Star Formation History of ω Centauri Imprinted in Elemental Abundance Patterns

The star formation history of the globular cluster Omega Centauri isinvestigated in the context of an inhomogeneous chemical evolution model inwhich supernovae induce star formation. The proposed model explains recentobservations for Omega Cen stars, and divides star formation into three epochs.At the end of the first epoch, ~ 70% of the gas was expelled by supernovae. AGBstars then supplied s-process elements to the remaining gas during the firstinterval of ~300 Myr. This explains the observed sudden increase in Ba/Feratios in Omega Cen stars at [Fe/H] ~ -1.6. Supernovae at the end of the secondepoch were unable to expel the gas. Eventually, Type Ia supernovae initiatedsupernova-induced star formation, and remaining gas was stripped when thecluster passed through the newly formed disk of the Milky Way. The formation ofOmega Cen is also discussed in the framework of globular cluster formationtriggered by cloud-cloud collisions. In this scenario, the relative velocity ofclouds in the collision determines the later chemical evolution in theclusters. A head-on collision of proto-cluster clouds with a low relativevelocity would have converted less than 1% of gas into stars and promoted thesubsequent chemical evolution by supernova-driven star formation. This isconsistent with present observed form of Omega Cen. In contrast, the otherGalactic globular clusters are expected to have formed from more intensehead-on collisions, and the resultant clouds would have been too thin forsupernovae to accumulate enough gas to form the next generation of stars. Thisexplains the absence of chemical evolution in these other globular clusters.Comment: 7 pages including 3 figures, to appear in Ap