More Lead Stars ⋆

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 λ4057.812Å 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.