Role of Supergiants in the Formation of Globular Clusters

Szécsi, Dorottya and Wunsch, R. (2019) Role of Supergiants in the Formation of Globular Clusters. ASTROPHYSICAL JOURNAL, 871 (1). ISSN 1538-4357

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Abstract

Multiple stellar populations are observed in almost all globular clusters, but the origin of this phenomenon is still debated. We investigate the role cool supergiants may have played. To do this, we combine two investigative methods: state-of-the-art massive stellar evolution and calculations of the hydrodynamic structure of the cluster gas. This approach allows us to study how star formation in young massive clusters depends on the energy and mass input of the first generation of stars, while predicting the chemical composition of the second generation. We find that the presence of massive (9-500M(circle dot)) metal-poor supergiants in the young cluster leads to a star formation episode within the first 4 Myr of the cluster's lifetime, that is, before the first core-collapse supernovae explode or the gas is expelled. The stellar winds accumulate in the cluster center, forming the second generation there. Its composition is predicted to show variations in O and Na abundances, consistently with observations. The abundance of helium is, similarly to other scenarios involving massive stars, higher than what is inferred from observations. Supposing dynamical removal of stars from the outskirts of the cluster, or applying a top-heavy initial mass function, we can predict a number ratio of the second generation as high as 20%-80%. The effect of metallicity is shown to be important, as the most luminous supergiants are only predicted at low metallicity, thus limiting-but not excluding-the extent of a polluted second generation at high metallicity. These massive stars becoming black holes suggest globular clusters hosting gravitational-wave progenitors. Our scenario predicts a correlation between the mass of the cluster and the extent of the multiple-population phenomenon.

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