Zero-Metallicity Stars and the Effects of the First Stars on Reionization.

We present stellar structure and atmosphere models of metal-free stars and examine them from a cosmological point of view. Metal-free stars exhibit high effective temperatures and small sizes relative to metal-enriched stars of equal mass. These unique physical characteristics enhance the ionizing photon production by metal-free stars, particularly in the He ii (hnu>/=4 ryd) continuum. The star formation rate of metal-free stars necessary to reionize the hydrogen in the universe by z=5 is consistent with the inferred star formation rate at that epoch. However, the hard stellar spectra are inconsistent with the observations of He ii opacity in the intergalactic medium (IGM) at z approximately 3, indicating that the period of metal-free star formation ended before that epoch. We examine the effects of these stars on the ionization balance of the IGM, the radiative feedback of the first luminous objects, and the extragalactic radiation field. We comment on the prospects for detecting metal-free stellar populations with the lambda1640 and lambda4686 recombination lines of He ii.