Astronomy & Astrophysics

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Issue		A&A Volume 385, Number 1, April I 2002
Page(s)		67 - 86
Section		Stellar clusters and associations
DOI		10.1051/0004-6361:20011771

A&A 385, 67-86 (2002)
DOI: 10.1051/0004-6361:20011771

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

S. Feltzing¹ and R. A. Johnson²

¹ Lund Observatory, Box 43, 221 00 Lund, Sweden
² Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK
e-mail: rjohnson@eso.org

(Received 20 July 2001 / Accepted 11 December 2001 )

Abstract
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 the resulting 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 $11\pm 2$ 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 $<V_{\rm 555} < 19$ (i.e. the red giant branch and horizontal branch) we find, for the Galactic bulge, $\sigma_{l}= 0.33\pm 0.03$ and $\sigma_{b}= 0.25\pm0.02$ arcsec per century. This give $\sigma_{l}/\sigma_{b}=1.32\pm0.16$ , 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.

Key words: Galaxy: globular clusters: individual: NGC 6528 -- Galaxy: bulge -- Galaxy: kinematics and dynamics

Offprint request: S. Feltzing, sofia@astro.lu.se

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