The s-process in low-metallicity stars - III. Individual analysis of CEMP-s and CEMP-s/r with asymptotic giant branch models

Abstract

We provide an individual analysis of 94 carbon enhanced metal-poor stars\nshowing an s-process enrichment (CEMP-s) collected from the literature. The\ns-process enhancement observed in these stars is ascribed to mass transfer by\nstellar winds in a binary system from a more massive companion evolving faster\ntoward the asymptotic giant branch (AGB) phase. The theoretical AGB\nnucleosynthesis models have been presented in Paper I. Several CEMP-s stars\nshow an enhancement in both s and r-process elements (CEMP-s/r). In order to\nexplain the peculiar abundances observed in CEMP-s/r stars, we assume that the\nmolecular cloud from which CEMP-s formed was previously enriched in r-elements\nby Supernovae pollution.\n A general discussion and the method adopted in order to interpret the\nobservations have been provided in Paper II. We present in this paper a\ndetailed study of spectroscopic observations of individual stars. We consider\nall elements from carbon to bismuth, with particular attention to the three\ns-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls]\nand [Pb/hs]. The presence of an initial r-process contribution may be typically\nevaluated by the [La/Eu] ratio. We found possible agreements between\ntheoretical predictions and spectroscopic data. In general, the observed\n[Na/Fe] (and [Mg/Fe]) provide information on the AGB initial mass, while\n[hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range\nof 13C-pocket strengths is required to interpret the observations. However,\nmajor discrepancies between models and observations exist. We highlight star by\nstar the agreements and the main problems encountered and, when possible, we\nsuggest potential indications for further studies. These discrepancies provide\nstarting points of debate for unsolved problems ...\n