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