Radiative Transfer in the Co - Moving

We discuss the formulation of the radiative transfer equation in the co-moving frame. For characteristic velocities larger than 2000 km s ?1 , the eeects of advection on the synthetic spectra are non-negligible, and hence, should be included in model calculations. We show that the time-independent or quasi-static approximation is adequate for most astrophysical problems, e.g. hot stars, novae, and supernovae. We examine the use of the Sobolev approximation in modeling moving atmospheres and nd that the number of overlapping lines in the co-moving frame make the approximation suspect in models that predict both lines and continua. We also discuss the form of the Rosseland mean opacity in the co-moving frame, and derive a formula that is easy to implement in radiation hydrodynamics calculations.