Hydrogenated amorphous carbon-coated silicate particles as a source of interstellar extinction

Abstract

A new model for interstellar grains has recently been proposed in which large and small silicate grains are mantled with thin layers of hydrogenated amorphous carbon (HAC or a-C:H) and amorphous carbon (a-C). A complete description of the grain model is presented for the first time. The optical properties of small inhomogenous particles are discussed and new expressions for carbon depletion, bump strength and albedo are derived. It is shown, in particular, that the wide variety of interstellar extinction curves can be explained by variations in carbon depletion, and model fits to normal and extreme cases of extinction are presented. The model enables a direct measurement of carbon depletion to be made from the extinction data and implies that carbon depletion in an interstellar cloud is determined by the recent history of that cloud. Thus, on this model, grains respond to and react with their environment, rather than being immutable, inert entities in the interstellar gas.