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

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, Brazil
e-mail: barbuy@astro.iag.usp.br
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: cristina.chiappini@aip.de
³ Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
⁴ Astrophysics Group, Keele University, ST5 5BG, Keele, UK
⁵ Dipartimento di Fisica e Astronomia, Università di Padova, 35122 Padova, Italy
⁶ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
⁸ Universidad Catolica de Chile, Instituto de Astrofisica, Casilla 306, Santiago 22, Chile
⁹ Millenium Institute of Astrophysics, Av. Vicuña Mackenna 4860, Macul, Santiago, Chile
¹⁰ Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, Santiago, Chile
¹¹ Instituto Nacional de Pesquisas Espaciais, Av. dos Astronautas 1758, 12227-010 São José dos Campos, Brazil
¹² Universidade Federal do Rio Grande do Sul, Departamento de Astronomia, CP 15051, 91501-970 Porto Alegre, Brazil
¹³ Observatoire de Paris-Meudon, GEPI, 92195 Meudon Cedex, France

Received: 30 May 2014
Accepted: 31 July 2014

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] ~ −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 ~ 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 ~ 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.