Extremely metal-poor stars in the milky way: A second generation formed after reionization

Abstract

Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M ≳ 100 M ·) 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 (MW) 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 Z · by winds from Population II stars. Our cosmological simulations of dark matter halos like the MW 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 T vir ∼ 104K. 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 Z. © 2010. The American Astronomical Society. All rights reserved.