A Two-moment Radiation Hydrodynamics Module in Athena Using a
  Time-explicit Godunov Method
  We describe a module for the Athena code that solves the gray equations of
radiation hydrodynamics (RHD), based on the first two moments of the radiative
transfer equation. We use a combination of explicit Godunov methods to advance
the gas and radiation variables including the non-stiff source terms, and a
local implicit method to integrate the stiff source terms. We adopt the M1
closure relation and include all leading source terms. We employ the reduced
speed of light approximation (RSLA) with subcycling of the radiation variables
in order to reduce computational costs. Our code is dimensionally unsplit in
one, two, and three space dimensions and is parallelized using MPI. The
streaming and diffusion limits are well-described by the M1 closure model, and
our implementation shows excellent behavior for a problem with a concentrated
radiation source containing both regimes simultaneously. Our operator-split
method is ideally suited for problems with a slowly varying radiation field and
dynamical gas flows, in which the effect of the RSLA is minimal. We present an
analysis of the dispersion relation of RHD linear waves highlighting the
conditions of applicability for the RSLA. To demonstrate the accuracy of our
method, we utilize a suite of radiation and RHD tests covering a broad range of
regimes, including RHD waves, shocks, and equilibria, which show second-order
convergence in most cases. As an application, we investigate radiation-driven
ejection of a dusty, optically thick shell in the interstellar medium (ISM).
Finally, we compare the timing of our method with other well-known iterative
schemes for the RHD equations. Our code implementation, Hyperion, is suitable
for a wide variety of astrophysical applications and will be made freely
available on the Web.
