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We present Lampray: a multi-group long characteristics ray tracing method foradaptive mesh radiation hydrodynamics in the Ramses code. It avoids diffusion,captures shadows, and treats colliding beams correctly, and thereforecomplements existing moment-based ray tracing in Ramses. Lampray includesdifferent options for interpolation between ray and cell domain, and use eitherintegral, Fourier, or an implicit method for hydrogen ionization to solve theradiative transfer. The opacity can either be tabulated or computed through acoupling to the general non-equilibrium astro-chemistry framework Krome. We usean H-He-C-O network with 36 species and 240 reactions to track thephoto-chemistry in the interstellar medium across 6 and 10 orders of magnitudein temperature and density. Self-shielding prescriptions for H$_2$ and CO areused together with a new model for the diffuse interstellar UV-field. We alsotrack the dust temperature, formation of H$_2$ on grains, and H$_2$O and COices in detail. Lampray is tested against standard benchmarks for molecularcloud and star formation simulations, including the formation of a Str\"omgrensphere, the expansion of an ionization front, the photo-evaporation of a denseclump, and the H-He-C-O chemistry in a static photo-dissociation front.Efficient parallelisation is achieved with a separate domain decomposition forrays where points along a ray reside in the same memory space, and datamovement from cell- to ray-domain is done with a direct hash-table lookupalgorithm. Point sources are treated without splitting rays, and therefore themethod currently only scales to a few point sources, while diffuse radiationhas excellent scaling.

Lampray: Multi-group long characteristics ray tracing for adaptive mesh radiation hydrodynamics