ARlogo Annu. Rev. Astron. Astrophys. 2013. 51:63-104
Copyright © 2013 by Annual Reviews. All rights reserved

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THREE-DIMENSIONAL DUST RADIATIVE TRANSFER*

Jürgen Steinacker 1,2, Maarten Baes 3, and Karl D. Gordon 4,3


1 UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble, F-38041, France;
2 Max-Planck-Institut für Astronomie, D-69117 Heidelberg, Germany
3Sterrenkundig Observatorium, Universiteit Gent, B-9000 Gent, Belgium;
4 Space Telescope Science Institute, Baltimore, Maryland 21218


Abstract: Cosmic dust is present in many astrophysical objects, and recent observations across the electromagnetic spectrum show that the dust distribution is often strongly three-dimensional (3D). Dust grains are effective in absorbing and scattering ultraviolet (UV)/optical radiation, and they re-emit the absorbed energy at infrared wavelengths. Understanding the intrinsic properties of these objects, including the dust itself, therefore requires 3D dust radiative transfer (RT) calculations. Unfortunately, the 3D dust RT problem is nonlocal and nonlinear, which makes it one of the hardest challenges in computational astrophysics. Nevertheless, significant progress has been made in the past decade, with an increasing number of codes capable of dealing with the complete 3D dust RT problem. We discuss the complexity of this problem, the two most successful solution techniques [ray-tracing (RayT) and Monte Carlo (MC)], and the state of the art in modeling observational data using 3D dust RT codes. We end with an outlook on the bright future of this field.


Keywords: scattering, Monte Carlo, ray tracing, computational astrophysics, numerical algorithms


Table of Contents

INTRODUCTION

THE THREE-DIMENSIONAL DUST RADIATIVE TRANSFER PROBLEM
The Radiative Transfer Equation
Primary Emission and Absorption
Including Scattering
Radiative Transfer in Dust Mixtures
Including Dust Emission
Radiative Transfer of Polarized Radiation

THE DISCRETE THREE-DIMENSIONAL DUST RADIATIVE TRANSFER PROBLEM
Spatial Grids
Local mean intensity storage grids
Density and source grids
Solution grids
Direction Grid
Wavelength and Dust Grain Grids

THE RAY-TRACING SOLUTION METHOD
Ray-Tracing Solution for a Single Ray
Beyond the spatial grid resolution
High optical depths
Ray location and global solution of the RTE
Thermal emission
Including scattered radiation
Ray-Tracing Error Analysis

THE MONTE CARLO SOLUTION METHOD
Simple MC RT
Step 1: birth.
Step 2: determination of the interaction point.
Step 3: absorption and scattering.
Weighted MC RT
Biased emission.
Absorption-scattering split.
Forced scattering
Peel-off technique.
Continuous absorption.
Instantaneous dust emission.
High optical depths.
Polychromatism.
Uncertainties for Monte Carlo

CHALLENGES IN MODELING OBSERVATIONS
Model Choice
Gridding
Comparison of Models and Data
Exploration of the Parameter Space
Error Analysis
Inverse RT

CODES AND BENCHMARKS
Available 3D codes
Benchmark efforts

THE FUTURE OF THE FIELD
Present Status
General Trends
Future Benchmarks
Data Modeling Future
Future Connections to Nondust Radiative Transfer Codes
Future Algorithms
Input Physics Improvements
Challenges

REFERENCES



*This review was the result of a collaboration of equals; the order in which authors are listed is not significant.

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