Stellar Sources for Heavyr‐Process Nuclei

Abstract

The stellar sites and the complete mechanism of r-process nucleosynthesis are still unresolved issues. From consideration of the observed abundances in metal-poor stars, it is proposed that the production of heavy r-process nuclei (r-nuclei with mass numbers A > 130) is not related to the production of Fe group elements or of elements with lower atomic numbers: Na, Mg, Al, Si, Ca, Sc, and Ti. This requires that the production of heavy r-nuclei not occur in supernovae with extended shell structure but be associated with either bare neutron stars or Type II supernovae (SNe II) in the mass range 8 M_☉ < M < 10 M_☉. From the observations of stars with [Fe/H] ~ -3 but with high abundances of r-elements, it is clear that these r-process enrichments cannot represent the composition of the interstellar medium from which the stars were formed but must represent very local contamination from binary companions. Further evidence for very high enrichments of s-process elements in metal-poor stars also requires binary systems for explanation. We propose that the accretion-induced collapse (AIC) of a white dwarf into a neutron star in a binary system may be associated with the production of heavy r-nuclei and may provide occasional coupling of high r-process and high s-process enrichments in the envelopes of low-mass stars with low [Fe/H]. If we assume that the bulk of the heavy r-nuclei are produced in AIC events, then these events would have produced ~1.6 × 109 neutron stars in the Galaxy. A much larger number of white dwarf binaries would have resulted from the evolution of other binary systems. The AIC scenario removes the assignment in our earlier model that SNe II provide the bulk of heavy r-nuclei and relegates r-process production in SNe II to light r-nuclei with A ≾ 130. This new assignment gives Fe yields that are in accord with the observed values for most SNe II.