Nuclei in the Cosmos - IX

Chemical evolution of C-Zn and r-process elements produced by the first generation stars

28 Jun 2006, 11:50

20m

CERN, Geneva

Oral contribution Evidences of nucleosynthesis in stars and in presolar grains 10 Element production & stellar evolution II

Speaker

Yuhri Ishimaru (Academic Support Center, Kogakuin University)

Description

Metal-poor stars record enrichment history of the Galaxy at the early epoch. Abundance analysis of these stars reveals large star-to-star scatters in r-process elements. This may be interpreted as a result of incomplete mixing of the interstellar medium (ISM) at the beginning of the Galaxy. However, recent studies also show considerable small dispersions for abundance ratios of C-Zn (Cayrel et al. 2004). We construct an inhomogeneous chemical evolution model, assuming supernova induced star formation. Then we discuss whether inhomogeneity of the ISM consistently accounts for observed differences between r-process and lighter elements, using several latest sets of supernova yields for metal-poor stars. If metal-poor stars are enriched by only one or a few supernovae, huge dispersions in r-process elements possibly imply that their yields are highly dependent on supernova progenitor masses. However, the site of r-process is still uncertain even from nucleosynthesis studies. We, then, attempt to determine the site of r-process, using an inhomogeneous chemical evolution model. In our previous study, we have shown that values of [Eu/Fe] of three metal poor stars given by Subaru observation strongly support the model where the r-process site is assumed as the low mass-end of supernova progenitors, such as 8-10 solar mass stars. On the other hand, a large dispersion has been found in [Sr/Ba] at lower metallicity (e.g., Ryan et al. 1996; Honda et al. 2004), suggesting that lighter elements such as Sr does not come from a universal process, which produces Ba and Eu, but from `weak' r-process. We show that this scenario well explains observations, when weak r-process produces ~60% of Sr but only ~1% of Ba in metal- poor stars. Intermediate mass elements between Sr and Ba must provide clues to understand the nucleosynthesis of weak r-process. We estimate Pd abundances of very metal-poor stars, using Subaru and also show that weak r-process pattern gradually decreases with atomic mass from Sr to Ba.