Proposed Models of the Production of the Diffuse Interstellar Features

significant polarisation. Although this may not mean that the diffuse features are not dust-related, it does suggest that they are not related to that population of interstellar grains which causes the polarisation of starlight.

1.5 Proposed Models of the Production of the Diffuse Interstellar Features

1.5.1 Observational Considerations

Any adequate theory of the production of the diffuse interstellar features has to satisfy the current consensus of observational evidence available. Observational evidence considered in this thesis as being conclusive is as follows:

o The rest wavelengths of the cores of the diffuse features remain constant with respect to line of sight.

o The breadths of the diffuse features (generally implying line profile) remain constant with respect to line of s i g h t .

o The diffuse features have not been observed in emission.

o There is a high positive correlation between the strengths of the diffuse features and reddening.

o There is a generally linear trend between the strengths of the diffuse features and reddening to beyond a reddening of E(B-V) = 2.0.

o There is scatter in the relationship between the strengths of the diffuse features and reddening which cannot be attributed to statistical errors in the data.

o There is a high positive correlation between the strengths of each of the diffuse features.

o There is scatter in the relationship between the strengths of each of the diffuse features which cannot be attributed to statistical errors in the data.

o There is a systematic variation in the diffuse feature strengths per unit reddening with respect to Galactic longitude.

o The profiles of the diffuse features reflect the velocity structure of interstellar clouds along each line of sight.

o There is no evidence for polarisation across the profiles of the diffuse features at 4430$, 5780$ and 6284$.

o There is no evidence for molecular rotational fine structure within the profiles of the sharp diffuse features to a resolution of 0.05$.

Observational evidence considered as requiring further investigation is as follows:

o Correlations between the strengths of the diffuse features and more redward colour excesses are systematically higher than their blueward counterparts.

o Correlations between the strengths of each of the diffuse features are systematically greater than correlations between the strengths of the diffuse features and r e d deni ng.

o The relationship between the strengths of the diffuse features and the strength of the 2200$ feature.

o The identification of anomalies in the relationship between the strengths of the diffuse features and reddening and their morphological correlates within the interstellar medium.

o The relationships between the strengths of the diffuse features and interstellar abundance data.

o The profiles of the diffuse features, unaffected by interstellar velocity structure.

Although it has been demonstrated that there is variation in the relative strengths of the diffuse features, and in the strengths of the diffuse features with respect to reddening, which cannot be attributed to statistical uncertainties in the data, these studies have largely been comparative. Similar statistical studies have been less conclusive.

A theory of the production of the diffuse features has to provide a link between the physical conditions understood to prevail in the diffuse interstellar medium (e.g. Section 1.3) and the behaviour of the diffuse features. Perhaps the most important of the parameters from which the physical state of the interstellar environment is inferred is the relative abundance of atoms, ions and molecules along particular lines of sight. These data, and the inferred atomic depletions, set useful constraints upon the likely composition of interstellar gas and dust along each line of sight. In addition to abundance information, the thermal temperatures within the diffuse interstellar clouds along the line of sight, the gas and grain density, and the ambient radiation density are also of importance. Sadly, this detailed information is usually not available for comparison with the diffuse feature observations.

Models of the production of the diffuse interstellar features fall into three categories:

(i) models where the production mechanism is in the gas phase;

(ii) models where the production mechanism is related to the intrinsic properties of the interstellar dust;

(iii) models where the production of the diffuse features occurs as a result of the interaction between gas and dust in the interstellar medium.

Several authors have reviewed the possible causes of the diffuse features: York (1971), Bromage (1972), Wu (1972), Savage (1976), Smith, Snow & York (1977). There follows a summary of the proposed mechanisms of diffuse feature production.

1.5.2 Gas Phase Models

Two gas phase mechanisms have been proposed for the diffuse features: unresolved molecular band structure in an unidentified interstellar molecule and the spectrum resulting from an atom or molecule in an excited state (i.e. auto-ionisation, p r e ­ ionisation or predissociation).

Any theory for the molecular origin of the diffuse interstellar features is required to satisfy element abundance constraints derived from observation. Wilson (1964) estimated a mean space density of 6x10 ^cm ^ for the carrier of the 4430$ diffuse feature, based upon the assumptions that the line is not saturated and that it has an oscillator strength of unity. This density may be compared with the density of atomic hydrogen in

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the local interstellar medium of approximately 1cm and 10 cm -5 -3