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A. Simple Approach to Radiative Transfer Through the Gas

In this simple approach, the extent of the ionized region (in spherical geometry) is determined by transporting the Lyman continuum photons (from the centrally located star / star cluster) through the cloud including the effect of the dust, where they can exist (as determined by their sublimation). The gas component of the cloud is assumed to consist of only hydrogen. Next, the radio continuum emission emerging from the cloud is calculated by transporting the radio photons (free-free emission from the ionized medium of the cloud), through the entire cloud without making any approximation about the optical thinness of the gas (i.e. self absorption is treated appropriately). The gas-dust coupling has been neglected.