More lead stars^*

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

Corresponding author: S. Van Eck, svaneck@astro.ulb.ac.be

Received: 18 July 2002
Accepted: 21 January 2003

Abstract

The standard model for the operation of the s-process in asymptotic giant branch (AGB) stars predicts that low-metallicity ([Fe/H] ) 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. [CITE]). 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] ). The line at  Å is detected and clearly resolved thanks to high-resolution spectra (). 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. [CITE]) 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.