Identifying Multiple Populations in M71 Using CN

Abstract

Abstract We have observed the CN features at ∼3800 and 4120 Å as well as the CH band at ∼4300 Å for 145 evolved stars in the Galactic globular cluster M71 using the multi-object spectrograph, Hydra, on the Wisconsin-Indiana-Yale-NOAO-3.5 m telescope. We use these measurements to create two δCN indices finding that both distributions are best fit by two populations: a CN-enhanced and CN-normal. We find that 42% ± 4% of the red giant branch stars in our sample are CN-enhanced. The percentage of CN-enhanced is 40% ± 13% for the asymptotic giant branch and 33% ± 9% for the horizontal-branch stars (HB stars), which suggests there are no missing second generation stars at these stages of stellar evolution. The two generations also separate in magnitude and color on the HB, which allows us to find the difference in He abundance between the two populations by fitting appropriate zero-age horizontal branches. The broad range of distances from the cluster’s center covered by our sample allows us to study the dependence of the ratio of the number of first to second population stars on the distance from the cluster’s center, and we find that this ratio does not vary radially and that the two populations are spatially mixed. Finally, we compare our identification of multiple populations with the classification based on the Na–O anti-correlation and the Hubble Space Telescope UV photometry, and we find good agreement with both methods.