  Motivated by theoretical predictions that first stars were predominantly very
massive, we investigate the physics of the transition from an early epoch
dominated by massive Pop III stars to a later epoch dominated by familiar
low-mass Pop II/I stars by means of a numerically-generated catalogue of dark
matter halos coupled with a self-consistent treatment of chemical and radiative
feedback. Depending on the strength of the chemical feedback, Pop III stars can
contribute a substantial fraction (several percent) of the cosmic star
formation activity even at moderate redshifts, z = 5. We find that the three z
= 10 sources tentatively detected in NICMOS UDFs should be powered by Pop III
stars, if these are massive; however, this scenario fails to reproduce the
derived WMAP electron scattering optical depth. Instead, both the UDFs and WMAP
constraints can be fulfilled if stars at any time form with a more standard,
slightly top-heavy, Larson IMF in the range 1 Msun < M < 100 Msun.
