Light Scattering by Graphite Core-Ice Mantle Grains

Abstract

Efficiency factors for extinction and albedo are computed for graphite spheres of radii 0.015 µ (0.0015 µ ) 0.03 µ , 0.04 µ (0.01 µ ) 0.07 µ covered with spherical concentric shells of ice up to 2 and 3 times the core radii. For a graphite sphere of radius ∼ 0.02 µ covered with ice up to twice its own radius the extinction in the infrared and red comes mainly from absorption by the graphite core. Graphite core absorption in the visible red contributes about 70% of the total extinction, and in the blue and ultraviolet to about 50% of the total extinction. Normalized extinction curves are constructed for individual composite grains as well as for graphite cores with equilibrium size distributions of ice mantles. General agreement with the observed regional extinction curves is quite satisfactory for cores of radii 0.05–0.07 µ covered with ice up to twice their own radii. The observed reddening law for the Cygnus region is reproduced very well by an equilibrium size distribution of ice mantles around graphite cores of radii less than ∼ 0.06 µ .