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Extremely metal-poor (EMP) stars in the Milky Way halo are the low-mass survivors of the first stars and their descendants that were born and first lit the universe during the “cosmological dark age”. We use their properties to study the star formation and galaxy evolution process in the early universe. From the analysis of their surface abundances, it has been shown that EMP stars with metallicity [Fe/H]<=-2.5 were born under the initial mass function (IMF) peaked around 10 M⊙ and that most of their relic stars currently observed are the secondary members of binary systems. We investigate the star formation and early chemical evolution with the derived IMF in the context of the hierarchical formation process of the Galaxy to demonstrate that the metallicity distribution function (MDF) of these halo stars is well reproduced in terms of the high-mass IMF and the binary origin of low-mass survivors. Our results suggest that the three most iron-poor stars of [Fe/H]<-4.5 known to date are Population III stars that were formed out of the primordial gas and have suffered the surface pollution due to the accretion of metal-rich gas. We also discuss the nature of the first stars and in particular the possible traces of pair-instability supernovae (PISNe). [ABSTRACT FROM AUTHOR]

We discuss the star formation history of the Galaxy, based on the observations of extremely metal-poor stars (EMP) in the Galactic halo, to gain an insight into the evolution and structure formation in the early universe. The initial mass function (IMF) of EMP stars is derived from the observed fraction of carbon-enhanced EXP (CEMP) stars among the EMP survivors, which are thought to originate from the evolution in the close binary systems with mass transfer. Relying upon the theory of the evolution of EMP stars and of their binary evolution, we find that stars of metallicity [Fe/H]<=-2.5 were formed at typical mass of ∼10 M⊙. The top heavy IMF thus obtained is applied to study the early chemical evolution of the Galaxy. We construct the merging history of our Galaxy semi-analytically and derive the metallicity distribution function (MDF) of low-mass EMP stars that survive to date with taking into account the contribution of binary systems. It is shown that the resultant MDF can well reproduce the observed distribution of EMP survivors, and, in particular, that they almost all stem from a less-mass companion in binary systems. We also investigate how first stars affect the MDF of EMP stars. [ABSTRACT FROM AUTHOR]