摘要：Context. Efforts to look for signatures of the first stars have
concentrated on metal-poor halo objects. However, the low end of the bulge metallicity
distribution has been shown to host some of the oldest objects in the Milky Way and hence
this Galactic component potentially offers interesting targets to look at imprints of the
first stellar generations. As a pilot project, we selected bulge field stars already
identified in the ARGOS survey as having [Fe/H] ≈−1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain
detailed abundances of key elements that are believed to reveal imprints of the first
stellar generations.
Aims. The main purpose of this study is to analyse selected ARGOS stars
using new high-resolution (R
~ 45 000) and high-signal-to-noise (S/N>
100) spectra. We aim to derive their stellar parameters and elemental
ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca,
and Ti, the odd-Z elements Na and Al, the neutron-capture s-process
dominated elements Y, Zr, La, and Ba, and the r-element Eu.
Methods. High-resolution spectra of five field giant stars were obtained
at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES
configuration. Spectroscopic parameters were derived based on the excitation and
ionization equilibrium of Fe i and Fe ii. The abundance analysis was
performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum
synthesis code.
Results. We confirm that the analysed stars are moderately metal-poor
(−1.04 ≤ [Fe/H]
≤−0.43),
non-carbon-enhanced (non-CEMP) with [C/Fe] ≤ +
0.2, and α-enhanced. We find that our three most metal-poor
stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed
from a gas enriched by core-collapse supernovae, and that the values are in agreement with
results in the literature for bulge stars in the same metallicity range. No abundance
anomalies (Na − O, Al
− O, Al − Mg anti-correlations) were detected in our
sample. The heavy elements Y, Zr, Ba, La, and Eu also exhibit oversolar abundances. Three
out of the five stars analysed here show slightly enhanced [Y/Ba] ratios similar to those
found in other metal-poor bulge globular clusters (NGC 6522 and M 62).
Conclusions. This sample shows enhancement in the first-to-second peak
abundance ratios of heavy elements, as well as dominantly s-process
element excesses. This can be explained by different nucleosynthesis scenarios: (a) the
main r-process plus extra mechanisms, such as the weak r-process; (b) mass
transfer from asymptotic giant branch stars in binary systems; (c) an early generation of
fast-rotating massive stars. Larger samples of moderately metal-poor bulge stars, with
detailed chemical abundances, are needed to better constrain the source of dominantly
s-process elements in the early Universe.
