A far Ultraviolet extinction law : what does it mean ?

Abstract

The dust grains in the diffuse cloud medium are characterized by a far-ultraviolet (FUV) extinction "law" so that all curves have the same functional form for inverse wavelengths λ^-1 ⪆ 6 μm^-1. A direct theoretical consequence of this fact is that there is very little scattering by either the particles responsible for the FUV extinction or the 2200 Å hump. A strong correlation between the strength of the 2200 Å hump and the visual extinction is confirmed. Poor correlation is found between the FUV extinction and either the 2200 Å hump or the visual extinction. All indications are that the dust grains responsible for the visual, the 2200 Å hump, and the FUV extinction are a trimodal distribution of independent groups of particles. Implications of the correlations and noncorrelations of the several components of the full extinction curve are used to derive some characteristics of the physical nature and evolution of grains outside of molecular clouds.