Formation History of Metal-Poor Halo Stars with Hierarchical Model and
  the Effect of ISM accretion on the Most Metal-Poor Stars
  We investigate the star formation and chemical evolution in the early
universe by considering the merging history of the Galaxy in the {\Lambda}CDM
scenario according to the extended Press-Schechter theory. We give some
possible constraints from comparisons with observation of extremely metal-poor
(EMP) stars. We demonstrate that (1) The hierarchical structure formation can
explain the characteristics of the observed metallicity distribution function
(MDF) including a break around [Fe/H]~-4. (2) A high mass IMF of peak mass
~10Msun with the contribution of binaries, derived from the statistics of
carbon enhanced EMP stars (Komiya et al. 2007), predicts the frequency of
low-mass survivors consistent with the number of EMP stars observed for
-4~<[Fe/H]~<-2.5. (3) The stars formed from primordial gas before the first
supernova explosions in their host mini-halos are assigned to the HMP stars
with [Fe/H]~-5. (4) There is no indication of significant changes in the IMF
and the binary contribution at metallicity -4~<[Fe/H]~<-2.5, or even larger as
long as the field stars of Galactic halo are concerned. We further study the
effects of the surface pollution through the accretion of ISM along the
chemical and dynamical evolution of the Galaxy for low-mass Pop.III and EMP
survivors. Because of shallower potential of smaller halos, the accretion of
ISM in the mini-halos in which these stars were born dominates the surface
metal pollution. This can account for the surface iron abundances as observed
for the HMP stars if the cooling and concentration of gas in their birth
mini-halos is taken into account. We also study the feedback effect from the
very massive Pop. III stars. The metal pre-pollution by PISNe is shown to be
compatible with the observed lack of their nucleosynthetic signatures when some
positive feedback on gas cooling works and changes IMF from being very massive
to being high mass.
