Person:

Dalgarno, Alexander

A comprehensive chemistry of the highly deuterated species D2, D+ 2, D2H+, 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 deuterated species are uncertain resulting in a corresponding uncertainty in their predicted abundances.

The excitation of the diatomic carbon molecule in diffuse interstellar clouds is discussed for a cloud with large density fluctuations of small linear filling factor along the line of sight. Possible implications for the chemistry of the diffuse cloud in front of Cygnus OB2 No. 12 are discussed.

Full close-coupled calculations are carried out of the cross sections for energy transfer between rotational levels of carbon monoxide in collision with 4He atoms with energies between 5 and 600 cm-1. At low energies, the cross sections are dominated by contributions from shape resonances. The calculated cross sections are in satisfactory agreement with the experimental data measured at an energy of 570 cm-1. Calculations using the infinite order sudden approximation are carried out of cross sections for energy transfer between vibrational levels of CO. Vibrational energy transfer is dominated by transitions in which the vibrational quantum number changes by unity.

We discuss the observational differences between lines of sight that intercept a group of turbulent dissipative structures and lines of sight that cross less-active regions. Using time-dependent calculations we show that the energy level distribution of the hydrogen molecule evolves in time in response to the local thermal phase. We find that relatively simple models can explain the observed properties of molecular hydrogen in diffuse interstellar clouds in terms of time evolution induced by collisional excitation in a low-density, high-temperature gas.

A model is constructed of the material in front of the star Cygnus OB2 no. 12 in which dense cores are embedded in diffuse clumps of gas. The model reproduces the measured abundances of Graphic C2 and CO, and predicts a column density of 9×1010 cm−2 for HCO+.