Enrichment of Very Metal Poor Stars with Bothr‐Process ands‐Process Elements from 8–10M⊙Stars

Recent spectroscopic studies have revealed the presence of numerouscarbon-enhanced, metal-poor stars with [Fe/H] < -2.0 that exhibit strongenhancements of s-process elements. These stars are believed to be the resultof a binary mass-transfer episode from a former asymptotic giant-branch (AGB)companion that underwent s-process nucleosynthesis. However, several such starsexhibit significantly lower Ba/Eu ratios than solar s-process abundances. Thismight 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 modelin which the double enhancements of r-process and s-process elements originatefrom a former 8-10 M_\odot companion in a wide binary system, which may undergos-processing during an AGB phase, followed by r-processing during itssubsequent supernova explosion. The mass of Eu (as representative of r-processelements) captured by the secondary through the wind from the supernova isestimated, which is assumed to be proportional to the geometric fraction of thesecondary (low-mass, main-sequence) star with respect to the primary(exploding) star. We find that the estimated mass is in good agreement with aconstraint on the Eu yield per supernova event obtained from a Galacticchemical evolution study, when the initial orbital separation is taken to be\sim 1 year. If one assumes an orbital period on the order of five years, theefficiency of wind pollution from the supernova must be enhanced by a factor of\sim 10. This may, in fact, be realized if the expansion velocity of thesupernova's innermost ejecta, in which the r-process has taken place, issignificantly slow, resulting in an enhancement of accretion efficiency bygravitational focusing.Comment: 10 pages, 1 figure, to be published in ApJ, January 200