Gray Radiation Hydrodynamics with the FLASH Code for Astrophysical Applications

We present the newly-incorporated gray radiation hydrodynamics capabilities of the FLASH code based on a radiation flux-limiter aware hydrodynamics numerical implementation designed specifically for applications in astrophysical problems. The newly incorporated numerical methods consist of changes in the unsplit hydrodynamics solver and adjustments in the flux-limited radiation diffusion unit. Our method can treat problems in both the strong and weak radiation-matter coupling limits as well as transitions between the two regimes. Appropriate extensions in the "Helmholtz" equation of state are implemented to treat two-temperature astrophysical plasmas involving the interaction between radiation and matter and the addition of a new opacity unit based on the OPAL opacity database, commonly used for astrophysical fluids. A set of radiation-hydrodynamics test problems is presented aiming to showcase the new capabilities of FLASH and to provide direct comparison to other codes like CASTRO. To illustrate the capacity of FLASH to simulate phenomena occuring in stellar explosions, such as shock break--out, radiative precursors and supernova ejecta heating due to the decays of radioactive nickel-56 and cobalt-56, we also present 1D supernova simulations and compare the computed lightcurves to those of the SNEC code. The latest public release of FLASH with these enhanced capabilities is available for download and use by the broader astrophysics community.