  The chemical abundances of the very metal-poor double-enhanced stars are
excellent information for setting new constraints on models of neutron-capture
processes at low metallicity. These stars are known as s+r stars, since they
show enhancements of both s-process and r-process elements. The observed
abundance ratios for the double-enhanced stars can be explained by those of
stars that were polluted by an AGB star and subsequently accreted very
significant amounts of r-process material out of an AIC (accretion-induced
collapse) or Type 1.5 supernova. In this paper we present for the first time an
attempt to fit the elemental abundances observed in the s- and r-rich, very
metal-poor stars using a parametric model and suggest a new concept of
component coefficients to describe the contributions of the individual
neutron-capture processes to double-enhanced stars. We find that the abundance
ratios of these stars are best fitted by enrichments of s- and r-process
material. The overlap factor in the AGB stars where the observed s-process
elements were produced lies between 0.1 and 0.81. Taking into account the
dependence of the initial-final mass relations on metallicity, this wide range
of values could possibly be explained by a wide range of core-mass values of
AGB stars at low metallicity. The component coefficient of the r-process is
strongly correlated with the component coefficient of the s-process for the
double-enhanced stars. This is significant evidence that the r-process material
in double-enhanced stars comes from an AIC or Type 1.5 supernova.
