FAR-ULTRAVIOLET MAPPING OF THE CYGNUS LOOP WITH THE VOYAGER-2 ULTRAVIOLET SPECTROMETER

We report far-ultraviolet maps of the Cygnus Loop made using spectroscopic data from the Voyager 2 Ultraviolet Spectrometer. Emission line features at approximately 980 and approximately 1035 angstrom dominate the 500-1700 angstrom spectra of the Cygnus Loop as observed with Voyager, and we have generated maps in the light of these two features. We have compared the 980 and 1035 angstrom maps with X-ray and optical images of the Cygnus Loop that have been sampled in the same manner. From this comparison, it is clear that the 980 angstrom feature arises mainly from regions of bright optical emission, whereas the 1035 angstrom emission more closely resembles the X-ray emission. This general impression can be confirmed by inspection of the spectra as a function of position. Spectra extracted from positions corresponding to the primary shock front show the 1035 angstrom feature to be relatively strong, while the 980 angstrom feature is brighter when substantial optical emission is present within the aperture. This distribution is consistent with the idea that the 980 angstrom feature is dominated by C III lambda-977 while the 1035 angstrom feature is dominated by O VI lambda-lambda-1032, 1038. Luminosities of the Cygnus Loop in C III and O VI are approximately 2.7 x 10(36) ergs s-1 and approximately 4.6 x 10(36) ergs s-1, respectively. The ubiquity of O VI emission implies that shocks with velocity greater than 160 km s-1 are widespread throughout the Cygnus Loop.