| Issue |
A&A
Volume 474, Number 1, October IV 2007
|
|
|---|---|---|
| Page(s) | 221 - 228 | |
| Section | Stellar atmospheres | |
| DOI | https://doi.org/10.1051/0004-6361:20066735 | |
| Published online | 13 August 2007 | |
Ruthenium and hafnium abundances in giant and dwarf barium stars *,**
Observatório do Valongo/UFRJ, Ladeira do Pedro Antonio 43, 20080-090 Rio de Janeiro, RJ, Brazil e-mail: d.moreira-allen@herts.ac.uk; gustavo@ov.ufrj.br
Received:
10
November
2006
Accepted:
6
July
2007
Aims.We present abundances for Ru and Hf, compare them to abundances of other heavy elements, and discuss the problems found in determining Ru and Hf abundances with laboratory gf-values in the spectra of barium stars.
Methods.We determined Ru and Hf abundances in a sample
of giant and dwarf barium stars, by the spectral synthesis of two
(λ4080.574 and λ4757.856) and two
(λ4080.437 and
λ4093.155) transitions. The stellar spectra were observed with
FEROS/ESO, and the stellar atmospheric parameters lie in the range
4300 < Teff/K < 6500, -1.2 < [Fe/H] ≤ 0 and
1.4 ≤ log g < 4.6.
Results.The λ4080 and the λ4758 observed
transitions result in a unreasonably high solar abundance,
given certain known uncertainties, when fitted with
laboratory gf-values. For these two transitions we determined
empirical gf-values by fitting the observed line profiles of the
spectra of the Sun and Arcturus. For the sample stars, this procedure
resulted in a good agreement of Ru and Hf
abundances given by the two available lines. The resulting Ru and
Hf abundances were compared to those of Y, Nd, Sm and Eu.
In the solar system Ru, Sm and Eu are dominated by the r-process
and Hf, Nd and Y by the s-process, and
all of these elements are enhanced in barium stars since they
lie inside the s-process path.
Ru abundances show large scatter when compared to other
heavy elements, whereas Hf abundances show less scatter and
closely follow the abundances of Sm and Nd, in good agreement with
theoretical expectations. We also suggest a possible, unexpected,
correlation of Ru and Sm abundances. The observed behaviour in
abundances is probably due to variations in the 13C pocket
efficiency in AGB stars, and, though masked by high uncertainties,
hint at a more complex scenario than proposed by theory.
Key words: stars: abundances / stars: chemically peculiar / stars: late-type
© ESO, 2007
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