EDP Sciences
Free Access
Volume 456, Number 1, September II 2006
Page(s) 313 - 321
Section Stellar atmospheres
DOI https://doi.org/10.1051/0004-6361:20054749
Published online 23 August 2006

A&A 456, 313-321 (2006)
DOI: 10.1051/0004-6361:20054749

Barium even-to-odd isotope abundance ratios in thick disk and thin disk stars

L. Mashonkina1, 2, 3 and G. Zhao3

1  Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, 81679 München, Germany
    e-mail: lyuda@usm.lmu.de
2  Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia
    e-mail: lima@inasan.ru
3  National Astronomical Observatories, Chinese Academy of Science, A20 Datun Road, Chaoyang District, Beijing 100012, PR China

(Received 22 December 2005 / Accepted 26 March 2006)

We present the Ba even-to-odd isotope abundance ratios in 25 cool dwarf stars with the metallicity [Fe/H] ranging between 0.25 and -1.35. Our method takes advantage of the hyperfine structure (HFS) affecting the $\ion{Ba}{ii}$ resonance line of the odd isotopes. The fractional abundance of the odd isotopes of Ba is derived from a requirement that Ba abundances from the resonance line $\lambda\,4554$ and subordinate lines $\lambda\,5853$ and $\lambda\,6496$ must be equal. The results are based on NLTE line formation and analysis of high resolution ( $R \sim 60\,000$) high signal-to-noise ( ${\it S/N} \ge 200$) observed spectra. We find that the fraction of the odd isotopes of Ba grows toward the lower Ba abundance (or metallicity) and the mean value in the thick disk stars equals $33 \pm 4\%$. This indicates the higher contribution of the r-process to barium in the thick disk stars compared to the solar system matter. The obtained fraction increases with the [Eu/Ba] abundance ratio growth in agreement with expectations. A significant fraction of the even isotopes of Ba found in old Galactic stars (the thick disk stars), ~67%, is in contrast to the prediction of the "classical" model of the s-process and favors the value predicted by the "stellar" models of Arlandini et al. (1999) and Travaglio et al. (1999).

Key words: line: formation -- nuclear reactions, nucleosynthesis, abundances -- stars: abundances -- stars: late-type -- Galaxy: evolution

© ESO 2006

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