Trust

Abstract

Context. 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. Aims. 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. Methods. The details of the external star, the slab itself, and the dust properties are provided. 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. The dust properties adopted are characteristic of the diffuse Milky Way interstellar medium. 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-distributionaveraged properties. A total of six Monte Carlo codes and one Ray Tracing code provide solutions to this benchmark. Results. The solution to this benchmark is given as global spectral energy distributions (SEDs) and images at select diagnostic wavelengths from the ultraviolet through the infrared. 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. Convergence tests using one of the Monte Carlo codes illustrate the sensitivity of the solutions to various model parameters. Conclusions. 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.

SKIRT 9: Redesigning an advanced dust radiative transfer code to allow kinematics, line transfer and polarization by aligned dust grains

“…Next to a material mix representing electrons, SKIRT includes a set of turn-key material mixes for various dust models that have been published in the literature (e.g., Zubko et al 2004;Draine and Li 2007;Jones et al 2017), and configurable dust mixes that allow specifying material compositions and grain size distributions for each component of the dust model. At the time of writing, the gas material type is provided but not actually implemented.…”

“…However, these techniques seem to fall short for certain model configurations with high optical depth. Gordon et al (2017) study a 3D benchmark model involving a point source and a cuboid-shaped dust slab. Comparing the results generated by several simulation codes for the radiation penetrating the slab for higher optical depths, they find a lack of agreement between the codes and a lack of convergence with increasing number of photon packets even for the same code.…”

“…To see these statistics at work in a 3D context, we configure a SKIRT 9 simulation with the 3D slab geometry defined by Gordon et al (2017) using a single, optical wavelength and a single instrument on the side of the slab opposite to the source (θ = 180 • ). The instrument frame covers a field of view across the length of the slab in the X direction and a slice of about 10% of the slab width in the Y direction; it is offset from the center so that it matches the X slice defined in figure 7 of Gordon et al (2017). The number of pixels is set to 300 × 1 so that each pixel spans the full width of the instrument frame, and all photon packet contributions along the width of the slice end up in the same bin.…”

Synthetic observations of protostellar multiple systems

“…In most cases, multiple codes must be tested against each other in a labour-intensive study (e.g. Bisbas et al 2015;Gordon et al 2017). We acknowledge that this should be examined in future work.…”

Unbiased Monte Carlo continuum radiative transfer in optically thick regions

“…Therefore, every photon package has to be tracked from its first emission to the moment it leaves the simulated space. The number of interactions it undergoes increases strongly with the optical depth of the medium, resulting in a high computational demand (e.g., Baes et al 2016;Gordon et al 2017;Camps & Baes 2018). This problem is usually handled by imposing a maximum number of interaction whose exceedance results in the removal of the photon package.…”