Effects of grain models on UV penetration into clouds

A derivation of the average scattering characteristics of interstellar grains in the ultraviolet based mainly on observations of extinction is shown to be more reliable than a determination from direct measures of scattered light. Several representative scattering models are constructed and their consequences for the transfer of UV radiation in clouds are discussed using the spherical harmonic expansion method (Flannery, Roberge & Rybicki 1980). Because of the presence of large grains (a ∼ 0.15 μm) in the size distribution, all models are characterized by strongly anisotropic scattering throughout the UV (g ≈ 0.7–0.9) with a relatively low albedo ($$\alpha \lt0.4$$ for $$\lambda \lt 2500$$ Å) resulting from the presence of very small grains ($$a \leqslant 0.01\enspace \mu \text m$$). An approximate algorithm replacing the exact solution of the radiative transfer equation by a greatly simplified solution for g = 1 is presented. It is shown to be accurate to within < 10 per cent at low and intermediate optical depths ($$\tau \lt 20$$) for $$g \gt 0.7$$ and $$\alpha \lt 0.4$$ and to give a deviation of less than a factor of 2 (for g as low as g = 0.5.