Examinando por Autor "Assmann, P."

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    A spectroscopic study of the globular Cluster NGC 4147
    (OXFORD UNIV PRESS, 2016-08) Villanova, S.; Monaco, L.; Moni Bidin, C.; Assmann, P.
    We present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium- and high-resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H] = -1.84 +/- 0.02 and an alpha-enhancement of +0.38 +/- 0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular, we found an Na-O anticorrelation and Na-Al correlation. The cluster contains only similar to 15 per cent of stars that belong to the first generation (Na-poor and O-rich). This implies that it suffered a severe mass-loss during its lifetime. Its [Ca/Fe] and [Ti/Fe] mean values agree better with the Galactic halo trend than with the trend of extragalactic environments at the cluster metallicity. This possibly suggests that NGC 4147 is a genuine Galactic object at odd with what claimed by some author that proposed the cluster to be member of the Sagittarius dwarf galaxy. An antirelation between the light s-process element Y and Na may also be present.
  • Miniatura
    Ítem
    NGC 6791: A Probable Bulge Cluster without Multiple Populations
    (Institute of Physics Publishing, 2018-11) Villanova, S.; Carraro, G.; Geisler, D.; Monaco, L.; Assmann, P.
    NGC 6791 is a unique stellar cluster, key to our understanding of both the multiple stellar population phenomenon and the evolution and assembly of the Galaxy. However, despite many investigations, its nature is still very controversial. Geisler et al. found evidence suggesting that it was the first open cluster to possess multiple populations, but several subsequent studies did not corroborate this. It has also been considered a member of the thin or thick disk or even the bulge, and either as an open or globular cluster or even the remnant of a dwarf galaxy. Here we present and discuss detailed abundances derived from high-resolution spectra obtained with UVES at VLT and HIRES at Keck of 17 evolved stars of this cluster. We obtained a mean [Fe/H] = +0.313 ±0.005, in good agreement with recent estimates, and with no indication of star-to-star metallicity variation, as expected. We also did not find any variation in Na, in spite of having selected the very same stars as in Geisler et al., where an Na variation was claimed. This points to the presence of probable systematics in the lower-resolution spectra of this very high metallicity cluster analyzed in that work. In fact, we find no evidence for an intrinsic spread in any element, corroborating recent independent APOGEE data. The derived abundances indicate that NGC 6791 very likely formed in the Galactic bulge and that the proposed association with the thick disk is unlikely, despite its present Galactic location. We confirm the most recent hypothesis suggesting that the cluster could have formed in the bulge and radially migrated to its current location, which appears to be the best explanation for this intriguing object. © 2018. The American Astronomical Society. All rights reserved.