Multi-dimensional Radiation Transport in Rapidly Expanding Envelopes

We discuss the current status of our HYDrodynamical RAdiation (Hydra) code for rapidly expanding, low density envelopes commonly found in core collapse and thermonuclear supernovae (+ novae and WR stars). We focus on our current implementation of multi-dimensional, non-relativistic radiation transport neglecting all terms of higher order than O(v/c). Line opacities are treated in the narrow line limit and consistency with the rate equations, radiation field and hydrodynamics is achieved iteratively in each time step via 'accelerated lambda iteration'. The solution of the transport is based on a hybrid scheme between a grid-based Variable Eddington Tensor and a Monte-Carlo method which is used for the auxiliary calculation to determine the Tensor elements. The advantages and limitation of our approach are discussed. We use this hybrid approach to reduce problems related to a grid-imposed directional dependence of the speed of light, and problems inherent to methods based on short and long characteristics for the Tensors. For short and long characteristics, the frequency or directional errors increase with the resolution or memory and computational requirements seem to be beyond feasibility, respectively. The limitations and the potential of our current approach is demonstrated by two simulations for thermonuclear supernovae.