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Photometric and spectroscopic analyses have shown that the Galactic bulgecluster Terzan 5 hosts several populations with different metallicities andages that manifest as a double red horizontal branch (HB). A recentinvestigation of the massive bulge cluster NGC 6569 revealed a similar, thoughless extended, HB luminosity split, but little is known about the cluster'sdetailed chemical composition. Therefore, we have used high-resolution spectrafrom the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemicalcompositions and radial velocity distributions of red giant branch and HB starsin NGC 6569. We found the cluster to have a mean heliocentric radial velocityof -48.8 km/s (sigma = 5.3 km/s; 148 stars) and a mean [Fe/H] =-0.87 dex (19stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significantmetallicity spread. NGC 6569 exhibits light- and heavy-element distributionsthat are common among old bulge/inner Galaxy globular clusters, including clear(anti)correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element datasuggest that NGC 6569 may be composed of at least two distinct populations, andthe cluster's low mean [La/Eu] = -0.11 dex indicates significant pollution withr-process material. We confirm that both HBs contain cluster members, butmetallicity and light-element variations are largely ruled out as sources forthe luminosity difference. However, He mass fraction differences as small asdelta Y ~ 0.02 cannot be ruled out and may be sufficient to reproduce thedouble HB.

Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569