Numerical models of the evolution of supernova remnants: the shell-formation stage

Numerical calculations of the behavior of supernova remnants have been carried out at several initial energies, with emphasis on the thermally unstable shell-formation phase. An approximate formula to fit the Cox cooling curves has been used. Thermal bremsstrahlung has been included exactly. The recombination region is fitted well, although La, the 1s-2p (0), and the 2s-2p (0, Si, S) transitions are underrepresented. The effect of this deviation is discussed. It is found that the time of onset of rapid shell formation is related to the time at which the temperature just behind the shock front in the Sedov phase decreases to the point that recombination processes exceed thermal bremsstrahlung (2 x 106 K). The time of onset of shell formation is only weakly dependent on the cooling coefficient, although the rate of formation is faster for a higher coefficient. For the best approximation to the Cox curves, the maximum density reached in the shell is higher for higher initial energy. In all cases, the resultant thin, dense, cool shell behaves like a momentum-limited piston which collides inelastically with the medium. This is in agreement with earlier calculations in this series and with earlier estimates made in general discussions by other authors. Subject headings: interstellar matter - shock waves - supernova remnants