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

摘要

Context. NGC6522 has been the firstmetal-poor globular cluster identified in the bulge by Baade. Despiteits importance, very few high-resolution abundance analyses of stars inthis cluster are available. The bulge metal-poor clusters may beimportant tracers of the early chemical enrichment of the Galaxy. Aims. The main purpose of this study is to determinemetallicity and elemental ratios in individual stars ofNGC6522. Methods. High-resolution spectra of8giants of the bulge's globular cluster NGC6522were obtained at the 8m VLT UT2-Kueyen telescope with theFLAMES+GIRAFFE spectrograph. Multibandphotometry was used to derive effective temperatures as referencevalues. Spectroscopic parameters were derived from Fe Iand Fe IIlines, and adopted forthe derivation of abundance ratios. Results. The present analysis provides a metallicity[Fe/H]=.The -elementsoxygen, magnesium and silicon show,[Mg/Fe]=[Si/Fe]=,whereas calcium and titanium show shallower ratios of[Ca/Fe]=[Ti/Fe]=+0.150.15.The neutron-capture r-process element europium appears to beoverabundant by [Eu/Fe]=+0.40.4.The neutron-capture s-elements lanthanum and barium are enhanced by[La/Fe]=+0.350.2 and[Ba/Fe]=+0.50.5. The large internal errors,indicating the large star-to-star variation in the barium and europiumabundances, are also discussed. Conclusions. The moderate metallicity combined to ablue horizontal branch (BHB), are characteristics similar to those ofHP1 and NGC6558, pointing to a population of veryold globular clusters in the Galactic bulge. Also, the abundance ratiosin NGC6522 resemble those in HP1 andNGC6558. The ultimate conclusion is that the bulge is old,and went through an early prompt chemical enrichment. Key words: stars: abundances - galaxy:bulge - galaxy: globular clusters: individual: NGC6522