Using two epochs of observations with HST/WFPC2 we obtain the stellar proper motions for all stars in the field. The proper motion are used to separate the bulge from the cluster stars. The stellar sequences in the resulting colour-magnitude diagram (CMD) are better defined than in any previously published CMD. Using a-enhanced stellar isochrones we find NGC 6528 to have a probable age of 11 +/- 2 Gyrs, this is the first attempt to establish the ab. solute 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. 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. From the measured velocities both the proper motion of the,cluster and the velocity dispersion in the Galactic Bulge are found. NGC 6528 is found to have a proper motion relative to the Galactic Bulge of < mu(l) > = 0.006 and < mu(b) > = 0.044 arcsec per century. Using stars with similar to 14 < V-555 < 19 (i.e. the red giant branch and horizontal branch) we find, for the Galactic Bulge, sigma(l) = 0.33 +/- 0.03 and sigma(b) = 0.25 +/- 0.02 arcsec per century. This give sigma(l)/sigma(b) = 1.32 +/- 0.16, consistent both with previous proper motion studies of K giants in the Galactic bulge as well as with predictions by models of the kinematics of bulge stars.
|Conference||Conference on Observed HR Diagrams and Stellar Evolution|
|Period||2001/06/18 → 2001/06/22|
- Astronomy, Astrophysics and Cosmology