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

We exploit the recent observations of extremely metal-poor (EMP) stars in the Galactic halo and investigate the constraints on the initial mass function (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. In previous study, the high-mass nature of the IMF with typical mass ≅10 M sun for the stars of the EMP population 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. We first examine the analysis to confirm their results for various assumptions on the mass-ratio distribution function of binary members. As compared with the uniform distribution they used, the increase or decrease function of the mass ratio gives a higher- or lower-mass IMF. For the independent distribution a lower-mass IMF results in both the members in the same IMF, but the derived ranges of typical mass differ less than by a factor of 2 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 (SN) models. We then apply the derived high-mass IMF with the binary contribution to show that the observed metallicity distribution function (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 hierarchical structure formation, and there is no indication of significant changes in the IMF for the EMP population. The present study indicates that three hyper-metal-poor/ultra-metal-poor 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 SNe.