Dynamics of the Primordial Hydrogen and Helium (HeI) Recombination in the Universe
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The dependences on z of fractional number densities of H+ and He+ ions are calculated with a proper allowance for two-photon decays of upper levels of hydrogen and parahelium and radiative transfer in intercombination line 2 3P_1 --> 1 1S_0 of helium. It is shown that for hydrogen this gives corrections for a degree of ionization in no more than a few percents but for helium this leads to a significant acceleration of recombination compared to the recent papers by Seager et al. (1999; 2000) where these effects were ignored.
10 pages, 2 figures, accepted for publication in Astronomy Letters
10 pages, 2 figures, accepted for publication in Astronomy Letters