  Modelling the interaction between ionizing photons emitted from massive stars
and their environment is essential to further our understanding of galactic
ecosystems. We present a hybrid Radiation-Hydrodynamics (RHD) scheme that
couples an SPH code to a grid-based Monte Carlo Radiative Transfer code. The
coupling is achieved by using the particle positions as generating sites for a
Voronoi grid, and applying a precise mapping of particle-interpolated densities
onto the grid cells that ensures mass conservation. The mapping, however, can
be computationally infeasible for large numbers of particles. We introduce our
tree-based algorithm for optimizing coupled RHD codes. Astrophysical SPH codes
typically utilize tree-building procedures to sort particles into hierarchical
groups (referred to as nodes) for evaluating self-gravity. Our algorithm
adaptively walks the gravity tree and transforms the extracted nodes into
pseudo-SPH particles, which we use for the grid construction and mapping. This
method allows for the temporary reduction of fluid resolution in regions that
are less affected by the radiation. A neighbour-finding scheme is implemented
to aid our smoothing length solver for nodes. We show that the use of
pseudo-particles produces equally accurate results that agree with benchmarks,
and achieves a speed-up that scales with the reduction in the final number of
particle-cell pairs being mapped.
