9 Discussion and summary

Colour-magnitude diagrams of the metal-rich globular cluster NGC 6528 are notoriously difficult to analyze. This is due to the fact the cluster is situated in the Galactic bulge and thus the fields stars belonging to the bulge have the same magnitudes as the stars in NGC 6528.

Using two epochs of observations with HST/WFPC2 we obtain the stellar proper motions for all stars in the field. The proper motions are used to separate the bulge from the cluster stars. The stellar sequences in the resulting colour-magnitude diagram are better defined than in any previously published colour-magnitude diagram.

Using $\alpha$-enhanced stellar isochrones we find NGC 6528 to have a probable age of $11\pm 2$ Gyrs, this is the first attempt to establish the absolute age of NGC 6528. Previous studies have only compared the fiducial ridge line for the cluster to that of other globular clusters of similar metallicities. Mainly the comparisons have been with regards to NGC 6553 and 47 Tuc. With the new metallicity determinations for individual stars in both NGC 6553 and NGC 6528 it is now clear that 47 Tuc (at -0.71 dex) has a significantly lower metallicity than NGC 6528 and NGC 6553 and is thus not a suitable comparison cluster as regards differential age determinations (see e.g. Stetson et al. 1996 and references therein).

The distance modulus obtained when fitting the stellar isochrones yields a distance to NGC 6528 of 7.2 kpc, fully compatible with previous derivations.

A comparison with the fiducial line for NGC 6553 confirms results in earlier studies, e.g. Ortolani et al. (1995), that the two clusters indeed have very similar ages.

The stellar proper motion also provide velocity dispersions for both the cluster and field stars. The velocity dispersion of the cluster is most likely dominated by measurement errors. The bulge dispersion can thus be found by deconvolution. The resulting dispersions are consistent with what has previously (Spaenhauer et al. 1992) been found for Bulge giants. Moreover, combining our results with those by Zoccali et al. (2001) we are able to confirm the difference in $\sigma_{\rm l}$ at two positions in the bulge as predicted by the model in Zhao (1996).

To our knowledge, our and Zoccali et al.'s study are the first to address the proper motions amongst bulge horizontal branch and fainter stars.

Finally we derive the mean proper motion of NGC 6528 relative to the Galactic bulge and also the space velocities. We find $<\mu_l>\,=\,\sim$+0.006 and $<\mu_b>\,=\,\sim$+0.044 arcsec per century and ($\Pi$, $\Theta$, W) = ( - 142, 303, 4) km s^-1.

Acknowledgements

We thank the Royal Swedish Academy of Sciences for a collaborative grant that enabled visits to Cambridge, for SF, and to Lund, for RAJ.