The Proper Motion of the Globular Cluster NGC 6553 and of Bulge Stars with HST

Abstract

WFPC2 images obtained with the Hubble Space telescope 4.16 years apart have allowed us to measure the proper motion of the metal rich globular cluster NGC 6553 with respect to the background bulge stars. With a space velocity of ($Π, Θ, W$) = (-3.5, 230, -3) km s$^{-1}$, NGC 6553 follows the mean rotation of both disk and bulge stars at a Galactocentric distance of 2.7 kpc. While the kinematics of the cluster is consistent with either a bulge or a disk membership, the virtual identity of its stellar population with that of the bulge cluster NGC6528 makes its bulge membership more likely. The astrometric accuracy is high enough for providing a measure of the bulge proper motion dispersion and confirming its rotation. A selection of stars based on the proper motions produced an extremely well defined cluster color-magnitude diagram (CMD), essencially free of bulge stars. The improved turnoff definition in the decontaminated CMD confirms an old age for the cluster (~13 Gyr) indicating that the bulge underwent a rapid chemical enrichment while being built up at in the early Universe. An additional interesting feature of the cluster color-magnitude diagram is a significant number of blue stragglers stars, whose membership in the cluster is firmly established from their proper motions.