The origin of light neutron-capture elements in very metal-poor stars

We obtained high resolution spectra of 40 very metal-poor stars, and measured the abundances of heavy elements. The abundance pattern of the heavy neutron-capture elements (56 <= Z <= 70) in r-process-enhanced, metal-poor stars are quite similar to that of the r-process component in solar-system material. In contrast, the abundance ratios of the light neutron-capture elements (38 <= Z <= 40) to heavier ones show a large dispersion. We investigated the correlation between Sr (Z = 38) and Ba (Z = 56) abundances, and obtained two clear results: (1) Ba-enhanced stars also show large excess of Sr (there is no object which is Ba-rich and Sr-poor); (2) stars with low Ba abundance show large scatter in Sr abundance. This trend is naturally explained by hypothesizing the existence of two processes, one Chat produces Sr without Ba and the other that produces Sr and Ba in similar proportions.