Preliminary Study of the Stellar Populations and Density Profile of NGC 6624 Using HST

Abstract

We present preliminary results from an ongoing study of the central 5 arcmin2 of NGC 6624 based on short F336W, F439W, and F555W (UBV) Hubble Space Telescope WFPC2 exposures (Yanny et al. 1996). NGC 6624 is a dense, metal rich, post core collapse globular cluster at a distance of 8.1 kpc. Nearly 5000 stars with V ≳ 21 (1.5 mag below the turnoff) are detected within the 34′′ × 34′′ area of the PC1 CCD which imaged the cluster center. Individual stars brighter than V = 20 are easily identifiable in the central image section shown in Fig. 1. Image simulations and similar data on the denser cluster M15 indicate that the effects of incompleteness and photometric error are negligible on the V < 19.5 sample of stars in NGC 6624. The stellar surface density profile derived from such a sample is well approximated by a power law of index α = −0.85 (Fig. 2). The density profile shows no hint of flattening towards smaller radii in the radial range over which it is reliably measured (r ≳ 0′′.3). A B vs. U – V color-magnitude diagram of the r < 15′′ region (Fig. 3) shows a well defined blue straggler sequence. Our preliminary findings are consistent with an earlier study of this cluster by Sosin and King (1995) using pre-repair FOC images.