Ultraviolet emission lines in supernova remnants (SNRs) provide important clues to the shock velocities, densities, grain destruction, and thermal structure of the cooling regions in these objects. However, several of the brightest ultraviolet emission lines seen in SNRs, including C II 1335, C IV 1550, and O VI 1034, are resonance lines whose brightness will be reduced by scattering if the column density along the line of sight is sufficiently large. Scattering is particularly important within the bright filaments of SNRs because these shock waves are seen nearly edge-on.
To assess the important of resonant scattering in the UV, we have taken advantage of the enormous diffuse source sensitivity of the Hopkins Ultraviolet Telescope to observe the UV emission from a face-on shock in the Vela supernova remnant. The position chosen is seen in projection near the center of the remnant and corresponds to a bright soft X-ray region. A 165 km/s shock with a 30% carbon depletion matches most of the line intensites, but the weakness of N V 1240 may suggest a departure from the simple shock models. We compare the spectrum of the face-on shock to that of a nearby bright filament and find some differences in shock velocity as well as in resonance line scattering.
We also derive the ram pressure of the shock from Fabry-Perot observations, and find that it greatly exceeds the thermal pressure derived from the [S II] doublet. This strongly suggests pressure support by magnetic fields or cosmic rays.