The bimodal metallicity distribution function of the globular clusters in the Galaxy: halo disc complementarity
1
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: eca@ll.iac.es
2
Consejo Superior de Investigaciones Científicas, Spain e-mail: jeb@ll.iac.es
Received:
7
August
2005
Accepted:
31
October
2005
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
and at [ Fe/H
. The first step is to verify the statistical
significance of these peaks, and the insignificance of a much smaller peak at
[ Fe/H
. 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.
© ESO, 2006