The DIRTY model. I. Monte Carlo radiative transfer through dust

We present the DIRTY (DustI Radiative Transfer, Yeah!) radiative transfer model in this paper and a companion paper. This model computes the polarized radiative transfer of photons from arbitrary distributions of stars through arbitrary distributions of dust using Monte Carlo techniques. The dust reemission is done self-consistently with the dust absorption and scattering and includes all three important emission paths: equilibrium thermal emission, nonequilibrium thermal emission, and the aromatic features emission. The algorithm used for the radiative transfer allows for the efficient computation of the appearance of a model system as seen from any viewing direction. We present a simple method for computing an upper limit on the output quantity uncertainties for Monte Carlo radiative transfer models that use the weighted photon approach.