2021RAA....21..111H

In the early Galaxy, elemental abundances of the extremely metal-poor (EMP) stars contain abundant information about the neutron-capture nucleosynthesis and the chemical enrichment history. In this work, we study the abundance characteristics of Sr and Ba for the EMP stars in the [Sr/Ba] vs. [Ba/Fe] space. We find that there are three boundaries for the distribution region of the EMP stars. The weak r-process star CS 22897-008 lies on the upper end and the main r-process stars lie on the right end of the region. Near the right boundary of the distribution region, there is an Fe-normal belt. For the EMP stars in the belt, element Fe dominantly originates from the normal massive stars. The low-Sr stars ([Sr/Fe]<=-0.3) distribute in the region of the lower left of the Fe-normal belt and their Fe should originate partly from the prompt inventory. We find that the formation of the lower boundary of the distribution region is due to the pollution of the main r-process material and the formation of the right boundary could be explained by the combination of the weak r- and main r-process material. Furthermore, the formation of the left boundary is due to the pollution of the weak r-process material. Although the [Sr/Ba] ratios are related to the relative importance of the weak r-process material, the scatter of [Sr/Ba] ratios for the EMP stars mainly depends on the abundance ratio of the weak r-process.