A three-dimensional Monte Carlo photoionization code for modelling diffuse ionized gas

We have developed a three-dimensional Monte Carlo photoionization code tailored for the study of Galactic HII regions and the percolation of ionizing photons in diffuse ionized gas. We describe the code, our calculation of photoionization, heating and cooling, and the approximations we have employed for the low-density HII regions we wish to study. Our code gives results in agreement with the Lexington HII region benchmarks. We show an example of a two-dimensional shadowed region and point out the very significant effect that diffuse radiation produced by recombinations of He has on the temperature within the shadow.