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J. Tumlinson, A. Venkatesan, J. M. Shull (University of Colorado, Boulder)
The first stars are increasingly thought to be the leading candidates for the sources that reionized the IGM by redshifts z ~6. Their metal-free composition restricts the stellar energy source to proton-proton burning rather than the more efficient CNO cycle. Consequently they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present recent results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Pop III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that ionizing radiation from these stars is substantially stronger and longer-lasting than that of Pop II stars, particularly in the He II continuum (E \geq 4 Ryd). We use a semi-analytic reionization model to assess the effects of metal-free stars on the epochs of H I and He II reionization, taking into account recent constraints on the H I reionization epoch. Metal-free stars may trigger an early epoch of partial He reionization and may enhance positive feedback on galaxy formation by promoting molecule formation in the IGM and early galaxies. We present initial results from an investigation of interstellar metal recycling and its importance for determining the duration of the first stars epoch. Finally, we suggest that He II recombination lines may provide a means of detecting metal-free stellar populations at high redshift.