Spherical relativistic radiation flows with variable Eddington factor

We solve spherically symmetric radiation flows under full special relativity with the help of a variable Eddington factor, f (tau,beta), where tau is the optical depth and beta is the flow velocity normalized by the speed of light. Relativistic radiation hydrodynamics under the moment formalism has several complex problems, such as a closure relation. Conventional moment equations closed with the traditional Eddington approximation in the comoving frame have singularity, beyond which the flow cannot be accelerated. In order to avoid such a pathological behavior inherent in the relativistic moment formalism, we propose a variable Eddington factor, which depends on the flow velocity as well as the optical depth, for the case of sperically symmetric one-dimensional flow. We then calculate the relativistic spherical flow with such variable Eddington factors to investigate the case that gas is accelerated by radiative force. As a result, it is shown that the gas speed reaches around the speed of light by radiation pressure.