VLT-FLAMES analysis of 8 giants in the bulge metal-poor globular cluster NGC 6522: oldest cluster in the Galaxy?. Analysis of 8 giants in NGC 6522

NGC 6522 has been the first metal-poor globular cluster identified in the bulge by Baade. Despite its importance, very few high-resolution abundance analyses of stars in this cluster are available. The bulge metal-poor clusters may be important tracers of the early chemical enrichment of the Galaxy. Aims: The main purpose of this study is to determine metallicity and elemental ratios in individual stars of NGC 6522. Methods: High-resolution spectra of 8 giants of the bulge's globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the FLAMES+GIRAFFE spectrograph. Multiband V, I, J, Ks photometry was used to derive effective temperatures as reference values. Spectroscopic parameters were derived from Fe i and Fe ii lines, and adopted for the derivation of abundance ratios. Results: The present analysis provides a metallicity [Fe/H] = -1.0 ± 0.2. The α-elements oxygen, magnesium and silicon show [O/Fe] = +0.4 ± 0.3, [Mg/Fe] = [Si/Fe] = +0.25±0.15, whereas calcium and titanium show shallower ratios of [Ca/Fe] = [Ti/Fe] = +0.15 ± 0.15. The neutron-capture r-process element europium appears to be overabundant by [Eu/Fe] = +0.4 ± 0.4. The neutron-capture s-elements lanthanum and barium are enhanced by [La/Fe] = +0.35 ± 0.2 and [Ba/Fe] = +0.5 ± 0.5. The large internal errors, indicating the large star-to-star variation in the barium and europium abundances, are also discussed. Conclusions: The moderate metallicity combined to a blue horizontal branch (BHB), are characteristics similar to those of HP 1 and NGC 6558, pointing to a population of very old globular clusters in the Galactic bulge. Also, the abundance ratios in NGC 6522 resemble those in HP 1 and NGC 6558. The ultimate conclusion is that the bulge is old, and went through an early prompt chemical enrichment.