2007AJ....134.1613B

We present, for the first time, a detailed abundance analysis of five giant stars in the moderately metal-poor bulge globular cluster NGC 6558. Spectra have been obtained at the VLT with the multifiber spectrograph FLAMES in GIRAFFE mode (R∼22,000). The resulting iron abundance is [Fe/H]=-0.97±0.15, in good agreement with the metallicity inferred from the slope of the red giant branch, but unusually high for a cluster with such a blue horizontal branch, possibly indicating an old age. A color-magnitude diagram in V and I, based on photometry obtained with the Wide-Field Imager at ESO, is also presented. An isochrone of 14 Gyr fits the evolutionary sequences, confirming an old age. NGC 6558 is another ``second-parameter'' bulge cluster. The metallicity derived is near the end of the low-metallicity tail of the bulge field-star distribution; hence, it presumably formed at the very early stages of the bulge formation. Abundance ratios show enhancements of the α-elements oxygen, magnesium, and silicon, with [O/Fe]=+0.38, [Mg/Fe]=+0.24, [Si/Fe]=+0.23, and solar calcium and titanium. The r-element europium is also enhanced by [Eu/Fe]=+0.36. The odd-Z elements sodium and aluminum, as well as the s-elements barium and lanthanum, show solar ratios. A heliocentric radial velocity of v^hel_r=-197.3±4 km/s is found for NGC 6558.: