  The chemical abundances of metal-poor stars are an excellent test bed by
which to set new constraints on models of neutron-capture processes at low
metallicity. Some r-process-rich (hereafter r-rich) metal-poor stars, such as
HD221170, show an overabundance of the heavier neutron-capture elements and
excesses of lighter neutron-capture elements. The study of these r-rich stars
could give us a better understanding of weak and main r-process nucleosynthesis
at low metallicity. Based on conclusions from the observation of metal-poor
stars and neutron-capture element nucleosynthesis theory, we set up a model to
determine the relative contributions from weak and main r-processes to the
heavy-element abundances in metal-poor stars. Using this model, we find that
the abundance patterns of light elements for most sample stars are close to the
pattern of weak r-process stars, and those of heavier neutron-capture elements
very similar to the pattern of main r-process stars, while the lighter
neutron-capture elements can be fitted by the mixing of weak and main r-process
material. The production of weak r-process elements appears to be associated
with the light elements, while the production of main r-process elements is
almost decoupled from that of the light elements. We compare our results with
the observed data at low metallicities, showing that the predicted trends are
in good agreement with the observed trends, at least for the metallicity range
[Fe/H] < -2.1. For most sample stars, the abundance patterns of both
neutron-capture elements and light elements could be best explained by a star
formed in a molecular cloud that has been polluted by both weak and main
r-process material.
