Models of Interstellar Dust
How are the observational data described in Chapter 2 used to generate a model of dust that is consistent with the constraints? The standard procedure in current models begins with a choice of specific materials for the dust, guided by the observational data. Complex fitting procedures are then used to generate interstellar extinction curves for particular lines of sight by varying the size distributions of the various dust components that have been adopted. Refinements are then introduced to meet other observational constraints. Chapter 3 introduces three current dust grain models. The model developed by Draine and his collaborators, involving separate dust components of silicates, carbons and PAHs, is widely used and has been very successful in accounting for many types of observational phenomena. The “unified” model favoured by the authors of this book, involving carbon layers deposited on silicate cores with free PAHs, indicates that dust grains should evolve both physically and chemically in the interstellar medium, and that dust properties must therefore be time-dependent. The “holistic” model of Jones and his collaborators has a more complex selection of grain materials: silicate grains containing iron inclusions and coated with carbon, small graphite grains, large amorphous carbon grains, but no PAHs. It has an implicit assumption of time-dependence. Although these three models have somewhat different characteristics, from the chemical perspective of this book each of them provides a similar range of dust grain surfaces on which heterogeneous chemistry may occur.