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[34.03] Observational Constraints on the Nature of the First Stars: From Nucleosynthesis to Reionization

J. Tumlinson (University of Chicago)

I will critique the hypothesis that the first stars were very massive stars (VMS; M > 140 M_\odot) by reviewing the two major lines of evidence for the existence of VMS: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment, and (2) that the high electron-scattering optical depth (\tau_e) to the CMB found by WMAP requires VMS for reionization in a concordance \LambdaCDM cosmology. Enrichment by Type II supernovae and/or ``hypernovae'' from zero-metallicity progenitors with M = 8 - 40 M_\odot can explain the observed trends nucleosynthetic trends better than VMS exploding as pair-instability SN. I use the nucleosynthesis results and stellar evolution models to construct an initial mass function (IMF) for reionization. Because the lifetime-integrated ionizing photon efficiency of metal-free stars peaks at ~ 120 M_\odot and declines at higher mass, an IMF with an approximate lower bound at M ~10 - 20~M_\odot and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. Semi-analytic reionization models for H~I and He~II reionization demonstrate that such an IMF can reproduce \tau_e ~q 0.10 - 0.14, consistent with the range from WMAP, without extreme astrophysical assumptions. I conclude, on the basis of these results, that VMS are not necessary to meet the existing constraints commonly taken to motivate them. I will also present recent FUSE observations of the He II Gunn-Peterson effect seen z ~ 3, and I will discuss how future applications of this technique can further constrain the nature of the first stars and quasars.

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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.