Ultraviolet emission of the Vela supernova remnant and the Cygnus Loop

Ultraviolet emission spectra of several filaments in the Vela supernova remnant and the Cygnus Loop are compared with theoretical models of radiative shocks. Shock velocities range from 85 to 130 km/s. Little or no grain depletion is apparent. Several of the observed filaments deviate from the steady-flow models in the sense that much of the photoionization-recombination zone is absent. It is shown that resonance line scattering within the emitting region can account for the weakness of the C II 1335 and C IV 1550 lines. Evaporation fronts, which are basic to the McKee and Ostriker picture of supernova remnant evolution, do not contribute significantly to the observed emission. Some discrepancies in UV emission predicted by different sets of shock models are traced to differing atomic rates used in the calculations. A fluorescent emission line of H2 may be observable in some supernova remnants.