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L. Kolokolova, B. A. S. Gustafson, M. Bourgeois (University of Florida, Gainesville)
The interpretation of optical (photometric, polarimetric, colorimetric, etc.) observations of dust, whether cometary, interplanetary, circum-stellar, or planetary aerosols, is hampered by the complicated structure of the grains that, most likely, include multi-compositional, aggregated particles. It is a common practice to use effective medium theories (EMT) to estimate average, "effective" optical constants of such inhomogeneous materials. A variety of EMTs were developed for different structures of the medium (aggregated, fractal, separated inclusions) and for a variety of sizes and shapes of the inhomogeneities. However, the validity of EMT for astronomical applications has only been demonstrated for extinction cross-sections that were checked using the DDA method. This paper compares angular distribution and wavelength dependence of intensity and polarization of scattered light obtained from EMT calculations with the results of microwave analog measurements at the microwave facilities of the University of Florida. We simulated the light-scattering by organic grains with silicate inclusions of size parameter x=0.075 (0.04 micron), 0.58 (0.3 micron), and 1.2 (0.6 micron). The conclusion is that all EMTs (including Maxwell-Garnett, Bruggeman, Looyenga, Stroud and Pan) yield similar results and work better for the intensity of the scattered light than for its polarization. The dependency of the intensity on the scattering angle is in good agreement with the EMTs. However, calculations overestimate the magnitude of the intensity when the size of the inclusions and/or their volume fraction in the mixture increases. We find that the EMTs cannot reproduce the angular dependence of polarization and yield errors in polarization of around 45-65% for all sizes and volume fractions of the inclusions. The spectral gradient of polarization (polarimetric color) calculated using EMTs also differ from the experiments by the same order. By contrast, the calculated spectral gradient of intensity (color) is in good agreement with the experiments. This work was supported by NASA grants NAG5-8944 and NAG5-6378.