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NGC 6558 is a bulge globular cluster with a blue horizontal branch (BHB),combined with a metallicity of [Fe/H]~-1.0. It is similar to HP 1 and NGC 6522,which could be among the oldest objects in the Galaxy. Element abundances inthese clusters could reveal the nature of the first supernovae. We aim to carryout detailed spectroscopic analysis for four red giants of NGC 6558, in orderto derive the abundances of the light elements C, N, O, Na, Al, thealpha-elements Mg, Si, Ca, Ti, and the heavy elements Y, Ba, and Eu.High-resolution spectra of four stars with FLAMES-UVES@VLT UT2-Kueyen wereanalysed. Spectroscopic parameter-derivation was based on excitation andionization equilibrium of FeI and FeII. This analysis results in a metallicityof [Fe/H] = -1.17+-0.10 for NGC 6558. We find the expected alpha-elementenhancements in O and Mg with [O/Fe]=+0.40, [Mg/Fe]=+0.33, and low enhancementsin Si and Ca. Ti has a moderate enhancement of [Ti/Fe]=+0.22. The r-element Euappears very enhanced with a mean value of [Eu/Fe]=+0.63. Ba appears to have asolar abundance ratio relative to Fe. NGC 6558 shows an abundance pattern thatcould be typical of the oldest inner bulge globular clusters, together with thepattern in the similar clusters NGC 6522 and HP 1. They show low abundances ofthe odd-Z elements Na and Al, and of the explosive nucleosynthesisalpha-elements Si, Ca, and Ti. The hydrostatic burning alpha-elements O and Mgare normally enhanced as expected in old stars enriched with yields fromcore-collapse supernovae, and the iron-peak elements Mn, Cu, Zn show lowabundances, which is expected for Mn and Cu, but not for Zn. Finally, thecluster trio NGC 6558, NGC 6522, and HP 1 show similar abundance patterns.

High-resolution abundance analysis of four red giants in the globular cluster NGC 6558