9319229 Zinn Recent investigations have indicated that the flattening of the galactic halo and the morphology of the horizontal branch (i.e. the relative frequencies of red and blue HB stars) of the halo vary with distance from the galactic center. The work of Dr. Zinn and other astronomers on the system of globular clusters suggests that this is due to the composite nature of the halo. The globular clusters of the inner halo have characteristics that are indicative of very old ages. While this Old halo population extends into the outer halo, at large galactocentric distances, the majority of clusters belong to a separate population, termed the Young halo. The properties of the Old halo clusters suggests that this system is a relic of the collapse of the Galaxy that ultimately produced the galactic disk and bulge, while those of the Young halo suggest that it is the debris of one or more large satellite galaxy (similar to the Large Magellanic Cloud) that merged with the Milky Way and was tidally disrupted in the process. The research carried out with this award is designed to test this hypothesis for the structure and formation of the galactic halo and to provide valuable information on the inner halo even if the hypothesis needs to be discarded. Observations of a selected sample of RR Lyrae variable stars will indicate if the stellar population of the inner halo is kinematically similar to the Old halo globular clusters. Samples of halo stars near the Sun will be examined for evidence that they are a mixture of the Old and Young halos. The results obtained by these investigations will provide a more detailed picture of the inner halo and the relationship between the globular cluster system and the stellar halo. They will therefore further our understanding of the evolution of our galaxy.
