On the formation of multiple stellar populations in globular clusters

Nearly all globular clusters (GCs) studied to date show evidence for multiple stellar populations, in stark contrast to the conventional view that GCs are a mono-metallic, coeval population of stars. This generic feature must therefore emerge naturally within massive star cluster formation. Building on earlier work, we propose a simple physical model for the early evolution (several 10⁸ yr) of GCs. We consider the effects of stellar mass loss, Type II supernovae (SNe II) and prompt Type Ia supernovae (SNe Ia), ram pressure, and accretion from the ambient interstellar medium (ISM) on the development of a young GCs own gas reservoir. In our model, SNe II from a first generation of star formation clears the GC of its initial gas reservoir. Over the next several 10⁸ yr, mass lost from asymptotic giant branch stars and matter accreted from the ambient ISM collect at the center of the GC. This material must remain quite cool (T ∼ 10² K), but does not catastrophically cool on a crossing time because of the high Lyman-Werner flux density in young GCs. The collection of gas within the GC must compete with ram pressure from the ambient ISM. After several 10⁸ yr, the Lyman-Werner photon flux density drops by more than three orders of magnitude, allowing molecular hydrogen and then stars to form. After this second generation of star formation, SNe II from the second generation and then prompt SNe Ia associated with the first generation maintain a gas-free GC, thereby ending the cycle of star formation events. Our model makes clear predictions for the presence or absence of multiple stellar populations within GCs as a function of GC mass and formation environment. While providing a natural explanation for the approximately equal number of first-and second-generation stars in GCs, substantial accretion from the ambient ISM may produce fewer chemically peculiar second-generation stars than are observed. Analyzing intermediate-age LMC clusters, we find for the first time evidence for a mass threshold of ∼10⁴ M_⊙ below which LMC clusters appear to be truly coeval. This threshold mass is consistent with our predictions for the mass at which ram pressure is capable of clearing gas from clusters in the LMC at the present epoch. Recently, claims have been made that multiple populations within GCs require that GCs form at the center of their own dark matter halos. We argue that such a scenario is implausible. Observations of the young and intermediate-age clusters in the LMC and M31 will provide strong constraints on our proposed scenario.