CAPOS: The bulge Cluster APOGEE Survey VI. Characterizing multiple stellar populations and chemical abundances in the bulge globular cluster NGC 6569

Abstract

Context. The CAPOS project aims to obtain accurate mean abundances and radial velocities for many elements, and it explores the multiple population (MP) phenomenon in Galactic bulge globular clusters (BGCs). NGC 6569 is one of the CAPOS targets. Aims. This study provides a detailed high-resolution spectroscopic analysis of NGC 6569 to derive precise abundances for elements of different nucleosynthetic origins and to unveil its MPs by focusing on key spectral features. Methods. We analyzed APOGEE-2 near-infrared spectra of 11 giant members with the BACCHUS code, deriving abundances for 12 elements (C, N, O, Mg, Si, Ca, Ti, Fe, Ni, Al, Ce, Nd). Isochrone fitting with Gaia+2MASS photometry was used to estimate atmospheric parameters, cluster distance, and extinction. Results. We obtained [Fe/H] = -0.91 $\pm$ 0.06, consistent with APOGEE pipeline values; the scatter lies within uncertainties. The cluster shows [$\alpha$/Fe] = 0.36 $\pm$ 0.06 dex, similar to other GCs. Al appears homogeneous, while N is strongly enriched ([N/Fe] = 0.68 $\pm$ 0.34) with a spread of 0.90 dex, yielding two populations anticorrelated in C. The n-capture elements Ce and Nd are overabundant compared to the Sun but consistent with GCs of similar metallicity, with $⟨$[Ce/Nd]$⟩$ = -0.17 $\pm$ 0.12. We also measure RV = -49.75 $\pm$ 3.68 km s${}^{-1}$, consistent with previous works, while d${}_{\odot }$ = 12.4 $\pm$ 1.45 kpc and E(B-V) = 0.68 are both higher than literature values. Conclusions. MPs in NGC 6569 are confirmed through a clear C-N anticorrelation. The cluster shows [$\alpha$/Fe] enhancement from Type II SNe and no Mg-Al-Si anticorrelation, suggesting rapid and homogeneous formation. The $⟨$[Ce/Nd]$⟩$ ratio points to contributions from r-process events such as neutron star mergers, while overall Ce and Nd abundances are reported here for the first time in this cluster.

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