A new, cleaner colour-magnitude diagram for the metal-rich globular cluster NGC 6528. Velocity dispersion in the Bulge, age and proper motion of NGC 6528

Abstract:
Using two epochs of HST/WFPC2 images of the metal-rich globular clusterNGC 6528 we derive the proper motions of the stars and use them toseparate the stars belonging to NGC 6528 from those of the Galacticbulge. The stellar sequences in the resulting colour-magnitude diagramfor the cluster are significantly better determined than in previouslypublished data. From comparison of the colour-magnitude diagram with thefiducial line for NGC 6553 from Zoccali et al. (cite{Zo01}) we concludethat 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 kpcaway from us. From the measured velocities both the proper motion of thecluster and the velocity dispersion in the Galactic bulge are found. NGC6528 is found to have a proper motion relative to the Galactic bulge of<mu<SUB>l</SUB> > = 0.006 and <mu <SUB>b</SUB>> = 0.044arcsec per century. Using stars with ~ 14 <V<SUB>555</SUB> < 19(i.e. the red giant branch and horizontal branch) we find, for theGalactic bulge, sigma <SUB>l</SUB>= 0.33+/- 0.03 and sigma <SUB>b</SUB>=0.25+/-0.02 arcsec per century. This give sigma <SUB>l</SUB>/sigma<SUB>b</SUB>=1.32+/-0.16, consistent both with previous proper motionstudies of K giants in the Galactic bulge as well as with predictions bymodels of the kinematics of bulge stars. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc. under NASA contract No. NAS5-26555. TablesA1, A2 and A3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/385/67