Colour Centres in Interstellar Grains

Abstract

ONE of us has shown^1,2 that the interstellar extinction curve in the visible spectral region may be represented to a high degree of accuracy by two straight lines intersecting at λ = 4300 Å. Although graphite flakes or graphite cores with dielectric mantles come close to fitting the observational curve³, it is clearly not possible to explain a very sharp knee on the basis of Mie scattering by solid particles. The role of impurity induced absorption bands has been discussed recently^4,5 in connexion with the 4430 å absorption band. The most widely studied impurity bands are the so-called F centres^6,7 in alkali halides which occur in the visible and near ultraviolet. Fig. 1 shows a typical absorption profile due to an F centre in KBr (ref. 7). Such a band, which could occur in any crystalline solid at a wavelength defined by the lattice, is caused by electrons trapped at vacancies. Features common to all such bands are: (a) f values close to unity; (b) symmetrical profiles; (c) widths of the order 1000 Å. We have already suggested⁵ that such a broad diffuse absorption band may be present, but escape resolution in stellar spectra. We consider here the possibility that the knee in the interstellar extinction at λ = 4300 Å may be caused by such a band superposed on the extinction curve derived from Mie scattering.