Globular cluster formation with multiple stellar populations: A comprehensive overview of a star-cloud interaction scenario

Abstract

We present a new scenario of globular cluster (GC) formation with multiple stellar populations (MPs) in which both the first and second populations (1P and 2P, respectively) of stars form from giant molecular clouds (GMCs) polluted by asymptotic giant branch (AGB) stars within and around the GMCs. Unlike previous GC formation scenarios with AGB stars being the primary polluters, the new scenario alleviates tensions with the mass-budget and dilution-timing problems The principal results based on idealized analytic models of the formation scenario are as follows. The observed fraction of 1P stars and the helium abundance spreads between the 1P and 2P as a function of GC masses can be well reproduced. The modelled GCs show O-Na, C-N, and Mg-Al anticorrelations and Si-Al, 25Mg-Al, 26Mg-Al correlations. The observed Mg-K anticorrelation can be reproduced, only if super AGB stars make a significant contribution to chemical enrichment within GMCs. The lack of correlations of Li abundances with [Na/Fe] and [Al/Fe] can be reproduced, only if about 20% of the polluting AGB stars produce Li-rich ejecta, which disfavours scenarios with polluters incapable of Li production. Iron-complex, Type-II GCs can be formed through merging of two GCs formed from two GMCs within a host dwarf galaxy at different epochs. The new scenario predicts young massive clusters formed in galaxy environments with surface star formation rate densities well below 1 M_sun/yr/kpc^2 are unlikely to evolve into GCs with MPs. It also predicts low 12C/13C ratios of 2P (~5), a [Na/Fe]-[F/Fe] anticorrelation, and P-rich star formation with [P/Fe]$>0.5$ and [N/Fe]>0.5. These predictions are tested against more than 30 observed properties of GCs with MPs, representing one of the most comprehensive observational benchmarks against a specific GC formation scenario to date.