Neutron‐Capture Elements in thes‐ andr‐Process‐rich Stars: Constraints on Neutron‐Capture Nucleosynthesis Processes

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.