SIMBAD references

We present the results of a new radiation hydrodynamics code called Maartje. This code describes the evolution of a flow in three spatial dimensions using an adaptive mesh, and contains a combination of a ray tracer and an atomic physics module to describe the effects of ionizing radiation. The code is parallelized using a custom threadpool library. We present an application in which we follow the ionization of two dense spherical clumps which are exposed to an ionizing radiation field from a 50000K black body. We study various configurations in which one of the clumps shields the other from the ionizing photons. We find that relatively long-lived filamentary structures with narrow tails are formed. This raises the possibility that cometary knots (such as are found in the Helix Nebula) may be the result of the interaction of an ionizing radiation field with an ensemble of clumps, as opposed to the identification of a single knot with a single clump.