Studies of the Multiple Stellar Populations in Globular Clusters

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Globular clusters (GCs) are nearly spherical aggregates that typically contain a few million stars. Until recently, it was thought that all of the stars in an individual GC formed at the same time (more than 10 billion years ago), with the same initial chemical composition. However, based largely on data obtained with the Hubble Space Telescope, observers have established that GCs harbour multiple stellar populations, with one or more younger generations formed from gas that was expelled by the preceding generation(s). Analyses of their spectra indicates that the younger stars probably formed from the ejecta of rapidly rotating massive stars during their core hydrogen-burning phase or from the winds of intermediate-mass stars that burn hydrogen and helium in thin shells (so-called "asymptotic-giant-branch stars") - but our understanding of these phenomena is still very crude. The main goal of the proposed research is to develop a satisfactory model to explain the origin of multiple stellar populations in GCs. To accomplish this, we will generate numerical models to follow the evolution of first-generation (FG) massive stars and try to adjust mixing parameters so that the chemical composition of their ejecta agrees with the observed metal abundances of the subsequent generation(s). Mixing, which can be caused by convection, rotation, or other factors, is important because it redistributes the relative abundances of the isotopes of elements that participate in nuclear reactions. Wherever possible, the adopted mixing prescriptions in our simulations will be based on the results of multi-dimensional hydrodynamical models. The chemical composition of the ejecta from FG stars will be the starting point for models of the chemical and dynamical evolution of GCs that fully take into account the cooling flows that are produced by the material that is ejected by massive and intermediate-mass stars. While developing this model, we will continue to make detailed comparisons of predicted and observed colour-magnitude diagrams in order to constrain, in particular, the helium abundances in the different stellar populations, but also the abundance variations of the most important heavy elements (including O, Mg, and Si).