Enrichment of Very Metal Poor Stars with Both r ‐Process and s ‐Process Elements from 8–10 M ⊙ Stars

Abstract

Recent spectroscopic studies have revealed the presence of numerouscarbon-enhanced, metal-poor stars with [Fe/H]{\lt}-2.0 that exhibitstrong enhancements of s-process elements. These stars are believed tobe the result of a binary mass transfer episode from a former asymptoticgiant branch (AGB) companion that underwent s-process nucleosynthesis.However, several such stars exhibit significantly lower Ba/Eu ratiosthan solar s-process values. This might be explained if there were anadditional contribution from the r-process, thereby diluting the Ba/Euratio by extra production of Eu. We propose a model in which the doubleenhancements of r-process and s-process elements originate from a former8-10 M_{solar} companion in a wide binary system, which mayundergo s-processing during an AGB phase, followed by r-processingduring its subsequent supernova explosion. The mass of Eu (asrepresentative of r-process elements) captured by the secondary throughthe wind from the supernova is estimated and is assumed to beproportional to the geometric fraction of the secondary (low-mass,main-sequence) star with respect to the primary (exploding) star. Wefind that the estimated mass is in good agreement with a constraint onthe Eu yield per supernova event obtained from a Galactic chemicalevolution study, when the initial orbital separation is taken to be ~1yr. If one assumes an orbital period on the order of 5 yr, theefficiency of wind pollution from the supernova must be enhanced by afactor of ~10. This may, in fact, be realized if the expansion velocityof the supernova's innermost ejecta, in which the r-process has takenplace, is significantly slow, resulting in an enhancement of accretionefficiency by gravitational focusing.