More lead stars.pdf

a r X i v : a s t r o - p h / 0 3 0 2 0 7 5 v 1 5 F e b 2 0 0 3 Astronomy Astrophysics manuscript no. newpaper February 2, 2008 (DOI: will be inserted by hand later) More lead stars? S. Van Eck1??, S. Goriely1???, A. Jorissen1???, and B. Plez2 1 Institut d’Astronomie et d’Astrophysique, Universite? Libre de Bruxelles, C.P. 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium 2 GRAAL, Universite? de Montpellier II, cc 072, F-34095 Montpellier Cedex 05, France Received date / Accepted date Abstract. The standard model for the operation of the s-process in asymptotic giant branch (AGB) stars predicts that low-metallicity ([Fe/H] ～ ? 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] ～ 1, hs being any of Ba, La or Ce) among CH stars has been reported in a previous paper (Van Eck et al., Nature 2001, 412, 793). 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]? 0.7). The Pb I line at λ4057.812 A? is detected and clearly resolved thanks to high-resolution spectra (R = λ/?λ = 135 000). 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, ApJ 561, 346) which do not conform to these predictions, however. 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. Nucleosynthesis, nuclear reactions, abundances – Stars: carbon – Stars: evolut