Dust grains definition

The detailed results of the photochemical models are dependent on the adopted UV radiation field as well as on the photodissociation and photoionisation cross-sections, which in principle can be obtained by laboratory or theoretical studies, and on the nature of the dust particles for which the chemical composition, size and hence optical properties are poorly known. In the absence of definitive information, detailed models of the photochemistry have relied on some rather simplistic approaches to the radiative transfer of UV photons through the envelope. Nejad and Millar (1987,1988) have used the approach developed by Jura and Morris (1981) which assumes that the dust grains absorb but do not scatter and leads to a numerically simple equation for the intensity of UV radiation at any point in the envelope. On the other hand, Glassgold and co-workers have used a formalism devised by Gerola and Glassgold (1978) in which the scale-length, dp for photodissociation of any species is used to diminish the intensity incident on the CSE, that is, the intensity is reduced by a quantity exp(-d(/r). These authors have also included the effects of both continuum and line self-shielding in their calculations. 

From infrared spectroscopy it is very difficult to obtain the composition of the amorphous silicates. This is because the spectral signature observed is a combination of grain composition, shape, size, and structure, making it difficult to isolate the pure amorphous silicate signal. This, in combination with the relatively small spectral changes caused by the composition of the silicates, makes it hard to get a definitive answer in most cases. In the case of interstellar dust we have a unique opportunity the grains are very small and (almost) all silicates are amorphous. 

The term particle (or grain) size refers to the structural make-up of such substances as granulates, powders, dusts, granular mixes, and suspensions. Knowledge of the particle size, in conjunction with the comminution process, determines such details as grinding efficiency and ultimate product fineness. To establish particle sizes and their distribution within powdered systems, the user can have recourse to a number of different measuring processes designed to indicate, with appropriate particle definition, details of the probable equivalent diameter of a particle. 

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