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Kinetic Scheme for Solving M1 Model of Radiative Transfer

We show a numerical scheme to solve the moment equations of the radiative transfer, i.e., M1 model which follows the evolution of the energy density, $ E $, and the energy flux, $ \mbox{\boldmath$F$} $. In our scheme we reconstruct the intensity from $ E $ and $ \mbox{\boldmath$F$} $ so that it is consistent with the closure relation, relation, $ χ= (3 + 4 f ^2)/(5 + 2 \sqrt{4 - 3 f ^2}) $. Here the symbols, $ χ$, $ f = |\mbox{\boldmath$F$}|/(cE) $, and $ c $, denote the Eddington factor, the reduced flux, and the speed of light, respectively. We evaluate the numerical flux across the cell surface from the kinetically reconstructed intensity. It is an explicit function of $ E $ and $ \mbox{\boldmath$F$} $ in the neighboring cells across the surface considered. We include absorption and reemission within a numerical cell in the evaluation of the numerical flux. The numerical flux approaches to the diffusion approximation when the numerical cell itself is optically thick. Our numerical flux gives a stable solution even when some regions computed are very optically thick. We show the advantages of the numerical flux with examples. They include flash of beamed photons and irradiated protoplanetary disks.