It is clear that NGC 6528 belongs to the most metal-rich globular clusters in the Milky Way.
Minniti (1995) compiled evidence that the metal-rich clusters termed "disk clusters'' by Zinn (1985) do belong to the Milky Way bulge rather than to the thick disk. While there may be still halo clusters among this sample, for instance NGC 6624, 6637 and 6496 (Richtler et al. 1994), NGC 6528 possesses all properties consistent with an origin in the Galactic bulge, as was already pointed out by OBB and by Ortolani et al. (1995), including its old age.
NGC 6528 possesses a further property that makes an origin as an object with disk properties very unlikey, namely its radial velocity. Zinn & West (1984) quote a value of +160 km/s, the value of Armandroff & Zinn (1988) is even higher (+180 km/s). Given the fact that the cluster is almost projected onto the Galactic center, a circular orbit can be excluded, since this would imply a an almost vanishing radial component. The circular velocity assuming at the cluster's galactocentric distance is about 200 km/s and thus still the cluster must have an appreciable radial component towards us. A distance of 7.5 kpc, as given by OBB, means a galactocentric distance of only 0.7 kpc and one could reasonably assume that the cluster then would be close to its perigalacticon. However, this distance does not fit to our finding that the cluster is located in the foreground with respect to the red field stars. Moreover, an orbit with low excentricity so close to the center would probably not allow the cluster to survive dynamically for more than 10 Gyr.
That leads to the conclusion that the cluster orbit must be highly excentric, a further reason against an origin in the disk. On the other hand, its high metallicity is an argument for a birth place not too far away from the Galactic center, i.e. in the bulge. One is tempted to speculate that the conditions at birth which put this cluster with a large initial velocity into an eccentric orbit, may best be realised in a chaotic environment with gaseous subunits of the central protogalaxy strongly interacting. This would fit to the meanwhile numerous observations that globular clusters are formed in dynamically hot environments, such as galaxy interactions.
Note added in proof: Meanwhile, the Hipparcos data have been used to recalibrate the relation between metallicity and HB-brightness of globular clusters. The presently available results from different workers differ by about 0.2 mag, thus a final answer is still pending (e.g. Gratton, et al., 1997, ApJ (in press); Pont, et al., 1997, A&A (in press)).
Acknowledgements
T.R. wants to thank the Humboldt Foundation for generous support and the Indian Institute of Astrophysics for hospitality and financial support. EKG acknowledges support by a fellowship of the DFG Graduiertenkolleg "Magellanic Clouds'' and by the German Space Agency (DARA) (grant 05 OR 9103 0). Furthermore, we thank S. Ortolani for mailing his photometric data. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.