Relative proper motions and the stellar velocity dispersion of the open cluster M67

Abstract

Relative proper motions for 663 stars in the field of the old open cluster M67 have been determined using 44 Yerkes 40-in refractor plates. The resulting proper-motion marginal distributions have been fit with a two-component model representing the sum of a cluster distribution and a much wider field distribution, both of which are assumed to be intrinsically Gaussian in form. The observed marginal distributions are actually quite non-Gaussian due to the effects of the proper-motion measurement errors, and thus a modeling procedure that realistically includes the measurement errors has been devised. The procedure yields excellent fits to the observed distributions, and thus allows reliable cluster-membership probabilities to be calculated. The cluster's intrinsic velocity dispersion is estimated from the proper motions of the about 80 brightest cluster stars to be 0.81 + or - 0.10 km/s. This is marginally higher than a dispersion estimate based on published radial-velocity measurements of a similar sample of cluster members.