THE ULTRAVIOLET-SPECTRUM OF A FACE-ON SHOCK-WAVE IN THE VELA SUPERNOVA REMNANT

Ultraviolet emission lines in supernova remnants (SNRs) provide import ant clues to the shock velocities, densities, grain destruction, and t hermal structure of the cooling regions in these objects. However, sev eral of the brightest ultraviolet emission lines seen in SNRs, includi ng C II lambda 1335, C IV lambda 1550, and O VI lambda 1034, are reson ance lines whose brightnesses will be reduced by scattering if the col umn density along the line of sight is sufficiently large. Scattering is particularly important in the bright filaments of SNRs because thes e shock waves are seen nearly edge-on. To assess the importance of res onant scattering in the UV, we have taken advantage of the enormous di ffuse source sensitivity of the Hopkins Ultraviolet Telescope to obser ve 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 remn ant and corresponds to a bright soft X-ray region. A 165 km s(-1) shoc k with a 30% carbon depletion matches most of the line intensities, bu t the weakness of the N V lambda 1240 line may suggest a departure fro m the simple shock models. We compare the spectrum of the face-on shoc k to that of a nearby bright filament and find some differences in sho ck velocity as well as in resonance-line scattering. We also derive th e ram pressure of the shock from Fabry-Perot observations, and find th at it greatly exceeds the thermal pressure derived from the [S II] dou blet. This result strongly suggests pressure support by magnetic field s or cosmic rays.