SPHRAY: A Smoothed Particle Hydrodynamics Ray Tracer for Radiative Transfer

TL;DR

SPHRAY is a Monte Carlo-based ray tracer for SPH simulations that accurately models 3D radiative transfer and ionization states without grid interpolation, supporting complex physics and parallel processing.

Contribution

It introduces SPHRAY, a novel SPH-based ray tracer that directly integrates through SPH kernels for 3D radiative transfer, enhancing flexibility and computational efficiency.

Findings

Successfully verified with Radiative Transfer Comparison Project tests

Supports multiple radiation sources including point, diffuse, and background

Parallelizable implementation with acceleration techniques

Abstract

We introduce SPHRAY, a Smoothed Particle Hydrodynamics (SPH) ray tracer designed to solve the 3D, time dependent, radiative transfer (RT) equations for arbitrary density fields. The SPH nature of SPHRAY makes the incorporation of separate hydrodynamics and gravity solvers very natural. SPHRAY relies on a Monte Carlo (MC) ray tracing scheme that does not interpolate the SPH particles onto a grid but instead integrates directly through the SPH kernels. Given initial conditions and a description of the sources of ionizing radiation, the code will calculate the non-equilibrium ionization state (HI, HII, HeI, HeII, HeIII, e) and temperature (internal energy/entropy) of each SPH particle. The sources of radiation can include point like objects, diffuse recombination radiation, and a background field from outside the computational volume. The MC ray tracing implementation allows for the quick introduction of new physics and is parallelization friendly. A quick Axis Aligned Bounding Box (AABB) test taken from computer graphics applications allows for the acceleration of the raytracing component. We present the algorithms used in SPHRAY and verify the code by performing all the test problems detailed in the recent Radiative Transfer Comparison Project of Iliev et. al. The Fortran 90 source code for SPHRAY and example SPH density fields are made available on a companion website (www.sphray.org).