Lampray: Multi-group long characteristics ray tracing for adaptive mesh
  radiation hydrodynamics
  We present Lampray: a multi-group long characteristics ray tracing method for
adaptive mesh radiation hydrodynamics in the Ramses code. It avoids diffusion,
captures shadows, and treats colliding beams correctly, and therefore
complements existing moment-based ray tracing in Ramses. Lampray includes
different options for interpolation between ray and cell domain, and use either
integral, Fourier, or an implicit method for hydrogen ionization to solve the
radiative transfer. The opacity can either be tabulated or computed through a
coupling to the general non-equilibrium astro-chemistry framework Krome. We use
an H-He-C-O network with 36 species and 240 reactions to track the
photo-chemistry in the interstellar medium across 6 and 10 orders of magnitude
in temperature and density. Self-shielding prescriptions for H$_2$ and CO are
used together with a new model for the diffuse interstellar UV-field. We also
track the dust temperature, formation of H$_2$ on grains, and H$_2$O and CO
ices in detail. Lampray is tested against standard benchmarks for molecular
cloud and star formation simulations, including the formation of a Str\"omgren
sphere, the expansion of an ionization front, the photo-evaporation of a dense
clump, and the H-He-C-O chemistry in a static photo-dissociation front.
Efficient parallelisation is achieved with a separate domain decomposition for
rays where points along a ray reside in the same memory space, and data
movement from cell- to ray-domain is done with a direct hash-table lookup
algorithm. Point sources are treated without splitting rays, and therefore the
method currently only scales to a few point sources, while diffuse radiation
has excellent scaling.
