Accuracy and efficiency of raytracing photoionisation algorithms
Abstract
Three nonequilibrium photoionisation algorithms for hydrodynamical gridbased simulation codes are compared in terms of accuracy, timestepping criteria, and parallel scaling. Explicit methods with firstorder time accuracy for photon conservation must use very restrictive timestep criteria to accurately track Rtype ionisation fronts. A secondorder accurate algorithm is described which, although it requires more work per step, allows much longer timesteps and is consequently more efficient. Implicit methods allow ionisation fronts to cross many grid cells per timestep while maintaining photon conservation accuracy. It is shown, however, that errors are much larger for multifrequency radiation than for monochromatic radiation with the implicit algorithm used here, and large errors accrue when an ionisation front crosses many optical depths in a single step. The accuracy and convergence rates of the different algorithms are tested with a large number of timestepping criteria to identify the best criterion for each algorithm. With these criteria selected, the secondorder explicit algorithm is the most efficient of the three, and its parallel scaling is significantly better than that of the implicit algorithm. The upgrade from first to secondorder accuracy in explicit algorithms could be made very simply to fixedgrid and adaptive meshrefinement codes which currently use a firstorder method.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 March 2012
 DOI:
 10.1051/00046361/201117984
 arXiv:
 arXiv:1201.5651
 Bibcode:
 2012A&A...539A.147M
 Keywords:

 radiative transfer;
 methods: numerical;
 Hii regions;
 Astrophysics  Galaxy Astrophysics;
 Physics  Computational Physics
 EPrint:
 14 pages, minor updates to match published version