Modeling primordial gas in numerical cosmology

We have reviewed the chemistry and cooling behavior of low-density (n ≲ 10 ⁴ cm ^-3) primordial gas and devised a model which involves 19 collisional and 9 radiative processes and is applicable for temperatures in the range 1 K < T < 10 ⁸K. In a companion paper (Anninos et al., 1997)[NewA, 2, 209] numerical methods are presented that unify the modeling of non-equilibrium primordial gas chemistry and cooling dicussed here with cosmological hydrodynamics. We derived new fits of rate coefficients for the photo-attachment of neutral hydrogen, the formation of molecular hydrogen via H ^-, charge exchange beween H ₂ and H ⁺, electron detachment of H ^- by neutral hydrogen, dissociative recombination of H ₂⁺ with slow electrons, photodissociation of H ₂⁺, and photodissociation of H ₂. Furthermore it was found that the molecular hydrogen produced through the gas-phase processes, H ₂⁺ + H → H ₂ + H ⁺, and H ^- + H → H ₂ + e ^-, is likely to be converted into its para configuration on a faster time scale than the formation time. We have tested the model extensively and shown it to agree well with former studies. We further studied the chemical kinetics in great detail and devised a minimal model which is substantially simpler than the full reaction network but predicts correct abundances. This minimal model shows convincingly that 12 collisional processes are sufficient to model the H, He, H ⁺, H ^-, He ⁺, He ⁺⁺, and H ₂ abundances in low density primordial gas for applications with no radiation fields.