A Monte Carlo model for light scattering by dark nebulae

Witt and Stephens (1974) have demonstrated that the Monte Carlo scattering model can be valuable in the investigation of very opaque dark nebulae that cannot be studied by conventional star counting techniques. In the Witt-Stephens model, density distributions are simulated by subdividing the cloud into concentric shells, each of uniform density. The present paper develops a model which treats such distributions as continuous density functions. In order to do so it is necessary to know the position and direction of flight of a photon in the model cloud at any point in time. The basic difficulty is to keep track of the geometrical coordinates; to achieve this a method has been developed that uses coordinate transformation matrices. Model results on the Thumbprint Nebula in Chamaeleon are discussed.