ASTROMETRY AND PHOTOMETRY IN THE GLOBULAR-CLUSTER M4

Reductions of PDS scans of 38 plates from six different telescopes over an epoch range of 90 years have yielded proper motions and photographic photometry for 530 stars down to V∼16 in the region of the nearby globular cluster M4. The cluster proper motion is significantly different from that of nearly all field stars, leading to a very clean segregation between cluster members and field stars. A color-magnitude diagram of the approximately 400 probable members is presented and discussed. We find strong evidence for a reddening gradient across the cluster. All of the previously known variables measured in this study are at least possible members, with all but one having very high probabilities of membership. A few blue straggler candidates are also found to be probable members. Our derivation of the cluster's absolute proper motion shows the space velocity to be large, as expected for a moderately metal-poor globular, but the W component of the velocity is surprisingly small.