Optical properties of interstellar graphite and silicate grains

The dielectric functions for graphite and astronomical silicate material are computed using available laboratory and astronomical data. It is noted that the magnetic dipole contribution to absorption in the infrared wavelengths can be important for conducting particles such as graphite. Formulas are given for evaluating electric and magnetic dipole cross-sections for small particles. Absorption cross-sections are evaluated for graphite and silicate particles with sizes between 0.003 and 1.0 microns, and wavelengths from 300 A to 1000 microns. On the basis of polarization profiles computed for both prolate and oblate graphite-silicate spheroids, it is concluded that interstellar silicate grain are predominantly oblate. Extinction curves are calculated for Mathis-Rumpl-Nordsieck graphite-silicate grain mixtures and are compared to observations. The model is found to be in good agreement with available infrared observations.