Variations in abundances among main sequence stars in M71

Abstract

CH and CN variations are definitely present among the main sequence stars in a narrow range of luminosity just below the turnoff in the globular cluster stars. We show that the same set of C, N and O abundances that are used to fit the CN band in the CN-strong and CN-weak giants and subgiants in this cluster also successfully reproduce the behavior of the CH and CN molecular bands seen in the main sequence sample. This strongly suggests that the variations seen in M71 are primordial in origin. A hint of variation in the strength of the NaD lines is also seen in the main sequence sample. A range in Na abundance of about a factor of 3 is required to explain this. Strong Na among the main sequence stars appears to be correlated with strong CN, which is also characteristic of Na variations among red giants in globular clusters.