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 transfer problem 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 benchmark composed of a slab of
dust with uniform density externally illuminated by a star. This simple 3D
benchmark is explicitly formulated to provide tests of the different components
of the radiative transfer problem including dust absorption, scattering, and
emission. This benchmark includes models with a range of dust optical depths
fully probing cases that are optically thin at all wavelengths to optically
thick at most wavelengths. This benchmark includes 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 depths is quantified for the first time.
We provide the first 3D dust radiative transfer benchmark and validate the
accuracy of this benchmark through comparisons between multiple independent
codes and detailed convergence tests.
