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argot: accelerated radiative transfer on grids using oct-tree




We present two types of numerical prescriptions that accelerate the radiative transfer calculation around point sources within a three-dimensional Cartesian grid by using the oct-tree structure for the distribution of radiation sources. In one prescription, distant radiation sources are grouped as a bright extended source when the group’s angular size, θs, is smaller than a critical value, θcrit, and radiative transfer is solved on supermeshes whose angular size is similar to that of the group of sources. The supermesh structure is constructed by coarse-graining the mesh structure. With this method, the computational time scales with Nmlog (Nm)log (Ns), where Nm and Ns are the number of meshes and that of radiation sources, respectively. While this method is very efficient, it inevitably overestimates the optical depth when a group of sources acts as an extended powerful radiation source and affects distant meshes. In the other prescription, a distant group of sources is treated as a bright point source ignoring the spatial extent of the group, and the radiative transfer is solved on the meshes rather than the supermeshes. This prescription is simply a grid-based version of start by Hasegawa & Umemura and yields better results in general with slightly more computational cost [inline image] than the supermesh prescription. Our methods can easily be implemented to any grid-based hydrodynamic codes and are well suited to adaptive mesh refinement methods.