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The physics of the interstellar medium (ISM) allows the coexistence of several density states, and observations confirm the widespread occurrence of a cloudy or clumpy structure of the ISM on many length scales. We have investigated the effects of such a clumpy structure upon the transfer of light through a dusty scattering medium by means of a Monte Carlo radiative transfer code. Application of this code can be found in dusty environments in star-forming regions, reflection nebulae, and dusty galaxies. The specific case studied here concerned the escape of continuum radiation originating from a central point source, propagating through a homogeneously clumpy medium which was terminated by a spherical boundary. We examined the transfer as a function of the ratio of the densities of the clumps and the inter-clump medium for two cases, the case of constant mass and the case of constant density of the clumps. A range of filling factors and typical clumpy sizes were examined. We characterize the scattering system by the distribution of radial optical depths and the mass spectrum of clumps, which result from the assumption of given filling factors and clump sizes. A general result is that transfer through a clumpy medium is equivalent to that through a homogeneous medium of substantially lower optical depth and lower dust albedo, as far as the escape of direct stellar and scattered radiation is concerned. The role of the interclump medium is important when clumps themselves are optically thick. The ratio of far-IR to optical radiation from a clumpy environment is substantially lower than that ratio from a homogeneous dust distribution containing the same dust mass.
This work was supported by NASA through LTSA Grant NAGW-3168 to The University of Toledo.
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