A proper-motion membership study of the old open cluster NGC 188

Relative proper motions have been determined for 1127 stars in the field of the open cluster NGC 188 from plates spanning a 60-year interval, taken with the 76-cm Thaw refractor of the Allegheny Observatory. Special attention has been paid to the corrections for systematic errors as a function of magnitude and color, and to the treatment of membership probabilities for stars with different proper-motion errors. The study is complete to a magnitude B = 16.2. Also some stars brighter than B = 12.5 were eliminated from the study because of the large uncertainty in estimating the magnitude equation at the bright end. The final relative proper motions have an accuracy of 0.5 mas/yr for well-exposed stars. From the cluster membership probabilities based on proper motions we obtain 360 cluster members, enabling us to better define the color-magnitude diagram. We provide a list of highly probable candidate blue stragglers in addition to the sample proposed by Eggen & Sandage (1969). In order to investigate mass segregation, we have analyzed the spatial distribution of the possible blue stragglers. Based on our proper-motion probabilities the spatial cumulative distributions show that the blue stragglers are not more centrally concentrated than the red giants.