Fossey, Stephen John; (1990) The diffuse interstellar features and interstellar relationships. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the results of a survey of the diffuse interstellar features towards about 120 stars. The observations, the data reduction and the measurement of the strengths of the diffuse features are described. The equivalent widths of nine diffuse features are then used in a comprehensive statistical analysis of the relationship between the carriers of the diffuse features and other components of the interstellar dust and gas. We re-examine some aspects of the well-known correlation of diffuse feature equivalent width with colour excess, E(B-V), and the extent of contamination of the diffuse feature measurements by stellar photosphere lines is assessed. The interrelationships of the diffuse features are examined with the technique of Principal Component Analysis and it is found that there are real variations in the relative strengths of some features from one line of sight to another. We perform a multivariate correlation analysis of the diffuse feature strengths using measurements of interstellar extinction in the ultraviolet and infrared, measurements of interstellar polarization, indicators of cloud density and temperature, and the abundances of a number of atomic and molecular species. Evidence is found that in general the diffuse features are weak with respect to reddening where the characteristic size of the dust grains is large. The strengths of a number of diffuse features per unit hydrogen column density ('abundances') are found to be diminished along lines of sight characterized by denser clouds. The carriers of several diffuse features appear to be distributed like the refractory atomic species in the diffuse medium, their abundances being well correlated with the gas-phase abundances of species such as Mg and Fe. The anti correlation of the abundances of a number of diffuse features with the abundances of several diatomic molecular species suggests that they are not a product of the network of gas-phase chemical reactions in quiescent clouds. Several pieces of evidence are presented to suggest that the diffuse feature carriers are produced in interstellar shocks. It is concluded that the diffuse feature carriers exist preferentially under the physical conditions which prevail in the envelopes or skins of diffuse clouds. We review some dust and gas-phase models for the origin of the diffuse features in the light of this evidence. We conclude that the diffuse features are most probably formed by a mixture of the radical and ionic species of poly atomic molecules whose abundance in the diffuse medium is sustained by their formation through grain fragmentation in interstellar shocks. We further suggest that the carriers of the narrower diffuse features, such as those studied here, are small polycyclic aromatic hydrocarbons of two to three rings in size, or linear carbon chain molecules consisting of about 10 atoms or less.

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