Spectroscopy of clump giants in NGC 2158

Abstract

Spectra have been obtained for ten clump giants in the open cluster NGC 2158, and indices reflecting the strength of the 3883-A and 4216-A CN bands, the Ca II H+K lines, and the G band have been measured. All of the stars have the same calcium index to within the observational uncertainty. There is marginal evidence for an intrinsic variation in the strength of the 3883-A CN and 4300-A G bands. It is clear, however, that NGC 2158 does not show the type of marked CN, CH inhomogeneities exhibited by the globular clusters. This supports previously available data which indicate that the older open clusters are relatively homogeneous by comparison with the globulars. This might be understood if the globular clusters, while still gaseous, had experienced a carbon and/or a nitrogen enrichment that was promoted by supernova explosions, whereas if such a process had been initiated within protoopen cluster gas clouds it might have resulted in their dispersal rather than their enrichment. Alternatively, globular cluster stars might be far more efficient at cycling their envelopes through the interior CN-processing shell than the younger, metal-richer open cluster stars.