More lead stars

TLDR

The standard s‑process model predicts that low‑metallicity AGB stars should exhibit large overabundances of Pb and Bi relative to other s‑elements. The authors investigate whether alternative proton‑mixing scenarios can explain low‑metallicity stars that deviate from the standard s‑process predictions. They discuss variations on the canonical proton‑mixing process below the convective hydrogen envelope that could account for the observed discrepancies. Eight CH stars were analyzed; five show lead enrichment ([Pb/Ce] ≈ 0.7) and their heavy‑element abundances match low‑metallicity s‑process predictions, except for two stars that do not conform.

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. Eight CH stars (with [Fe/H] ranging from -1.5 to -2.5) are studied in the present paper using high-resolution spectra (R = lambda/Delta lambda = 135 000), and five of them appear to be enriched in lead (with [Pb/Ce] ~ 0.7). Abundances of heavy elements in these `lead stars' (defined as stars enriched in s-elements and with [Pb/hs] >= 1, hs being any of Ba, La or Ce) 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. Two among the studied stars (HD 189711 and V Ari) add to a growing number of low-metallicity stars 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.