The bimodal metallicity distribution function of the globular clusters in the Galaxy: halo disc complementarity
Aims.Our aim in this paper is to present an explanatory scenario for the formation of the observed relatively metal rich globular clusters associated with the thick disc of the Galaxy, distinct from the mode of formation of the lower metallicity halo clusters.Methods.The observations to be accounted for here are the two peaks in the metallicity distribution of the thick disc globular clusters, at [ Fe/H] ∼ -0.7 and at [ Fe/H] ∼ -0.4. The first step is to verify the statistical significance of these peaks, and the insignificance of a much smaller peak at [ Fe/H] ∼ -0.2. The basic model assumption is that these globular clusters were formed as the most massive long term survivors of a much larger set of open clusters whose epochs of formation coincided with the main epochs of star formation in the thin disc. These latter are identified using established data sets giving the local stellar frequency distribution in time based on stellar activity indices.Results.Our simple stellar accretion model accounts reasonably for the presence of the observed peaks in the cluster metallicity distribution, and the long time constant for the accretion as a massive cluster moves through the stellar environment explains qualitatively why the most recent peak in the local star formation rate has not yet given rise to a corresponding peak in the globular cluster distribution. It also explains in broad terms how a uniform process of cluster formation originating both open clusters and disc globular clusters can yield the observed high numbers of open clusters and the few surviving globulars.