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 } ] \lesssim - 2.5 were formed at typical mass of \sim 10 M _ { \odot } .
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 .