PURE-OXYGEN RADIATIVE SHOCKS WITH ELECTRON THERMAL CONDUCTION

We calculate steady state radiative shock models in gas composed entirely of oxygen, with the purpose of explaining observations of fast-moving knots in Cas A and other oxygen-rich supernova remnants (SNRs). Models with electron thermal conduction differ significantly from models in which conduction is neglected. Conduction reduces postshock electron temperatures by a factor of 7-10 and flattens temperature gradients. The O III ion, whose forbidden emission usually dominates the observed spectra is present over a wide range of shock velocities, from 100 to 170 km s-1. The electron temperature in the [O III] line formation region is 3 × 104 K, in agreement with the 2 × 104 K derived from observations. All models with conduction have extensive warm (T > 4000 K) photoionization zones, which provides better agreement with observed optical [O I] line strengths. However, the temperatures in these zones could be lowered by [Si II] 34.8 μm and [Ne II] 12.8 μm cooling if Si and Ne are present in appreciable abundance relative to O. Such low temperatures would be inconsistent with the observed [O I] emission in oxygen-rich SNRs.