Chemistry and physical processes in early Universe structure formation

Abstract

Chemistry and cooling are explored in the early Universe. This includes the topics of recombination chemistry and chemistry and cooling during collapse towards the first objects. After the first stars have formed, the direct photodissociation of H_2 becomes important. Therefore, this process is explored in greater detail.

A comprehensive chemistry of the highly deuterated species D_2, D_2^+ , D_2H^+, and D_3^+ in the early Universe is presented. Fractional abundances for each are calculated as a function of redshift z in the recombination era. The abundances of the isotopologues are found to display similar behavior. Fractionation enhances the abundances of most of the more highly deuterated species as the redshift decreases due to the closing of some reaction channels as the gas temperature cools. Rate coefficients for the majority of the reactions involving the ions are uncertain resulting in a corresponding uncertainty in their predicted abundances.

Chemistry and cooling are investigated in collapsing primordial clouds for total baryonic densities up to ~10^6 cm^−3. The hydrodynamic evolution of the gas is modeled under the assumptions of free-fall and isobaric collapse as well as for the central regions of ~10^5 M_Sun objects in hierarchical scenarios drawn from three-dimensional cosmological hydrodynamical simulations that include the effects of nonequilibrium hydrogen chemistry and cooling as well as dark matter dynamics. The dominant processes in the reaction network are identified, as well as the most important cooling mechanisms. In all collapse models, the temperature evolution is influenced by the choice of the adopted H_2 radiative cooling function.

Ab initio potential curves and dipole transition moments were used to calculate cross sections for direct photodissociation through the Lyman and Werner transitions of H2. Partial cross sections were calculated within a range of wavelengths between 100 Angstroms and the dissociation threshold between the upper electronic states and all 301 rovibrational levels (v', J') within the ground electronic state. Influence of the process on a model of a photodissociation region is explored, and a data truncation prescription is presented.