Subsweep: Extensions to the Sweep method for radiative transfer

TL;DR

Subsweep is a novel radiative transfer postprocessing code that enhances the transport sweep method with sub-timesteps, significantly reducing computational costs while accurately modeling coupled radiation chemistry in various astrophysical scenarios.

Contribution

It introduces a hierarchical sub-timestep extension to the transport sweep method, enabling more efficient and accurate radiative transfer simulations with coupled chemistry.

Findings

Successfully modeled HII region expansion.

Accurately simulated shadow formation behind dense objects.

Demonstrated effectiveness with periodic boundary conditions.

Abstract

We introduce the radiative transfer postprocessing code Subsweep. The code is based on the method of transport sweeps, in which the exact solution to the scattering-less radiative transfer equation is computed in a single pass through the entire computational grid. The radiative transfer module is coupled to radiation chemistry, and chemical compositions as well as temperatures of the cells are evolved according to photon fluxes computed during radiative transfer. Subsweep extends the method of transport sweeps by incorporating sub-timesteps in a hierarchy of partial sweeps of the grid. This alleviates the need for a low, global timestep and as a result Subsweep is able to drastically reduce the amount of computation required for accurate integration of the coupled radiation chemistry equations. We succesfully apply the code to a number of physical tests such as the expansion of HII regions, the formation of shadows behind dense objects, and its behavior in the presence of periodic boundary conditions.