More lead stars

Abstract

The standard model for the operation of the s-process in asymptotic giant branch (AGB) stars predicts that low-metallicity ([Fe/H] less than or similar to -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] greater than or similar to1, 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] similar or equal to 0.7). The Pb. line at lambda4057.812 Angstrom is detected and clearly resolved thanks to high-resolution spectra (R = lambda/Deltalambda = 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) 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