Photometric Detection of Multiple Populations in Globular Clusters Using
  Integrated Light
  We investigate the multiple stellar populations of the globular clusters 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 globular clusters throughout
the Local Group can be used to characterize their multiple populations, and
thereby constrain the formation history of globular clusters in different
galactic environments.
