Nucleosynthesis, Reionization, and the Mass Function of the First Stars

Abstract

We critique the hypothesis that the first stars were very massive stars (VMS; M > 140 Msun). We review 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. The supernova yield patterns of VMS are incompatible with the Fe-peak and r-process abundances in halo stars. Models including Type II supernovae and/or ``hypernovae'' from metal-free progenitors with 8 - 40 Msun can better explain the observed trends. With a simple metal-transport model, we estimate that halo enrichment curtails metal-free star formation after ~10^8 yr at z ~ 20. Because the ionizing photon efficiency of metal-free stars peaks at ~120 Msun and declines at higher mass, an IMF with a lower bound at M ~ 10 - 20 Msun and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. An IMF devoid of low-mass stars is justified independently by models of the formation of primordial stars. Using a semi-analytic model for H I and He II reionization, we find that such an IMF can reproduce tau_e = 0.10 - 0.14, consistent with the range from WMAP. We conclude, on the basis of these results, that VMS are not necessary to meet the existing constraints commonly taken to motivate them.Comment: final version accepted for ApJ Volume 612, September 200

Probing Star Formation at Low Metallicity: The Radio Emission of Super Star Clusters in SBS 0335–052

“…However, despite these strong links between globular clusters and SSCs, we do not know how the extremely low metallicity in the early universe affected massive star cluster formation. A low metal content may affect star formation in a variety of ways, including cooling and pressure in the birth cloud, hardness of the stellar spectra, and evolution of the stars themselves (e.g., Schaerer 2002;Smith et al 2002;Tumlinson et al 2004;Bate 2005). One way to approach this question is to study SSC formation in extremely low-metallicity systems in the local universe.…”

Gravitational waves from the first stars

“…As these stars produced heavier elements, the universe achieved a critical metallicity (∼ 10 −4 times solar metallicity) [4], at which point the massive mode of star formation yielded to a more normal distribution of stellar masses with a SFR peaked at z ≈ 3. A more detailed understanding of the first epoch of star formation will rely on the phenomenological consequences of the models such as element enrichment and supernova rates [5,6,7].…”

Can Very Massive Stars Avoid Pair-Instability Supernovae?

“…However, using those yields, Tumlinson et al (2004) have shown that it provides only a very poor fit to the abundances pattern observed in the metal-poor stars known today. The odd-even effect is not observed, and the models significantly over-produce Cr and under-produce V, Co and Zn.…”