Identifying Multiple Populations in M71 Using CN

Jeffrey M. Gerber, Eileen D. Friel, Enrico Vesperini

Published 2020-01-31Version 1

We have observed the CN features at $\sim$3800 ~\AA~ and 4120~ \AA~ as well as the CH band at $\sim$4300~\AA~ for 145 evolved stars in the Galactic globular cluster M71 using the multi-object spectrograph, Hydra, on the WIYN-3.5m telescope. We use these measurements to create two $\delta$CN indices finding that both distributions are best fit by two populations, a CN-enhanced and CN-normal. We find that 42 $\pm$ 4\% of the RGB stars in our sample are CN-enhanced. The percentage of CN-enhanced is 40 $\pm$ 13\% for the AGB and 33 $\pm$ 9\% for the 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 ZAHBs. 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 HST UV photometry, and we find good agreement with both methods.