START: Smoothed particle hydrodynamics with tree-based accelerated radiative transfer

TL;DR

START is a new SPH-based radiation hydrodynamics code that employs tree-based acceleration for radiative transfer, enabling faster simulations with many sources while maintaining accuracy, crucial for cosmic reionization studies.

Contribution

The paper introduces START, a novel SPH code with tree-based accelerated radiative transfer, significantly improving computational efficiency for complex radiation-hydrodynamics problems.

Findings

Radiative transfer calculations are accelerated, scaling as Np log Ns.

START accurately reproduces results of non-accelerated codes with a tolerance < 1.0.

The code successfully simulates shadowing and diffuse ionizing photon effects.

Abstract

We present a novel radiation hydrodynamics code, START, which is a smoothed particle hydrodynamics (SPH) scheme coupled with accelerated radiative transfer. The basic idea for the acceleration of radiative transfer is parallel to the tree algorithm that is hitherto used to speed up the gravitational force calculation in an N-body system. It is demonstrated that the radiative transfer calculations can be dramatically accelerated, where the computational time is scaled as Np log Ns for Np SPH particles and Ns radiation sources. Such acceleration allows us to readily include not only numerous sources but also scattering photons, even if the total number of radiation sources is comparable to that of SPH particles. Here, a test simulation is presented for a multiple source problem, where the results with START are compared to those with a radiation SPH code without tree-based acceleration. We find that the results agree well with each other if we set the tolerance parameter as < 1.0, and then it demonstrates that START can solve radiative transfer faster without reducing the accuracy. One of important applications with START is to solve the transfer of diffuse ionizing photons, where each SPH particle is regarded as an emitter. To illustrate the competence of START, we simulate the shadowing effect by dense clumps around an ionizing source. As a result, it is found that the erosion of shadows by diffuse recombination photons can be solved. Such an effect is of great significance to reveal the cosmic reionization process.