Molecules in diffuse interstellar clouds

Smith, Colin Adrian (1988). Molecules in diffuse interstellar clouds. MPhil thesis The Open University.



The existence of diatomic molecules in the diffuse interstellar gas has been known since the late thirties and from that time numerous observations have been made, at visible and ultraviolet wavelengths, of interstellar molecular absorption lines in the spectra of bright stars. The derivation of quantitative data on the column densities of the various molecules observed depends on a knowledge of certain molecular parameters, particularly the oscillator strength or f value of the transition being studied. Unfortunately the use of widely differing f values for each transition observed has led to confusion in calculating the true column densities of molecules along lines of sight.

This thesis attempts to overcome the problem by scaling the quantitative data available in the published literature using a common set of oscillator strengths. Lists of molecular oscillator strengths for the major transitions observed are included in a review of molecular spectroscopy. The result is a catalogue of lines of sight toward over two hundred stars detailing the column densities of CH, CH+, 13 CH+, CO, 13CO, CN, C2, C3, H, H2, HD, OH, H2O, HCl, NH, N2 , CS+, CO+ and H2O+. Tables of each stars characteristics, molecular radial velocities and references for each line of sight are also included.

Following a review of the current state of interstellar chemistry, the relationships between the molecules in the catalogue are examined using sample correlation coefficients, and correlation diagrams. The results of this large data set confirm modern chemical theories and show that ion molecule reactions are the dominant mechanism for the formation of molecules in diffuse interstellar clouds.

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