Extinction and Polarization by Ellipsoidal Particles in the Infrared

Abstract

The properties of extinction and polarization in the near-infrared wavelength region are investigated for ellipsoidal particles composed of amorphous carbon and of silicate on the basis of the Fredholm integral equation method (FIM). We find that the wavelength dependence of extinction for moderately elongated ellipsoidal particles is quite similar to that for spherical ones but differs from that for infinite cylinders. The dependence of polarization on wavelength is shown to be similar to that of extinction if the particle size is relatively small. Recent observations show that the amount of extinction is proportional to λ<SUP>-1.7</SUP> both in dense and diffuse clouds, and a similar proportionality is also found for interstellar polarization. Our calculation shows that an ellipsoidal particle of amorphous carbon or of silicate can reproduce the observed power-law index when the size of the particle is about 0.1 μm. Infinite cylindrical particles of amorphous carbon give values for spectral indices which are too low to explain the observations. <P />We have calculated spectral indices for ellipsoidal and spherical particles in a power-law size distribution and for spherical ones whose size distribution is represented by a power law with exponential decay (PED). The spectral index in PED is less sensitive to a variation of mean size of the particles than the power-law model. This shows that the observed invariable nature of infrared extinction may be better explained by the PED.