Extremely Metal-Poor Stars and a Hierarchical Chemical Evolution Model
  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.
