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2010 Spring Symposium

Multiple Populations in Globular Clusters: Massive Binaries as the Source of Abundance Anomalies

Presented by: Selma de Mink  (Utrecht University)
Category: Science Symposium   Duration: 20 minutes   Broadcast date: May 03, 2010
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Various features in the color magnitude diagrams of globular clusters indicate the presence of multiple stellar populations within one cluster differing in chemical composition and possibly age. It has been proposed that the more massive stars in the cluster enriched their surroundings with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch (AGB) stars and massive stars rotating near the break-up limit ("spin stars"). A challenge for all proposed scenarios is to provide the large amount of ejecta required to form subsequent stellar populations that are equally or even more numerous than the first population. We propose massive binaries as a promising additional source of enrichment and we argue why this source may be more important than AGB stars and "spin stars", at least in terms of the amount of ejected mass. To demonstrate the principle we compute the evolution of a typical 20 solar mass star in a close binary considering the effects of mass transfer, rotation and tidal interaction. We find that this system sheds about 10 solar masses of material, about 5 times than in the "spin star"-scenario. This material is probably ejected at low velocity. We expect that it remains inside the potential well of the cluster and becomes available for the formation or pollution of a second stellar generation. Given the high fraction of close binaries among massive stars in nearby loose OB associations, -- a fraction which may be even higher in the dense center of the young progenitor of massive globular clusters --, it is likely that the majority of massive stars interact with a companion and contribute to the self enrichment of the cluster. This scenario strongly relieves the need to adopt commonly made assumptions such as strong preferential loss of the first generation of stars form the cluster, a very high fraction of fast rotating stars among massive stars, external pollution or an anomalous initial mass function.

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Selma de Mink PowerPoint (.ppt)