Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........17t&link_type=abstract
Thesis (PhD). UNIVERSITY OF COLORADO AT BOULDER, Source DAI-B 63/06, p. 2876, Dec 2002, 132 pages.
Mathematics
Logic
Scientific paper
The first stars hold intrinsic interest for their uniqueness and for their potential importance to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining populations of large bright quasars at z > 3 imply that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by redshifts z ˜ 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 results from a 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 H- ionizing photon production from metal-free stellar clusters takes twice as long as that of Pop II to decline to 1/10 its peak value. In addition, metal-free stars produce ˜105 times more photons than Pop II in the He II (E > 4 Ryd) continuum, for a characteristic time of 2.5 Myr after an instantaneous burst. Large Lyα equivalent widths may provide a means of detecting metal-free stellar populations at high redshift, and He II recombination lines may confirm identifications of Population III. While Pop III clusters are intrinsically bluer than their Pop II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Pop III stars with broadband colors. Using our evolving spectra, we examine the significance of zero-metallicity stellar populations for the cosmological reionization of H I and He II. We find that Population III stars by themselves can reionize H I and He II at redshifts z ≃ 9 (4.7) and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More realistic scenarios involving combinations of Population III and Population II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these latter cases, but we find that the ionization fraction of He III reaches a maximum of about 60% at z ˜ 5.6 if Population III star formation lasts for 109 yr. We suggest that future data on H I reionization can test the amount of small-scale power available to the formation of the first objects. Future observations of the He II Gunn-Peterson effect in the IGM at z ≳ 3, particularly in void regions that may contain relic ionization from early Pop III activity, may reveal the significance of Pop III stars for He II reionization.
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