EXTREMELY METAL-POOR STARS IN THE MILKY WAY: A SECOND GENERATION FORMED AFTER REIONIZATION

Cosmological simulations of Population III star formation suggest an initial mass function ( IMF) biased toward very massive stars (M greater than or similar to 100 M(circle dot)) formed in minihalos at redshift z greater than or similar to 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 greater than or similar to 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 less than or similar to 10 in pockets of gas enriched to metallicity Z greater than or similar to 10(-3.5) Z(circle dot) 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 less than or similar to 13 in halos with virial temperature T(vir) similar to 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 approximate to 10(-3.5) Z(circle dot)