2003A&A...404..291V

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., 2001Natur.412..793V). 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 PbI line at λ4057.812Å is detected and clearly resolved thanks to high-resolution spectra (R=λ/Δλ=135000). 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., 2001ApJ...561..346A) 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.