EARLY-AGE EVOLUTION OF THE MILKY WAY RELATED BY EXTREMELY METAL-POOR STARS

Abstract

We exploit the recent observations of extremely metal-poor (EMP) stars in the Galactic halo and investigate the constraints on the IMF of the stellar population that left these low-mass survivors of [Fe/H]<-2.5 and the chemical evolution that they took part in. A high-mass IMF with the typical mass~10Msun and the overwhelming contribution of low-mass members of binaries to the EMP survivors are derived from the statistics of carbon-enriched EMP stars with and without the enhancement of s-process elements (Komiya et al. 2007). We first examine the analysis to confirm their results for various assumptions on the mass-ratio distribution function. As compared with the uniform distribution, the increase or decrease function of the mass ratio gives a higher- or lower-mass IMF, and a lower-mass IMF results for the independent distribution with the both members in the same IMF, but the derived ranges of typical mass differ less than by a factor of two and overlap for the extreme cases. Furthermore, we prove that the same constraints are placed on the IMF from the surface density of EMP stars estimated from the surveys and the chemical evolution consistent with the metal yields of theoretical supernova models. We then apply the derived high-mass IMF with the binary contribution to show that the observed MDF of EMP stars can be reproduced not only for the shape but also for the number of EMP stars. In particular, the scarcity of stars below [Fe/H]<-4 is naturally explained in terms of the hierarchical structure formation, and there is no indication of significant changes in the IMF for the EMP Population. The present study indicates that 3 HMP stars of [Fe/H]<-4 are the primordial stars that were born as the low-mass members of binaries before the host clouds were polluted by their own supernovae.