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

• Autor(a) principal: Barbuy, Beatriz
• Data de Publicação: 2007
• Outros Autores: Zoccali, Manuela, Ortolani, Sergio, Minniti, Dante, Hill, Vanessa, Renzini, Alvio, Bica, Eduardo Luiz Damiani, Gómez, Ana
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Institucional da UFRGS
• Texto Completo: http://hdl.handle.net/10183/108242
• topic: Aglomerados estelares globulares; Evolucao estelar; Espectros estelares; Estrelas gigantes; Globular clusters: general; Globular clusters: individual (NGC 6558)

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 Vand 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 showenhancements of the -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-ˡ is found for NGC 6558.