A survey of proper-motion stars. VIII - On the Galaxy's third population

Results from a survey of proper-motion stars (Laird et al., 1988) are used to show that a third discrete population may exist within the Galaxy. This population has a mean metallicity m/H of about -0.5, and a metallicity distribution function that appears to be independent of height above the plane. Based on an apparent blue limit to the colors of stars with metallicities near -0.5, this population is found to be very old: similar to the disk globular cluster 47 Tuc. It is suggested that this third population could be the signature of a merger event early in the Galaxy's history, occurring shortly after the disk had formed.