Recent spectroscopic studies have revealed the presence of numerous carbon-enhanced, metal-poor stars with [Fe/H] < -2.0 that exhibit strong enhancements of s -process elements. These stars are believed to be the result of a binary mass transfer episode from a former asymptotic giant branch (AGB) companion that underwent s -process nucleosynthesis. However, several such stars exhibit significantly lower Ba/Eu ratios than solar s -process values. This might be explained if there were an additional contribution from the r -process, thereby diluting the Ba/Eu ratio by extra production of Eu. We propose a model in which the double enhancements of r -process and s -process elements originate from a former 8-10 M ☉ companion in a wide binary system, which may undergo s -processing during an AGB phase, followed by r -processing during its subsequent supernova explosion. The mass of Eu (as representative of r -process elements) captured by the secondary through the wind from the supernova is estimated and is assumed to be proportional to the geometric fraction of the secondary (low-mass, main-sequence) star with respect to the primary (exploding) star. We find that the estimated mass is in good agreement with a constraint on the Eu yield per supernova event obtained from a Galactic chemical evolution study, when the initial orbital separation is taken to be ~1 yr. If one assumes an orbital period on the order of 5 yr, the efficiency of wind pollution from the supernova must be enhanced by a factor of ~10. This may, in fact, be realized if the expansion velocity of the supernova's innermost ejecta, in which the r -process has taken place, is significantly slow, resulting in an enhancement of accretion efficiency by gravitational focusing.
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Wanajo et al. (2006) studied this question.