DISCOVERY OF A FAST RADIATIVE SHOCK-WAVE IN THE CYGNUS LOOP USING THE HOPKINS-ULTRAVIOLET-TELESCOPE

We have observed the far-ultraviolet spectrum of a bright radiative filament on the eastern edge of the Cygnus Loop supernova remnant using the Hopkins Ultraviolet Telescope aboard the Astro-1 space shuttle mission in 1990 December. These are the first data to resolve the sub-Lyman-alpha emission of an interstellar shock wave. We observe strong emission lines of C III lambda-977, N III lambda-991, and O VI lambda-lambda-1032, 1038 and many fainter lines, as well as lines seen above Lyman-alpha in IUE spectra of supernova remnants. Comparison of this spectrum with shock model calculations indicates that the emission at this position in the Cygnus Loop is dominated by a approximately 170 km s-1 shock wave, although emission from lower shock velocities may also contribute. Both faster and slower shocks have been identified in the Cygnus Loop, but little attention has been given to the possible presence and importance of fast radiative shocks such as the one reported here. We briefly compare these results to Voyager UVS spectra of the Cygnus Loop and discuss the more global implications of these data for the understanding of the Cygnus Loop.