Mapping UV radiation in dark clouds

The penetration of ultraviolet radiation inside dark clouds is discussed, and calculations of model-dependent transmissivities are presented. It is shown that, in the high-RV regime, the interiors of dark clouds can be much better illuminated than their nominal optical depth would suggest. The spherical harmonics method is used to find the solution to the radiative transfer equation appropriate for media having radially varying extinction and scattering coefficients. To describe variations in the optical properties of the dust mixture inside a cloud, we derive the scattering parameters for large RV environments with a procedure similar to the one used to obtain the standard MRN model optical constants; the cloud is then modelled with the introduction of two synthetic dust populations, one characteristic of the diffuse medium and the other possessing extinction properties of dense molecular clouds. Local sources of radiation (cosmic-ray-induced and decaying dark matter photons), together with the residual interstellar field, are computed in order to map the distribution of radiation inside the model clouds.