Ground-based NUV-Optical Photometry Reveals Abundance Variations in Red Giant Stars of Galactic Bulge Globular Clusters
The signatures of CNONa-bearing molecules in the atmospheres of red giant stars can be used to identify and characterize multiple stellar populations in globular clusters. Abundance studies over the last two decades have revealed Na-O, Mg-Al, and other anticorrelations among globular cluster red giant stars that are commonly interpreted as evidence of two or more episodes of star formation. Meanwhile, photometric studies have shown that CNONa abundance variations in cluster red giants can be detected using blue and NUV filters because various strong molecular absorption bands (OH, CN, NH, CH) are found blueward of ~4000 Å. We present the photometric detection of multiple RGB sequences in 14 Milky Way bulge globular clusters, which demonstrates the efficacy of ground-based de-reddened optical SDSS ugriz photometry, and in particular the (u-g)-(g-i) (Cugi) pseudo-color index, to separate giant stars with varying atmospheric light element abundances, even when the clusters are heavily obscured. We find that the spread in Cugi colors of the red giant stars correlates with their CNONa atmospheric abundances, and that the derived stellar population fraction of each cluster is correlated with the cluster absolute visual magnitude, in agreement with recent HST photometric studies of halo globular clusters. Data used in this paper comes from the Blanco DECam Survey Collaboration and we gratefully acknowledge financial support from the National Science Foundation under grants AST-1413755 and AST-1412673.