Molecular hydrogen in the cosmic recombination epoch
Abstract
The advent of precise measurements of the cosmic microwave background (CMB) anisotropies has motivated correspondingly precise calculations of the cosmic recombination history. Cosmic recombination proceeds far out of equilibrium because of a "bottleneck" at the n=2 level of hydrogen: atoms can only reach the ground state via slow processes: two-photon decay or Lyman-α resonance escape. However, even a small primordial abundance of molecules could have a large effect on the interline opacity in the recombination epoch and lead to an additional route for hydrogen recombination. Therefore, this paper computes the abundance of the H2 molecule during the cosmic recombination epoch. Hydrogen molecules in the ground electronic levels X1+g can either form from the excited H2 electronic levels B1+u and C1u or through the charged particles H2+, HeH+ and H-. We follow the transitions among all of these species, resolving the rotational and vibrational sub-levels. Since the energies of the X1+g--B1+u (Lyman band) and X1+g-C1u (Werner band) transitions are near the Lyman-α energy, the distortion of the CMB spectrum caused by escaped H Lyman-line photons accelerates both the formation and the destruction of H2 due to this channel relative to the thermal rates. This causes the populations of H2 molecules in X1+g energy levels to deviate from their thermal equilibrium abundances. We find that the resulting H2 abundance is 10-17 at z=1200 and 10-13 at z=800, which is too small to have any significant influence on the recombination history.
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