Light Scattering and Extinction Properties of Dust Grains in Interstellar and Interplanetary Medium

Dust grains are ubiquitous in nature and an important constituent of the interstellar, 
interplanetary medium and comets. Inhomogeneity or roughness in the grain structure, composition and the ambient medium modify the extinction and scattering properties of the grains. This talk focuses on studying the optical properties such as scattering, extinction, and the physical properties (size, shape and composition) of the dust grains lying in different interstellar environments. We make use of a composite grain model to explain the extinction properties of dust grains around objects observed by International Ultraviolet Explorer (IUE). We obtain reasonable fits to the observed data using spheroidal grains of axial ratio 1.33 and 2.0, with a size distribution a = 0.005­-0.250 μm and a = 0.001­0.100 μm. In the latter part of the talk, I will discuss about the fractal signatures in interplanetary dust aggregates, possibly constituted in proto­planetary disks and comets etc. Their physical and optical characteristics are significantly influenced by the morphology of the core of these aggregates. We present some statistical tools to extract the morphological features such as the size, texture, and fractal dimension of the aggregates by calculating the structure factor. Further, we investigate the single and multiple light­scattering by two classes of fractal aggregates of dimensions 1.78 and 2.5, numerically simulated using diffusion aggregation mechanism. Our focus here is on the fractal regime of the structure factor curve. We conclude that this regime allows us for a direct estimation of the fractal dimension, and is robust against moderate multiple­scattering by small particles.