The IMF of Extremely Metal-Poor Stars and the Probe into the Star-Formation Process of the Milky Way

TL;DR

This paper investigates the initial mass function of extremely metal-poor stars in the Milky Way's halo, using binary star evolution and observed star fractions to understand early galaxy formation and chemical evolution.

Contribution

It derives a top-heavy IMF for EMP stars at [Fe/H]<-2.5 based on binary evolution theory and observed CEMP fractions, providing new insights into early star formation.

Findings

EMP stars with [Fe/H]<-2.5 formed around 10 solar masses

The derived IMF is top-heavy and influences early chemical evolution models

The metallicity distribution function matches observed EMP survivor distributions

Abstract

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 initialmass 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 ~10M_sun. 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.