Astron. Astrophys. 363, 526-536 (2000)

1. Introduction

Galactic globular clusters (hereafter GCs) are fossil records of the formation of the Galaxy. The understanding of their formation process would certainly shed light on the early galactic evolution. However, at the present time, there is no widely accepted theory of GC formation. In Parmentier et al. (1999) (hereafter Paper I), we suggest a formation scenario based on a self-enrichment process such as proposed by Cayrel (1986) and further developed by Brown et al. (1991, 1995).

Our self-enrichment scenario takes place within the Fall & Rees (1985) description of the protoGalaxy, namely cold clouds embedded in a hot protogalactic background. These cold clouds are assumed to be the progenitors of galactic halo GCs. Since they are made up of primordial gas, the main advantage of a self-enrichment scenario is that it explains in parallel the formation of the clusters and the origin of their metal contents.

The main target of Paper I was to demonstrate conclusively that the gaseous progenitors of galactic halo GCs are able to sustain a few hundreds of Type II Supernovae (hereafter SNII) without being disrupted. This result is in contrast with the widespread idea according to which a few supernovae are able to disrupt a Proto-Globular Cluster Cloud (hereafter PGCC). Furthermore, the large number of SNeII allowed by our model can explain the amount of metals currently observed in galactic halo globular clusters, and this without any requirement of pre-enrichment of the gas.

The aim of the present paper is to explore further an interesting consequence of Paper I, which is also the main difference existing between our self-enrichment model and the one developed by Brown et al. (1995). The metallicity that a PGCC can reach through self-enrichment depends on the pressure exerted by the medium surrounding the progenitor cloud and, therefore, on the cloud location in the protoGalaxy. The deepest in the protoGalaxy the PGCC is located, the highest the final metallicity induced by self-enrichment will be. Therefore, we expect to find a metallicity gradient throughout the galactic halo.

The paper is organised as follows.

In Sect. 2, we briefly review the self-enrichment model presented in Paper I, focusing on the link between the final metallicity of the PGCC and the pressure exerted on it by the surrounding hot protogalactic background. In Sect. 3, we examine the different arguments suggesting the existence of halo substructures, in order to isolate to which one the self-enrichment model can be safely compared. In Sect. 4, we compare the model with the observations. Sect. 5 explores the putative link between GC metallicities and their perigalactic distances. Finally, we present our conclusions in Sect. 6.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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