TRUST I: A 3D externally illuminated slab benchmark for dust radiative transfer

The radiative transport of photons through arbitrary three-dimensional (3D) structures of dust is a challenging problem due to the anisotropic scattering of dust grains and strong coupling between different spatial regions. The radiative transferproblem in 3D is solved using Monte Carlo or Ray Tracing techniques as no full analytic solution exists for the true 3D structures. We provide the first 3D dust radiative transfer benchmarkcomposed of a slab of dust with uniform density externally illuminated by a star. This simple 3D benchmarkis explicitly formulated to provide tests of the different components of the radiative transferproblem including dust absorption, scattering, and emission. This benchmarkincludes models with a range of dust optical depthsfully probing cases that are optically thin at all wavelengths to optically thick at most wavelengths. This benchmarkincludes solutions for the full dust emission including single photon (stochastic) heating as well as two simplifying approximations: One where all grains are considered in equilibrium with the radiation field and one where the emission is from a single effective grain with size-distribution-averaged properties. A total of six Monte Carlo codes and one Ray Tracing code provide solutions to this benchmark. Comparison of the results revealed that the global SEDs are consistent on average to a few percent for all but the scattered stellar flux at very high optical depths. The image results are consistent within 10%, again except for the stellar scattered flux at very high optical depths. The lack of agreement between different codes of the scattered flux at high optical depthsis quantified for the first time. We provide the first 3D dust radiative transfer benchmarkand validate the accuracy of this benchmarkthrough comparisons between multiple independent codes and detailed convergence tests.