Extremely-Metal Poor Stars in the Milky Way: A Second Generation Formed after Reionization

TL;DR

This paper proposes that the initial mass function of second-generation stars formed before reionization was deficient in low-mass stars due to the CMB temperature floor, explaining the observed properties of extremely-metal poor stars.

Contribution

It introduces a new model where the IMF of early stars is shaped by the CMB temperature, reconciling simulations with observed EMP star abundances and suggesting a focus on later formation epochs.

Findings

EMP stars formed at z<10 show signatures of Population II enrichment.

Current EMP star samples mainly constrain late-time Population III IMF.

Probing higher redshift Population III stars requires large-scale EMP star surveys.

Abstract

Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M>100Msun) formed in minihalos at redshift z>20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely-metal poor (EMP) stars in the Milky Way halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose a new solution to the problem in which the IMF of second-generation stars formed at z>10, before reionization, is deficient in sub-solar mass stars, owing to the high cosmic microwave background temperature floor. The observed EMP stars are formed preferentially at z<10 in pockets of gas enriched to metallicity Z>10^{-3.5} Zsun by winds from Population II stars. Our cosmological simulations of dark matter halos like the Milky Way show that current samples of EMP stars can only constrain the IMF of late-time Population III stars, formed at z<13 in halos with virial temperature Tvir~10^4 K. This suggests that pair instability supernovae were not produced primarily by this population. To begin probing the IMF of Population III stars formed at higher redshift will require a large survey, with at least 500 and probably several thousand EMP stars of metallicities Z~10^{-3.5} Zsun.