Extremely Metal-Poor Stars and a Hierarchical Chemical Evolution Model

Abstract

Early phases of the chemical evolution and formation history of extremely metal poor (EMP) stars are investigated using hierarchical galaxy formation models. We build a merger tree of the Galaxy according to the extended Press-Schechter theory. We follow the chemical evolution along the tree, and compare the model results to the metallicity distribution function (MDF) and abundance ratio distribution of the Milky Way halo. We adopt three different initial mass functions (IMFs). In a previous studies, we argue that typical mass of EMP stars should be high-mass(~10Msun) based on studies of binary origin carbon-rich EMP stars. In this study, we show that only the high-mass IMF can explain a observed small number of EMP stars. For relative element abundances, the high-mass IMF and the Salpeter IMF predict similar distributions. We also investigate dependence on nucleosynthetic yields of supernovae (SNe). The theoretical SN yields by Kobayashi et al.(2006) and Chieffi & Limonge (2004) show reasonable agreement with observations for $\alpha$-elements. Our model predicts significant scatter of element abundances at [Fe/H]<-3. Best fit yields for one zone chemical evolution model by Francois et al.(2004) well reproduces the trend of the typical abundances of EMP stars but our model with their yield predicts much larger scatter of abundances than the observations. The model with hypernovae predicts Zn abundance in agreement with observations but other models predict lower [Zn/Fe]. Ejecta from the hypernovae with large explosion energy is mixed in large mass and decreases scatter of the element abundances.