EDP Sciences
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Volume 404, Number 1, June II 2003
Page(s) 291 - 299
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20030447

A&A 404, 291-299 (2003)
DOI: 10.1051/0004-6361:20030447

More lead stars

S. Van Eck1, S. Goriely1, A. Jorissen1 and B. Plez2

1  Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
2  GRAAL, Université de Montpellier II, cc 072, 34095 Montpellier Cedex 05, France

(Received 18 July 2002 / Accepted 21 January 2003 )

The standard model for the operation of the s-process in asymptotic giant branch (AGB) stars predicts that low-metallicity ([Fe/H] $\la-1$) AGB stars should exhibit large overabundances of Pb and Bi as compared to other s-elements. The discovery of the first three such "lead stars" (defined as stars enriched in s-elements with [Pb/hs] $\ga $1, hs being any of Ba, La or Ce) among CH stars has been reported in a previous paper (Van Eck et al. 2001). Five more CH stars (with [Fe/H] ranging from -1.5 to -2.5) are studied in the present paper, and two of them appear to be enriched in lead (with [Pb/Ce]  $\simeq 0.7$). The $\ion{Pb}{i}$ line at $\lambda4057.812$ Å is detected and clearly resolved thanks to high-resolution spectra ( $R = \lambda/\Delta\lambda = 135\ts000$). The abundances for these two stars (HD 198269 and HD 201626) are consistent with the predictions for the s-process operating in low-metallicity AGB stars as a consequence of the "partial mixing" of protons below the convective hydrogen envelope. Another two stars (HD 189711 and V Ari) add to a growing number of low-metallicity stars (also including LP 625-44 and LP 706-7, as reported by Aoki et al. 2001) which do not conform to these predictions. Variations on the canonical proton-mixing scenario for the operation of the s-process in low-metallicity stars, that could account for these discrepant stars, are briefly discussed.

Key words: nuclear reactions, nucleosynthesis, abundances -- stars: carbon -- stars: evolution -- stars: population II

Offprint request: S. Van Eck, svaneck@astro.ulb.ac.be

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© ESO 2003