A NEW LOOK AT THE GLOBULAR-CLUSTER M28

Reductions of PDS microdensitometer scans of 20 plates from three different telescopes over an epoch range of 92 yr have yielded proper motions and photometry for more than 250 stars down to V approximately 16 in the region of the poorly studied globular cluster M28 (NGC 6626). Membership probabilities for these stars, including several variables, are derived from the proper motions, and a color-magnitude diagram of probable members is presented and discussed. We find strong evidence of systematic errors in previous photographic photometry. Some questions about the metal abundance of M28 are addressed; in particular, a dependence upon method of the derived metallicity. All previously known variables included in this study are at least possible members. We find evidence of variability of a few stars not previously known to vary. Our derivation of the cluster's absolute proper motion shows the space velocity to be surprisingly disklike for a moderately metal-poor cluster.