Radiation Hydrodynamics using Characteristics on Adaptive Decomposed
  Domains for Massively Parallel Star Formation Simulations
  We present an algorithm for solving the radiative transfer problem on
massively parallel computers using adaptive mesh refinement and domain
decomposition. The solver is based on the method of characteristics which
requires an adaptive raytracer that integrates the equation of radiative
transfer. The radiation field is split into local and global components which
are handled separately to overcome the non-locality problem. The solver is
implemented in the framework of the magneto-hydrodynamics code FLASH and is
coupled by an operator splitting step. The goal is the study of radiation in
the context of star formation simulations with a focus on early disc formation
and evolution. This requires a proper treatment of radiation physics that
covers both the optically thin as well as the optically thick regimes and the
transition region in particular. We successfully show the accuracy and
feasibility of our method in a series of standard radiative transfer problems
and two 3D collapse simulations resembling the early stages of protostar and
disc formation.
