Examinando por Autor "Zoccali, M"

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  • Miniatura
    Ítem
    DISCOVERY OF RR LYRAE STARS IN THE NUCLEAR BULGE OF THE MILKY WAY
    (IOP PUBLISHING, 2016-10) Minniti, D; Ramos, RC; Zoccali, M; Rejkuba, M; Gonzalez, OA; Valenti, E; Gran, F
    Galactic nuclei, such as that of the Milky Way, are extreme regions with high stellar densities, and in most cases, the hosts of a supermassive black hole. One of the scenarios proposed for the formation of the Galactic nucleus is merging of primordial globular clusters. An implication of this model is that this region should host stars that are characteristically found in old Milky Way globular clusters. RR Lyrae stars are primary distance indicators, well known representatives of old and metal-poor stellar populations, and therefore are regularly found in globular clusters. Here we report the discovery of a dozen RR Lyrae type ab stars in the vicinity of the Galactic center, i.e., in the so-called nuclear stellar bulge of the Milky Way. This discovery provides the first direct observational evidence that the Galactic nuclear stellar bulge contains ancient stars (>10 Gyr old). Based on this we conclude that merging globular clusters likely contributed to the build-up of the high stellar density in the nuclear stellar bulge of the Milky Way.
  • Miniatura
    Ítem
    Heavy elements Ba, La, Ce, Nd, and Eu in 56 Galactic bulge red giants
    (EDP SCIENCES, 2016-02) Van der Swaelmen, M; Barbuy, B; Hill, V; Zoccali, M; Minniti, D; Ortolani, S; Gomez, A
    Aims. The aim of this work is the study of abundances of the heavy elements Ba, La, Ce, Nd, and Eu in 56 bulge giants (red giant branch and red clump) with metallicities ranging from 1 : 3 dex to 0 : 5 dex. Methods. We obtained high-resolution spectra of our giant stars using the FLAMES-UVES spectrograph on the Very Large Telescope. We inspected four bulge fields along the minor axis. Results. We measure the chemical evolution of heavy elements, as a function of metallicity, in the Galactic bulge. Conclusions. The [Ba; La; Ce; Nd = Fe] vs. [Fe = H] ratios decrease with increasing metallicity, in which aspect they differ from disc stars. In our metal-poor bulge stars, La and Ba are enhanced relative to their thick disc counterpart, while in our metal-rich bulge stars La and Ba are underabundant relative to their disc counterpart. Therefore, this contrast between bulge and discs trends indicates that bulge and (solar neighbourhood) thick disc stars could behave differently. An increase in left perpendicularLa; Nd = Euright perpendicular with increasing metallicity, for metal-rich stars with [Fe = H] > 0 dex, may indicate that the s-process from AGB stars starts to operate at a metallicity around solar. Finally, [Eu = Fe] follows the [alpha = Fe] behaviour, as expected, since these elements are produced by SNe type II.