1998A&A...335..403G

The process of molecule formation in the primordial gas is considered in the framework of Friedmann cosmological models from redshift z=10⁴ to z=0. First, a comprehensive analysis of 87 gas phase reaction rates (both collisional and radiative) relevant in the physical environment of the expanding universe is presented and critically discussed. On this basis, calculations are carried out of the abundance of 21 molecular species as function of redshift, for different values of the cosmological parameters {OMEGA}₀, η and H₀, evaluating consistently the molecular heating and cooling due to H₂, HD and LiH molecules. One of the major improvements of this work is the use of a better treatment of H recombination that leads to a reduction of a factor 2-3 in the abundance of electrons and H⁺ at freeze-out, with respect to previous studies. The lower residual ionization has a negative effect on the chemistry of the primordial gas in which electrons and protons act as catalysts in the formation of the first molecules. We find that in the standard model (h=0.67, η₁₀=4.5, {OMEGA}₀=1 and [D/H]=4.3x10^–5), the residual fractional ionization at z=1 is [e/H]=3.02x10^–4, and the main molecular species fractional abundances [H₂/H]=1.1x10^–6, [HD/H₂]=1.1x10^–3, [HeH⁺/H]=6.2x10^–13, [LiH⁺/H]=9.4x10^–18 and [LiH/LiH⁺]=7.6x10^–3. We devise a reduced chemical network that reproduces with excellent accuracy the numerical results of the complete model and allows to follow the chemical compositions and the thermal properties of a primordial gas in the presence of an external radiation field. Finally, we provide accurate cooling functions of H₂, HD and LiH in a wide range of density and temperature that can be conveniently used in a variety of cosmological applications.