Title: Diversity of Abundance Patterns of Light Neutron-capture Elements in Very-metal-poor Stars,
Abstract: We determine the abundances of neutron-capture elements from Sr to Eu for
five very-metal-poor stars (-3<[Fe/H]<-2) in the Milky Way halo to reveal the
origin of light neutron-capture elements. Previous spectroscopic studies have
shown evidence of at least two components in the r-process; one referred to as
the "main r-process" and the other as the "weak r-process," which is mainly
responsible for producing heavy and light neutron-capture elements,
respectively. Observational studies of metal-poor stars suggest that there is a
universal pattern in the main r-process, similar to the abundance pattern of
the r-process component of solar-system material. Still, it is uncertain
whether the abundance pattern of the weak r-process shows universality or
diversity, due to the sparseness of measured light neutron-capture elements. We
have detected the key elements, Mo, Ru, and Pd, in five target stars to give an
answer to this question. The abundance patterns of light neutron-capture
elements from Sr to Pd suggest a diversity in the weak r-process. In
particular, scatter in the abundance ratio between Ru and Pd is significant
when the abundance patterns are normalized at Zr. Our results are compared with
the elemental abundances predicted by nucleosynthesis models of supernovae with
parameters such as electron fraction or proto-neutron-star mass, to investigate
sources of such diversity in the abundance patterns of light neutron-capture
elements. This paper presents that the variation in the abundances of observed
stars can be explained with a small range of parameters, which can serve as
constraints on future modeling of supernova models.