Rees, Paul Christopher Tudor; (1990) The diffuse interstellar features and their interstellar environment. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis addresses the problem of the identification of the cause of the diffuse interstellar absorption features, observed in the spectra of reddened stars. Two approaches have been adopted in this investigation: a statistical study towards approximately one hundred stars using intermediate resolution (45km/s) photographic spectra obtained at the Lick Observatory, California; and a high resolution (1.5km/s) study using photo-electric spectra obtained at the Mt. Stromlo Observatory, Canberra. For the statistical study, a thorough bivariate statistical analysis was made of the measured strengths of the diffuse features at 5780Å, 5797Å, 6196Å, 6203Å, 6270Å, 6284Å and 6379Å. This study also included the development of a computational procedure to rectify the alpha band of telluric oxygen in the blue-ward wing of the diffuse feature at 6284Å. Other data used in this study were Johnson UBV photometry, ultraviolet photometry from the TD-1 satellite, and existing published column densities for H, Na, Ca⁺, H₂, CH, CH⁺ and CO. For the high resolution study, data for the sodium D lines and the 6614Å diffuse feature were used. The sodium data were used to determine the velocity structure, column densities and velocity dispersions towards each star. The data for the 6614Å diffuse feature were used to examine the hypothesis that this diffuse feature is caused by unresolved rotational fine structure within an electronic molecular transition. A careful appraisal of the results reported herein, and in existing published studies, suggests that the most likely cause of the diffuse interstellar features is auto-ionisation, preionisation or predissociation within the warm envelopes of diffuse interstellar clouds. No specific carriers have been identified.

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