A scheme for radiation pressure and photon diffusion with the M1 closure   in RAMSES-RT
  We describe and test an updated version of radiation-hydrodynamics (RHD) in the RAMSES code, that includes three new features: i) radiation pressure on gas, ii) accurate treatment of radiation diffusion in an unresolved optically thick medium, and iii) relativistic corrections that account for Doppler effects and work done by the radiation to first order in v/c. We validate the implementation in a series of tests, which include a morphological assessment of the M1 closure for the Eddington tensor in an astronomically relevant setting, dust absorption in a optically semi-thick medium, direct pressure on gas from ionising radiation, convergence of our radiation diffusion scheme towards resolved optical depths, correct diffusion of a radiation flash and a constant luminosity radiation, and finally, an experiment from Davis et al. of the competition between gravity and radiation pressure in a dusty atmosphere, and the formation of radiative Rayleigh-Taylor instabilities. With the new features, RAMSES-RT can be used for state-of-the-art simulations of radiation feedback from first principles, on galactic and cosmological scales, including not only direct radiation pressure from ionising photons, but also indirect pressure via dust from multi-scattered IR photons reprocessed from higher-energy radiation, both in the optically thin and thick limits. 
J. Rosdahl and R. Teyssier
astro-ph.IM astro-ph.HE