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

Abstract

Context. 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.