4 research outputs found
Using 3D Voronoi grids in radiative transfer simulations
Probing the structure of complex astrophysical objects requires effective
three-dimensional (3D) numerical simulation of the relevant radiative transfer
(RT) processes. As with any numerical simulation code, the choice of an
appropriate discretization is crucial. Adaptive grids with cuboidal cells such
as octrees have proven very popular, however several recently introduced
hydrodynamical and RT codes are based on a Voronoi tessellation of the spatial
domain. Such an unstructured grid poses new challenges in laying down the rays
(straight paths) needed in RT codes. We show that it is straightforward to
implement accurate and efficient RT on 3D Voronoi grids. We present a method
for computing straight paths between two arbitrary points through a 3D Voronoi
grid in the context of a RT code. We implement such a grid in our RT code
SKIRT, using the open source library Voro++ to obtain the relevant properties
of the Voronoi grid cells based solely on the generating points. We compare the
results obtained through the Voronoi grid with those generated by an octree
grid for two synthetic models, and we perform the well-known Pascucci RT
benchmark using the Voronoi grid. The presented algorithm produces correct
results for our test models. Shooting photon packages through the geometrically
much more complex 3D Voronoi grid is only about three times slower than the
equivalent process in an octree grid with the same number of cells, while in
fact the total number of Voronoi grid cells may be lower for an equally good
representation of the density field. We conclude that the benefits of using a
Voronoi grid in RT simulation codes will often outweigh the somewhat slower
performance.Comment: 9 pages, 7 figures, accepted by A
Efficient 3D NLTE dust radiative transfer with SKIRT
We present an updated version of SKIRT, a 3D Monte Carlo radiative transfer
code developed to simulate dusty galaxies. The main novel characteristics of
the SKIRT code are the use of a stellar foam to generate random positions, an
efficient combination of eternal forced scattering and continuous absorption,
and a new library approach that links the radiative transfer code to the DustEM
dust emission library. This approach enables a fast, accurate and
self-consistent calculation of the dust emission of arbitrary mixtures of
transiently heated dust grains and polycyclic aromatic hydrocarbons, even for
full 3D models containing millions of dust cells. We have demonstrated the
accuracy of the SKIRT code through a set of simulations based on the edge-on
spiral galaxy UGC 4754. The models we ran were gradually refined from a smooth,
2D, LTE model to a fully 3D model that includes NLTE dust emission and a clumpy
structure of the dusty ISM. We find that clumpy models absorb UV and optical
radiation less efficiently than smooth models with the same amount of dust, and
that the dust in clumpy models is on average both cooler and less luminous. Our
simulations demonstrate that, given the appropriate use of optimization
techniques, it is possible to efficiently and accurately run Monte Carlo
radiative transfer simulations of arbitrary 3D structures of several million
dust cells, including a full calculation of the NLTE emission by arbitrary dust
mixtures.Comment: 15 pages, 7 figures, accepted for publication in ApJ
Three-dimensional continuum radiative transfer simulations of dusty systems
3D continuum radiative transfer is one of the remaining grand challenge problems in computational astrophysics. Yet it is key to understanding the three-dimensional structure of astronomical objects for which we can observe only two-dimensional projections on the plane of the sky. We present SKIRT, a Monte Carlo code designed to treat 3D continuum radiative transfer problems in dusty systems. It has been used to study the effects of dust absorption, scattering and emission on the observed properties of galaxies, circumstellar discs and AGNs. The code can handle arbitrary geometries and different dust mixtures, even at large optical depths and/or including very small grains. We present a number of recent computational techniques that have been instrumental in making the code fast and reliable, even for complex geometries. We also show some recent 3D simulations to highlight the possibilities of modern radiative transfer codes