Formation History of Metal-Poor Halo Stars with Hierarchical Model and the Effect of ISM accretion on the Most Metal-Poor Stars

TL;DR

This paper models the formation and chemical evolution of metal-poor halo stars within a hierarchical galaxy formation framework, examining the impact of ISM accretion and Pop III feedback on observed stellar metallicities.

Contribution

It presents a hierarchical model explaining EMP star metallicity distribution, surface pollution effects, and the role of Pop III stars in early galaxy evolution.

Findings

Hierarchical formation explains MDF features including the [Fe/H]~-4 break.

High-mass IMF with binaries matches observed EMP star frequencies.

ISM accretion accounts for surface pollution in HMP stars.

Abstract

We investigate the star formation and chemical evolution in the early universe by considering the merging history of the Galaxy in the {\Lambda}CDM scenario according to the extended Press-Schechter theory. We give some possible constraints from comparisons with observation of extremely metal-poor (EMP) stars. We demonstrate that (1) The hierarchical structure formation can explain the characteristics of the observed metallicity distribution function (MDF) including a break around [Fe/H]~-4. (2) A high mass IMF of peak mass ~10Msun with the contribution of binaries, derived from the statistics of carbon enhanced EMP stars (Komiya et al. 2007), predicts the frequency of low-mass survivors consistent with the number of EMP stars observed for -4~<[Fe/H]~<-2.5. (3) The stars formed from primordial gas before the first supernova explosions in their host mini-halos are assigned to the HMP stars with [Fe/H]~-5. (4) There is no indication of significant changes in the IMF and the binary contribution at metallicity -4~<[Fe/H]~<-2.5, or even larger as long as the field stars of Galactic halo are concerned. We further study the effects of the surface pollution through the accretion of ISM along the chemical and dynamical evolution of the Galaxy for low-mass Pop.III and EMP survivors. Because of shallower potential of smaller halos, the accretion of ISM in the mini-halos in which these stars were born dominates the surface metal pollution. This can account for the surface iron abundances as observed for the HMP stars if the cooling and concentration of gas in their birth mini-halos is taken into account. We also study the feedback effect from the very massive Pop. III stars. The metal pre-pollution by PISNe is shown to be compatible with the observed lack of their nucleosynthetic signatures when some positive feedback on gas cooling works and changes IMF from being very massive to being high mass.