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(Reference retrieved automatically from Web of Science through information on FAPESP grant and its corresponding number as mentioned in the publication by the authors.)

High-resolution abundance analysis of red giants in the globular cluster NGC 6522

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Author(s):
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Barbuy, B. [1] ; Chiappini, C. [2] ; Cantelli, E. [1] ; Depagne, E. [2] ; Pignatari, M. [3] ; Hirschi, R. [4] ; Cescutti, G. [2] ; Ortolani, S. [5, 6] ; Hill, V. [7] ; Zoccali, M. [8, 9] ; Minniti, D. [9, 10] ; Trevisan, M. [11] ; Bica, E. [12] ; Gomez, A. [13]
Total Authors: 14
Affiliation:
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[1] Univ Sao Paulo, IAG, BR-05508900 Sao Paulo - Brazil
[2] Leibniz Inst Astrophys Potsdam AIP, D-14482 Potsdam - Germany
[3] Univ Basel, Dept Phys, CH-4056 Basel - Switzerland
[4] Keele Univ, Astrophys Grp, Keele ST5 5BG, Staffs - England
[5] Osserv Astron Padova, INAF, I-35122 Padua - Italy
[6] Univ Padua, Dipartimento Fis & Astron, I-35122 Padua - Italy
[7] Univ Nice Sophia Antipolis, CNRS, Observ Cote Azur, Lab Lagrange UMR7293, F-06304 Nice 4 - France
[8] Univ Catolica Chile, Inst Astrofis, Santiago 22 - Chile
[9] Millenium Inst Astrophys, Santiago - Chile
[10] Univ Andres Bello, Dept Ciencias Fis, Santiago - Chile
[11] Inst Nacl Pesquisas Espaciais, BR-12227010 Sao Jose Dos Campos - Brazil
[12] Univ Fed Rio Grande do Sul, Dept Astron, BR-91501970 Porto Alegre, RS - Brazil
[13] Observ Paris, GEPI, F-92195 Meudon - France
Total Affiliations: 13
Document type: Journal article
Source: Astronomy & Astrophysics; v. 570, OCT 2014.
Web of Science Citations: 26
Abstract

Context. The {[}Sr/Ba] and {[}Y/Ba] scatter observed in some galactic halo stars that are very metal-poor and in a few individual stars of the oldest known Milky Way globular cluster NGC 6522 have been interpreted as evidence of early enrichment by massive fast-rotating stars (spinstars). Because NGC 6522 is a bulge globular cluster, the suggestion was that not only the very-metal poor halo stars, but also bulge stars at {[}Fe/H] similar to-1 could be used as probes of the stellar nucleosynthesis signatures from the earlier generations of massive stars, but at much higher metallicity. For the bulge the suggestions were based on early spectra available for stars in NGC 6522, with a medium resolution of R similar to 22 000 and a moderate signal-to-noise ratio. Aims. The main purpose of this study is to re-analyse the NGC 6522 stars reported previously by using new high-resolution (R similar to 45 000) and high signal-to-noise spectra (S/N > 100). We aim at re-deriving their stellar parameters and elemental ratios, in particular the abundances of the neutron-capture s-process-dominated elements such as Sr, Y, Zr, La, and Ba, and of the r-element Eu. Methods. High-resolution spectra of four giants belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe I and Fe II. Results. Our analysis confirms a metallicity {[}Fe/H] = -0.95 +/- 0.15 for NGC 6522 and the overabundance of the studied stars in Eu (with +0.2 < {[}Eu/Fe] < +0.4) and alpha-elements O and Mg. The neutron-capture s-element-dominated Sr, Y, Zr, Ba, and La now show less pronounced variations from star to star. Enhancements are in the range 0.0 < {[}Sr/Fe] < +0.4, +0.23 < {[}Y/Fe] < +0.43, 0.0 < {[}Zr/Fe] < +0.4, 0.0 < {[}La/Fe] < +0.35, and 0.05 < {[}Ba/Fe] < +0.55. Conclusions. The very high overabundances of {[}Y/Fe] previously reported for the four studied stars is not confirmed with the new high-quality spectra. The moderate enhancement in {[}Sr/Fe] previously reported for one of the re-studied stars is confirmed, but the strong enhancements of this ratio for the other two stars are not confirmed. Despite the moderate enhancements found for the neutron-capture s-element-dominated species, none of the four stars studied here show positive values for all {[}Sr/Ba], {[}Y/Ba] and {[}Zr/Ba] ratios. The re-studied stars are now compatible not only with the interpretation that the s-process enhancements in these very old stars are due to spinstars, but also with alternative models such as mass transfer from s-process-rich AGB stars. Note, however, that when our results are interpreted in the context of more extended datasets from the literature, the spinstar scenario still seems to be favoured. (AU)

FAPESP's process: 12/05142-5 - Assembling Baryons to Dark Halos: Stellar Content and the Star Formation History of Early-­type Galaxies
Grantee:Marina Trevisan
Support type: Scholarships in Brazil - Post-Doctorate