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2.1. The Dust Component

The physical properties and the size distribution of the interstellar dust grains have been fixed to a very widely accepted variety for all our models. An equal mixture (50% : 50%) of Graphite and Astronomical Silicate grains, and a gas to dust ratio of 100:1 by mass, have been considered. All physical properties of the grains, viz., absorption, scattering efficiencies and linear anisotropy factor have been taken from Draine & Lee (1984) and Laor & Draine (1993). Averaging over 29 discrete sizes have been performed, following a size distribution of n(a)da ~ a-mda, amin leq a leq amax, with m = 3.5 (Mathis et al., 1977). The lower and upper limits of the grain size distribution, amin (0.01 µm) and amax (0.25 µm), have been taken from Mathis et al. (1983).