Interstellar dust models for extinction and emission. Commentary
A large fraction of the power radiated by dust in the interstellar (IS) medium is observed in the near and mid infrared (l 80 mm) This emission is a strong evidence for very small grains in the interstellar medium from a few tenths to a few tens of nanometer radius. This interpretation implies a major revision of earlier dust models that were mainly based on the properties of the interstellar extinction curve. We propose a new coherent interpretation of both the interstellar extinction and the infrared emission, using an empirical dust model made of three components: big grains, very small grains (3 dimensional) and PAHs (2 dimensional) We argue that the big grain component is probably made of a size distribution of silicates with some dark refractory mantle and very small grains are probably carbon dominated. Allowing the dust abundances and sizes to be fixed by the average interstellar extinction curve and emission spectrum in the Solar neighbourhood, we investigate the variations of IR colours with the intensity of the incident radiation field in various astrophysical situations, in order to assess the validity of the model. We predict some correlations between anomalous extinction curves and anomalous infrared colours. Extensive tables and figures are given in order to facilitate comparison with observations.