The effect of r-process enhancement in binary CEMP-s/r stars (1012.0456v1)

Abstract: About half of carbon and s-process enhanced metal-poor stars (CEMP-s) show a high r-process enrichment (CEMP-s/r), incompatible with a pure s-process contribution. CEMP-s stars are of low mass (M < 0.9 Msun) and belong to binary systems. The C and s-process enrichment results from mass transfer by the winds of the primary AGB companion (now a white dwarf). The nucleosynthesis of the r-process, instead, is believed to occur in massive stars exploding as Supernovae of Type II. The most representative r-process element is Eu (95% of solar Eu). We suggest that the r-process enrichment was already present by local SNII pollution in the molecular cloud from which the binary system formed. The initial r-enrichment does not affect the s-process nucleosynthesis. However, the s-process indicators hs/ls and [Pb/hs] may depend on the initial r-enhancement. For instance, the hs-peak has to account of an r-process contribution estimated to be 30% for solar La, 40% for solar Nd, and 70% for solar Sm. A large spread of [Eu/Fe] is observed in unevolved halo stars up to [Eu/Fe] ~ 2. In presence of a very high initial r-enrichment of the molecular cloud, the maximum [hs/Fe] predicted in CEMP-s/r stars may increase up to 0.3 dex. Instead, the spread of [Y,Zr/Fe] observed in unevolved halo stars reaches a maximum of only ~ 0.5 dex, not affecting much the predicted [ls/Fe]. This is in agreement with observations of CEMP-s/r stars that show an observed [hs/ls] in average higher than that observed in CEMP-s. Preliminary results are presented.