2.1. What constitutes an interstellar dust model?
An interstellar dust model is completely characterized by the abundance of the different elements locked up in the dust, and by the composition, morphology, and size distribution of its individual dust particles. This seemingly simple definition hides the complexities involved in deriving such a dust model.
First and foremost, any dust model must specify the total mass of the different refractory elements that are locked up in the solid phase of the ISM. These elements can form many different solid or molecular compounds with different optical and physical properties. In addition, the morphology of the dust particles whether spherical, ellipsoidal, cylindrical, platelike, or amorphous has an important effect on these properties. Finally, the size distribution of these dust particles will determine their collective properties and interactions with the ambient gas and radiation field. These interactions play a major role in the radiative appearance of galaxies, and in the thermal and chemical balance of their interstellar medium.