Photometric Detection of Multiple Populations in Globular Clusters Using Integrated Light

Abstract

We investigate the multiple stellar populations of the globular clusters (GCs) M3, M5, M13, and M71 using ${g}^{{\prime} }$ and intermediate-band CN-$\lambda 3883$ photometry obtained with the WIYN 0.9 m telescope on Kitt Peak. We find a strong correlation between red giant stars' CN$-{g}^{{\prime} }$ colors and their spectroscopic sodium abundances, thus demonstrating the efficacy of the two-filter system for stellar population studies. In all four clusters, the observed spread in red giant branch CN$-{g}^{{\prime} }$ colors is wider than that expected from photometric uncertainty, confirming the well-known chemical inhomogeneity of these systems. M3 and M13 show clear evidence for a radial dependence in the CN-band strengths of its red giants, while the evidence for such a radial dependence of CN strengths in M5 is ambiguous. Our data suggest that the dynamically old, relatively metal-rich M71 system is well mixed, as it shows no evidence for chemical segregation. Finally, we measure the radial gradients in the integrated CN$-{g}^{{\prime} }$ color of the clusters and find that such gradients are easily detectable in the integrated light. We suggest that photometric observations of color gradients within GCs throughout the Local Group can be used to characterize their multiple populations, and thereby constrain the formation history of GCs in different galactic environments.