Chemistry of heavy elements in the Dark Ages

TL;DR

This paper investigates the formation of primordial molecules involving heavy elements during the Universe's Dark Ages, using complex chemistry models and nucleosynthesis scenarios to predict molecular abundances.

Contribution

It presents a detailed chemical model of heavy-element molecules in the early Universe, considering non-standard nucleosynthesis effects on their abundances.

Findings

CH and OH are the most abundant heavy-element molecules.

Non-standard nucleosynthesis can increase heavy-element molecule abundances by two orders of magnitude.

Heavy-element molecules can surpass some primordial hydrogen molecules in abundance.

Abstract

Primordial molecules were formed during the Dark Ages, i.e. the time between recombination and reionization in the early Universe. 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. 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. We find that the most abundant Dark Ages molecules based on heavy elements are CH and OH. 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 compared to the standard case, leading to a CH relative abundance higher than that of HD+ or H2D+.