NGC 6558: A blue horizontal branch moderately metal-poor globular cluster in the bulge

B. Barbuy, M. Zoccali, S. Ortolani, D. Minniti, V. Hill, A. Renzini, E. Bica, A. Gómez

Research output: Contribution to journalArticlepeer-review

39 Citations (Scopus)


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 vr hel = -197.3 ±4 km s-1 is found for NGC 6558.

Original languageEnglish
Pages (from-to)1613-1625
Number of pages13
JournalAstronomical Journal
Issue number4
Publication statusPublished - 2007


  • Globular clusters: general
  • Globular clusters: individual (NGC 6558)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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