Comoving-frame radiation transport for nonrelativistic fluid velocities

We investigate the comoving-frame treatment of radiation transport for use in problems of nonrelativistic radiation hydrodynamics. The transport equation is re-derived, to O( v/ c), retaining a term that is traditionally omitted. This reduced equation is not just formally, but also physically, correct to O( v/ c), inasmuch as it keeps the wave speed bounded by light speed and can transformed to the correct Eulerian-frame transport equation in any physical regime. We strongly recommend that this equation, and its moments, be used in all situations where the fluid-flow speeds are nonrelativistic.