Cosmological effects of the first stars: Evolving spectra of Population III

The first stars hold intrinsic interest for their uniqueness and for their potentially important contributions to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining population of large bright quasars at redshifts z > 3 implies that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by z similar to 6. The metal-free composition of the first stars 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 new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Population III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Population II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Population II in the He II ( E > 4 ryd) continuum. We suggest that large Lyalpha equivalent widths (W-Lyalpha > 400 Angstrom) may provide a means of detecting metal-free stellar populations at high redshift and that He II recombination lines (lambdalambda1640, 4686) may confirm identifications of Population III. While Population III clusters are intrinsically bluer than their Population II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Population III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Population III for the reionization of the IGM in both Hand He.