Using two epochs of HST/WFPC2 images of the metal-rich globular cluster NGC 6528 we derive the proper motions of the stars and use them to separate the stars belonging to NGC 6528 from those of the Galactic bulge. The stellar sequences in theresulting colour-magnitude diagram for the cluster are significantly better determined than in previously published data. From comparison of the colour-magnitude diagram with the fiducial line for NGC 6553 from Zoccali et al. (2001) we conclude that the two globular clusters have the same age. Further, using alpha-enhanced stellar isochrones, NGC 6528 is found to have an age of 11+-2 Gyr. This fitting of isochrones also give that the cluster is 7.2 kpc away from us. 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 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.
تحميل البحث