CN Variations in High-Metallicity Globular and Open Clusters1

Abstract

We present a comparison of CN bandstrength variations in thehigh-metallicity globular clusters NGC 6356 and NGC 6528 with thosemeasured in the old open clusters NGC 188, NCG 2158 and NGC 7789.Star-to-star abundance variations, of which CN differences are a readilyobservable sign, are commonplace in moderate-metallicity halo globularclusters but are unseen in the field or in open clusters. We find thatthe open clusters have narrow, unimodal distributions of CNbandstrength, as expected from the literature, while the globularclusters have broad, bimodal distributions of CN bandstrength, similarto moderate-metallicity halo globular clusters. This result hasinteresting implications for the various mechanisms proposed to explainthe origin of globular cluster abundance inhomogeneities, and suggeststhat the local environment at the epoch of cluster formation plays avital role in regulating intracluster enrichment processes.