2025A&A...699A.128B

Context. The CAPOS project aims to obtain accurate mean abundances for many elements and their mean radial velocities, and it explores the multiple population (MP) phenomenon in Galactic bulge globular clusters (BGCs). NGC 6569 is one of the clusters observed by CAPOS.
Aims. This study presents a detailed high-resolution spectroscopic analysis of NGC 6569 to derive high-precision mean abundances for a number of elements with various nucleosynthetic origins and to unveil its MPs by focusing on key spectral features. Our aim is to complement previous suggestions of the presence of MPs in this cluster based on the typical Na-O anticorrelation and the presence of a double horizontal branch.
Methods. We analyzed the near-infrared APOGEE-2 spectra of 11 giant member stars in NGC 6569 using the code BACCHUS. We derived abundances for 12 elements, including light elements (C, N), α-elements (O, Mg, Si, Ca, Ti), iron-peak elements (Fe, Ni), the odd-Z element (Al), and s-process elements (Ce, Nd). We also performed an isochrone fitting using photometric data (Gaia + 2MASS) to estimate atmospheric parameters, the cluster distance, and its extinction.
Results. We derived a mean metallicity of [Fe/H] = -0.91 ± 0.06, which is consistent with the values from the APOGEE pipeline and slightly more metal poor than previous findings. The scatter lies within the observational uncertainties. The cluster shows enhanced α-element abundances ([α/Fe] = 0.36 ± 0.06 dex) similar to other Galactic globular clusters (GCs). We find no significant variation in Al, suggesting a homogeneous distribution within the cluster. In contrast, we find considerable N-enrichment ([N/Fe] = 0.68 ± 0.34 dex) and a large spread of 0.90 dex, which enabled us to distinguish at least two separate populations based on N that have anticorrelated C abundances. The n-capture elements Ce and Nd are overabundant compared to the Sun, but are similar to those of GCs in this metallicity regime, and also show an average ratio of ⟨[Ce/Nd]⟩ = -0.17 ± 0.12. Finally, we estimated a mean radial velocity of RV = -49.75 ± 3.68 km s^–1, which is consistent with previous measurements, but the heliocentric distance (d_⊙ = 12.4 ± 1.45 kpc) and interstellar reddening (E(B-V) = 0.68) are higher than reported in the literature.
Conclusions. The analysis confirms the presence of MPs in NGC 6569, evidenced by a significant spread in N and a clear C-N anticorrelation. This supports the previously established Na-O anticorrelation. MPs are characterized through this pattern for the first time. NGC 6569 exhibits chemical signatures typical of BGCs, without a significant spread in metallicity. The cluster α-element enhancement (consistent with early enrichment by type II supernovae) and the absence of a Mg-Al-Si anticorrelation agree with expectations for relatively high-metallicity GCs and suggests a rapid and homogeneous star formation history. The overabundance of n-capture elements indicates contributions from r-process events and might be linked to neutron star mergers. These n-capture elements are reported in NGC 6569 for the first time.