Most Galactic globular clusters are (even paradigmatic) representatives of the Galactic halo population. However, there is a sample of globular clusters, for which considerable uncertainty exists with respect to their role in evolution and structure of the Milky Way.
Clusters more metal-rich than -0.8 dex (on the scale of Zinn & West 1984) have been assigned to the "disk'' population of globular clusters (Zinn 1985; Armandroff 1993). Other authors (Ortolani et al. 1992; Minniti 1995) in contrast see them as "bulge'' clusters. Burkert & Smith (1997) even discuss substructure among this group. Apart from 47 Tuc and M 71, few spectroscopic abundance determinations based on individual stars have been carried out. François (1991) obtained dex for one star in NGC 5927 and Barbuy et al. (1992) measured dex for one star in NGC 6553. Fullton et al. (1995) investigated NGC 6352 and found it very similar to 47 Tuc. Many of these clusters are seen in projection onto the Galactic bulge, which means severe contamination by field stars. For these clusters, abundances are still mainly based on integrated colours if available at all. Additionally, the clusters are strongly reddened, which complicates the interpretation of both colour-magnitude diagrams (CMDs) and spectroscopy. Therefore, the metallicity scale of globular clusters in the metal-rich domain is not yet well established.
Richtler et al. (1994) investigated the clusters NGC 6496, NGC 6624, and NGC 6637 (M 69) and found them to be distinctly more metal-poor than what had been previously suggested. Also their minimal orbit inclinations do not indicate an association with the Galactic disk. More and better information about these difficult objects is clearly needed to understand their role within the Galactic globular cluster system.
NGC 6528 is particularly difficult to analyze. On the scale of Zinn & West (1984), it holds the record with a metallicity of +0.2 dex. An early photographic study from van den Bergh & Younger (1979) found the cluster to be quite metal-rich, but the usefulness of photographic data in such crowded fields is very limited. Armandroff & Zinn (1988) determined the metallicity of NGC 6528 to be -0.23 dex based on the infrared Ca triplet in integrated cluster spectra. Ortolani et al. (1992) (herafter OBB) give results for metallicity, reddening, and distance together with other basic cluster data.
The turn-off region is not clearly identifiable even in the CCD CMDs of OBB. Thus the question of whether NGC 6528 could be younger than the majority of galactic globular clusters, as has been suggested for disk globular clusters, remained unanswered. After the first version of our paper was submitted, Ortolani et al. (1995) published HST observations of NGC 6528, showing unambiguously the turn-off of the main sequence location. They also estimated the age placing NGC 6528 among the old globular clusters of the Galaxy. Also for NGC 6352, another "disk cluster'', an old age has been found (Fullton et al. 1995).
Regarding the turn-off location, the HST data is clearly superior over ground based data. We thus dropped the discussion of the age completely. However, the metallicity could not be quantified in the HST study beyond the statement that it is a quite metal-rich cluster.
A remarkable feature pointed out in the study of OBB is the "curved giant branch'', which OBB attribute to strong blanketing due to the high metallicity of that cluster. They even locate the red giant branch tip at a V-band brightness fainter than the horizontal branch. These peculiarities together with the fact that this cluster may be at the upper end of the metallicity scale of globular clusters makes an independent study worthwhile.
|E(B-V)||0.55||Ortolani et al. (1992)|
|Distance [kpc]||7.5||Ortolani et al. (1992)|
|Armandroff & Zinn (1988)|
|Armandroff & Zinn (1988)|