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>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.
