Radiation Hydrodynamics in Supernovae

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. In supernovae, one of the main issues is the coupling between a radiation field and properties of the matter. Due to the low densities, nonthermal excitation by high-energy photons from radioactive decays and the time dependence of the problem, significant departures from local thermodynamical equilibrium (LTE) are common throughout the envelope even at large optical depths. This effect must be taken into account to simulate the evolution of spectra and light curves which are the basic tools to link between explosion physics and observations. The large velocity fields and the non-LTE problem result in a coupling of spatial, frequency space and the level population. This physical system can be described by a large system of coupled integro-differential equations for which the spatial and energy discretization (and its errors) are coupled. For the numerical solution, we use variable separation, analytic solutions and approximations, and iterative schemes. The need for adaptive mesh refinement (AMR) is demonstrated. As example, we show detailed spectra and light curves for the thermonuclear Supernova SN99by.