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

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

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Título NGC 6558 : a blue horizontal branch moderately metal-poor globular cluster in the bulge
Autor Barbuy, Beatriz
Zoccali, Manuela
Ortolani, Sergio
Minniti, Dante
Hill, Vanessa
Renzini, Alvio
Bica, Eduardo Luiz Damiani
Gómez, Ana
Abstract 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.
Contido em The astronomical journal. Vol. 134, no. 4 (Oct. 2007), p. 1613-1625
Assunto Aglomerados estelares globulares
Espectros estelares
Estrelas gigantes
Evolucao estelar
[en] Globular clusters: general
[en] Globular clusters: individual (NGC 6558)
Origem Estrangeiro
Tipo Artigo de periódico
URI http://hdl.handle.net/10183/108242
Arquivos Descrição Formato
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