MULTIPLE-SCATTERING IN CLUMPY MEDIA .1. ESCAPE OF STELLAR RADIATION FROM A CLUMPY SCATTERING ENVIRONMENT

We studied the radiative transfer in a spherical, two-phase clumpy med ium, in which coherent, non-conservative scattering is the dominant op acity source and where the source of photons is situated at the center . The structure of the medium is random but statistically homogeneous and is characterized by the density ratio between the low- and high-de nsity phases, the optical depth radius of the equivalent homogeneous d ust distribution, the filling factor of high-density clumps, and the l ength scale of individual clumps. We examined in detail the cloud mass spectrum, the distribution of optical depths, and the apparent fracta l nature of the projected cloud structures. The photometric characteri stics of the clumpy scattering system are studied as a function of den sity contrast between the two phases, of the filling factor, and of th e length scale of high-density clumps, and they are compared with thos e of homogeneous, constant-density distributions of equal effective op tical depth. Direction-averaged surface brightness distributions of th e scattered light are studied for both optically thick and optically t hin cases, which reveal the important role of scattering by the optica lly thin interclump medium. The conversion of UV/optical/near-IR radia tion into thermal far-IR dust radiation in a dusty system is profoundl y affected by the structure of the medium; the homogeneous, constant-d ensity distribution always provides the highest conversion efficiency for any given geometry and dust mass. The effective optical depth of a clumpy distribution is known not to scale linearly with the equivalen t optical depth of a homogeneous distribution of equal dust mass; this leads to effective attenuation laws that differ from the original opa city law assumed for the dust in the system. The expected reddening is substantially reduced for clumpy media. Finally, since the scattering response of a clumpy system is consistently that of an equivalent sys tem of lower effective optical depth and lower effective albedo, effor ts to determine the dust albedo of real systems with clumpy dust distr ibutions by employing models, which are homogeneous, can lead to a bia s toward albedo values that are too low.