Polarized radiative transfer: Monte Carlo code Ventspils RTMC. Testing of it and RADMC3D

Abstract

We have created Monte Carlo code Ventspils RTMC for polarized radiative transfer modelling in an isotropic, optically inactive medium of arbitrary configuration and morphology, and made it public. Its main advantages are simplicity of structure and usage, which can be a benefit for simple physical problems, together with use of sound and fully defined basic equations describing the scattering process. We compared the numeric results of Ventspils RTMC with those of RADMC3D created by Dullemond et al., as well as with the direct solution of Fredholm integral equations for source functions. A model of homogeneous sphere with Rayleigh scattering and concentric star inside it was used for comparison. The conclusion is that all three codes give essentially the same results if the radial optical thickness of the sphere does not exceed 5. If the optical thickness is between 5 and 10 then the statistical noise increases, and some small but systematic trends of differences appear between the results of three codes; however, the three codes are compatible. For optical thickness exceeding 10, the current version of Ventspils RTMC is too slow. There is a theoretical uncertainty about the adequacy of diffusion approximation of polarized radiative transfer used in RADMC3D for optical thickness exceeding 15. While RADMC3D is a very complex code usable for broad scope of applications, our Ventspils RTMC is a much simpler code usable for continuum transfer in case of modest optical thickness.