Diversity of Abundance Patterns of Light Neutron-capture Elements in Very Metal-poor Stars

Misa Aoki, Yuhri Ishimaru, Wako Aoki, Shinya Wanajo

Published 2017-01-30Version 1

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 evidences of at least two components in the r-process; one referred to as the "main r-process" and the other the "weak r-process," mainly responsible in 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 show 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 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 small range of parameters, which can serve as constraint on future modelling of supernova models.