Tree-based solvers for adaptive mesh refinement code FLASH -- III: a
  novel scheme for radiation pressure on dust and gas and radiative transfer
  from diffuse sources
  Radiation is an important contributor to the energetics of the interstellar
medium, yet its transport is difficult to solve numerically. We present a novel
approach towards solving radiative transfer of diffuse sources via backwards
ray tracing. Here we focus on the radiative transfer of infrared radiation and
the radiation pressure on dust. The new module, \textsc{TreeRay/RadPressure},
is an extension to the novel radiative transfer method \textsc{TreeRay}
implemented in the grid-based MHD code {\sc Flash}. In
\textsc{TreeRay/RadPressure}, every cell and every star particle is a source of
infrared radiation. We also describe how gas, dust and radiation are coupled
via a chemical network. This allows us to compute the local dust temperature in
thermal equilibrium, leading to a significantly improvement over the classical
grey approximation. In several tests, we demonstrate that the scheme produces
the correct radiative intensities as well as the correct momentum input by
radiation pressure. Subsequently, we apply our new scheme to model massive star
formation from a collapsing, turbulent core of 150 ${\rm M}_\odot$. We trace
the effects of both, ionizing and infrared radiation on the dynamics of the
core. We find that the newborn massive star(s) prevent fragmentation in their
proximity through radiative heating. Over time, dust and radiation temperature
equalize, while the gas temperature can be either warmer due to shock heating
or colder due to insufficient dust-gas coupling. Compared to gravity, the
effects of radiation pressure become significant on the core scale only at an
evolved stage.
