 omega 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.