Playing CHESS with stars II. Homogeneous analysis of high-resolution optical spectra for a sample of 135 giants in 14 bulge globular clusters

Abstract

Bulge globular clusters are key tracers of the complex formation history of the Galactic bulge, as they retain information on its past star-formation episodes. Their chemical abundances serve as empirical `fossil records'' of the early evolution of the bulge. We present a homogeneous analysis of archival high-resolution UVES spectra for 135 giants in 14 Galactic bulge globular clusters. We explore the results in the context of the phenomenon of multiple stellar populations and discuss what the abundances reveal about the chemical evolution of the bulge. We employed the CHEmical Survey analysis System (CHESS) pipeline to derive atmospheric parameters on an accurate scale defined by reference stars. Abundances for Li, O, Na, Mg, Al, Si, Ca, Sc, Y, Zr, Ba, Ce, Nd, and Eu were derived by spectrum synthesis. Deviations from local thermodynamic equilibrium (i.e., non-LTE or NLTE) were taken into account for Na, Mg, Al, Si, Ca, Y, Ba, and Eu. We serendipitously discovered a Li-rich giant with A(Li) = 1.68 (after 3D NLTE corrections) in NGC 6723, which was hidden in the archival data. Our sample includes both first- and second-generation stars, as indicated by the observed spread in Na abundances. Compared to halo clusters, the bulge clusters show a lower [α/Fe] plateau at low metallicity and a more gradual decline in [α/Fe] toward higher metallicities. The Sc abundances are below solar and lower than in disk stars of the same metallicity. This difference could be important for solving open questions about the nucleosynthesis of this element. None of the clusters exhibit any significant spreads in heavy elements. Most clusters exhibit [Ba/Eu] \sim 0.0, indicating a contribution from both s- and r-processes. Clusters with [Fe/H] < -0.5 show negative [Ce/Nd] ratios, supporting the idea that these elements were predominantly produced by the r-process in these clusters.