The influence of absorption on light-scattering by non-spherical particles

Abstract

Cometary dust is composed of a mixture of silicates, sulfides, metals, and organics. Although the organic material is the most abundant component, there is no consensus as to its composition. An estimate of the absorption coefficient for an analogue organic sample varies from one experiment to another. Using the discrete-dipole approximation, we study how light scattering by agglomerated debris particles depend on the imaginary part of refractive index Im(m) in the range from 0 to 1.3. We fix the real part of the refractive index Re(m)=1.5 and consider the size parameters x=5, 10, and 20. We found that Umov's law is valid in both metallic and dielectric domains, but not in the transition regime. The phase angle of the maximum polarization almost monotonically increases with Im(m). Non-absorbing particles reveal a deep branch of negative polarization at small phase angles. Particles with Im(m) ~ 0.2 have no negative polarization, but large particles with Im(m) ~ 1 exhibit the negative polarization. By comparing our results with observations of comets, we can constrain the imaginary part of refractive index for organics in cometary dust.