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5.2 Formation rate

The formation rate of Population III stars can be indirectly estimated from the re-ionization fraction of the early universe, which was re-ionized by light from these stars [11747Jump To The Next Citation Point]. Using estimates of the ultraviolet light emission from Population III stars, their ionization efficiency, and the re-ionization fraction of the early universe, one can determine that about 0.01% - 1% of the universe’s baryonic matter was found in these very massive stars. This corresponds to 4 7 ~ 10 -10 Population III stars in a 11 10 Mo. galaxy and thus a collapse rate that is -3 -1 <~ 10 yr. Thus, a reasonable occurrence rate can be found for an observation (luminosity) distance of ~ 50 Gpc (which corresponds to a redshift of z = 5, in the cosmology used by [86Jump To The Next Citation Point]). However, uncertainties in the assumptions make this formation rate uncertain by a few orders of magnitude [1Jump To The Next Citation Point96Jump To The Next Citation Point286Jump To The Next Citation Point]. Indeed, the latest results suggest that the initial mass function begins to fall off dramatically above 100Mo . and the number of stars fomred with masses above 25Mo . may be many orders of magnitude below this rate[189]. Instead, Population III stars may be dominated by the collapse of stars in the 20- 100 Mo .. These stars may also collapse ultimately to black holes. For discussion of these objects, we refer the reader back to Section 4.
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