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.
Observations collected at the European Southern Observatory, Paranal,
Chile (ESO), under programmes 88.D-0398A.Appendix A is available in
electronic form at http://www.aanda.org
