Abundances of neutron-capture elements in stars of the Galactic disk substructures

Abstract

Aims. The aim of this work is to present and discuss the observations of the iron peak (Fe, Ni) and neutron-capture element (Y, Zr, Ba, La, Ce, Nd, Sm, and Eu) abundances for 276 FGK dwarfs, located in the Galactic disk with metallicity −1 < [Fe/H] < +0.3. Methods. Atmospheric parameters and chemical composition of the studied stars were determined from an high resolution, high signal-to-noise echelle spectra obtained with the echelle spectrograph ELODIE at the Observatoire de Haute-Provence (France). Effective temperatures were estimated by the line depth ratio method and from the H α line-wing fitting. Surface gravities (log g) were determined by parallaxes and the ionization balance of iron. Abundance determinations were carried out using the LTE approach, taking the hyperfine structure for Eu into account, and the abundance of Ba was computed under the NLTE approximation. Results. We are able to assign most of the stars in our sample to the substructures of the Galaxy thick disk, thin disk, or Hercules stream according to their kinematics. The classification of 27 stars is uncertain. For most of the stars in the sample, the abundances of neutron-capture elements have not been measured earlier. For all of them, we provide the chemical composition and discuss the contribution from different nucleosynthesis processes. Conclusions. The [Ni/Fe] ratio shows a flat value close to the solar one for the whole metallicity range, with a small scatter, pointing to a nearly solar Ni/Fe ratio for the ejecta of both core-collapse SN and SNIa. The increase in the [Ni/Fe] for metallicity higher than solar is confirmed, and it is due to the metallicity dependence of 56 Ni ejecta from SNIa. Under large uncertainty in the age determination of observed stars, we verified that there is a large dispersion in the AMR in the thin disk, and no clear trend as in the thick disk. That may be one of the main reasons for the dispersion, observed for the s-process elements in the thin disk (e.g., Ba and La), whereas much narrower dispersion can be seen for r-process elements (e.g., Eu). Within the current uncertainties, we do not see a clear decreasing trend of [Ba/Fe] or [La/Fe] with metallicity in the thin disk, except maybe for super-solar metallicities. We cannot confirm an increase in the mentioned ratios with decreasing stellar age.

Heavy elements in old very metal-rich stars

“…7, using iron as the reference element. We compare our results with data from the literature for halo stars (François et al 2007;Ishigaki et al 2013;Nissen & Schuster 2011) and disk stars (Edvardsson et al 1993;Reddy et al 2003Reddy et al , 2006Bensby et al 2005;Nissen & Schuster 2011;Mishenina et al 2013). The halo data show a large scatter, as pointed out by François et al (2007), and the trend of element-to-Fe ratio for these four elements becomes tighter above [Fe/H] > −2.8.…”

“…Previous heavyelement abundance derivations in metal-rich stars were carried out for thin-disk (Edvardsson et al 1993;Reddy et al 2003;Allende Prieto et al 2004), and thick-disk stars (Reddy et al 2006;Bensby et al 2005). Recently, Mishenina et al (2013) derived heavy-element abundances of thin-, thick-disk and Hercules-stream stars, and abundances of bulge-field stars were determined by Bensby et al (2013) and Johnson et al (2012Johnson et al ( , 2013. In addition, heavy-element abundances were derived for open-cluster stars (e.g.…”

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…This is because the NLTE abundance corrections for lines of Sr II, Zr II, and Eu II are small in the stellar parameter range, with which we are concerned (see Section 3.3). The upward trend in [Zr/Fe] was reported by Mashonkina et al (2007b) and Mishenina et al (2013), although the latter paper studied a narrow range of metallicity, down to [Fe/H];−1. Enhancement of Eu relative to Fe and Ba in the halo and/or thick-disk stars was obtained earlier by Mashonkina & Gehren (2000), Burris et al (2000), Barklem et al (2005), Bensby et al (2005), and Mishenina et al (2013).…”

“…The upward trend in [Zr/Fe] was reported by Mashonkina et al (2007b) and Mishenina et al (2013), although the latter paper studied a narrow range of metallicity, down to [Fe/H];−1. Enhancement of Eu relative to Fe and Ba in the halo and/or thick-disk stars was obtained earlier by Mashonkina & Gehren (2000), Burris et al (2000), Barklem et al (2005), Bensby et al (2005), and Mishenina et al (2013). With a different stellar sample, Mashonkina et al (2007b) found Galactic trends for Zr/Ba and Zr/Sr that are very similar to ours.…”

“…In particular, the outer halo of the Galaxy may be dominated by the merging imprints of nearby dwarf galaxies, where the chemical evolution proceeds at a lower rate than that of the inner region of the Galaxy. Moreover, even for stars in the solar neighborhood, they have distinct abundance distributions (see, e.g., Nissen & Schuster 1997, 2010, indicating the complex chemical evolution of the Galaxy at different locations. To decipher the spectral fingerprints in terms of abundances requires realistic models for the stellar atmospheres and the line-formation processes.…”