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

6. Conclusions

The self-enrichment model of galactic halo GCs (see Paper I for a detailed description) has been compared to the observational situation and the conclusions are as follow:

1. The final metallicity induced by the self-enrichment process depends on , the pressure exerted on the PGCC by the hot protogalactic background medium. The result is in agreement with galactic halo GC metallicities.

2. There is a range of halo GC metallicities due to the variations in with the galactocentric distance. Considering the clusters located between 1 and 30 kpc, the ranges of metallicities [Fe/H] are 0.75 and 1.50 dex when the self-enrichment model is combined with and respectively. The second solution is in better agreement with the observed metallicity range. Moreover, when the Murray & Lin (1992) pressure distribution is used, the theoretical metallicity interval over the galactocentric range is . This result is in nice agreement with the observations in the galactic halo GCs.

3. Because of the expected decrease in with increasing galactocentric distance, the model induces a metallicity gradient throughout the galactic halo. Such a metallicity gradient is indeed observed once the GCs suspected to have been accreted by the Milky Way are removed. These clusters were born in fragments which evaded the initial protogalactic collapse and have experienced their own chemical evolution. The division between the galactic and accreted components of the halo is mainly based on the BHB/RHB (OH/YH) classification introduced by Zinn (1993). Indeed, it seems likely that the halo GCs consist of clusters with more than one origin. Since the accreted clusters did not take part in the formation and early evolution of the galactic halo, their progenitor clouds did not share the same external pressure distribution as the genuine galactic proto-GCs. Therefore, they should not be taken into account in the self-enrichment model.

   Again, the observed radial distribution of GC abundances favours a background pressure profile scaling as rather than .

   However, it should be noted that the scatter of the data about the model lines in Fig. 2 (and also in Fig. 4) exceeds the observational uncertainties. As already stated, this can be due to the GC orbital motions which carry them away from their formation sites. Furthermore, it is certainly an oversimplification to consider that is the only parameter determining the GC metallicity. Other parameters must interfere (e.g. stellar mass range, SNII yields,...).

4. In our model, no age-metallicity relation is required to explain the different GC metallicities. Actually, there is no compelling evidence for an age-metallicity relationship among halo GCs (Buonanno et al. 1998), and especially once the sample is limited to the BHB/OH group (Sarajedini et al. 1997). In this group, all GCs are coeval according to Rosenberg et al. (1999). This is in agreement with our self-enrichment model where we see an enhanced chemical enrichment with decreasing galactocentric distance rather with time.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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