Chemistry of heavy elements in the Dark Ages

Context. Primordial molecules were formed during the Dark Ages, i.e. the time between recombination and reionization in the early Universe. They were the constitutents of the first proto-stellar clouds. Standard Big Bang nucleosynthesis predicts the abundances of hydrogen, helium, lithium, beryllium, and their isotopes in the early Universe. Heavier nuclei such as carbon, nitrogen, or oxygen are formed in trace amounts only. In non-standard Big Bang nucleosynthesis models, it is possible to synthesize larger quantities of these heavier elements. The latter are interesting because they can form molecules with a high electric dipole moment which can increase the cooling in collapsing protostellar structures. Aims. The purpose of this article is to analyze the formation of primordial molecules based on heavy elements during the Dark Ages, with elemental abundances taken from different nucleosynthesis models. Methods. We present calculations of the full non-linear equation set governing the primordial chemistry. We consider the evolution of 45 chemical species and use an implicit multistep method of variable order of precision with an adaptive stepsize control. Results. For the first time the cosmological recombination of heavy elements is presented. We find that the most abundant Dark Ages molecules based on heavy elements are CH and OH. When considering initial conditions given by the standard Big Bang nucleosynthesis model, we obtain relative abundances [CH] = nCH/nb = 6.2 × 10−21 and [OH] = nOH/nb = 1.2 × 10−23 at z = 10, where nb is the total number density. But non-standard nucleosynthesis can lead to higher heavy element abundances while still satisfying the observed primordial light abundances. In that case, we show that the abundances of molecular species based on C, N, O and F can be enhanced by two orders of magnitude, leading to a CH relative abundance higher than that of HD or H2D.