  (Abridged) 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 the heavy
r-process nuclei (with mass numbers A>130) is not related to the production of
the Fe group elements or of the elements with lower atomic numbers: Na, Mg, Al,
Si, Ca, Sc, and Ti. This requires that the production of the heavy r-nuclei not
occur in supernovae with extended shell structure, but be associated with
either bare neutron stars or SNe II in the range of 8-10 solar masses. 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 the 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 about 1.6 billion
neutron stars in the Galaxy.
