A FAR-ULTRAVIOLET STUDY OF THE CYGNUS LOOP USING THE VOYAGER ULTRAVIOLET SPECTROMETERS

We have used the Voyager 1 and 2 Ultraviolet Spectrometers to study th e far-ultraviolet emissions from different types of shock waves in the Cygnus Loop. In the southeast and northern parts of the supernova rem nant (SNR), we have measured the O VI A1035 surface brightness from th e main blast wave. This value is several times below the average and m ore than one order of magnitude below the peak O VI brightness in the SNR as measured with Voyager. A simple blast wave model appears able t o reproduce the observations in the southeast and the northern parts o f the Cygnus Loop but can only account for 10%-15% of the total O VI e mission from the Cygnus Loop. The brightest O VI and C III lambda977 e mission is found coincident with optical filamentation and X-ray enhan cements in the northeast. We interpret the observations in the northea st in terms of nonradiative and incomplete shocks whose surface area r ises in the optical filamentary regions. We conclude that the bulk of the O VI emission from the Cygnus Loop arises from optically bright cl ouds within which intermediate-velocity (200 + 50 km s-1) nonradiative and incomplete shocks are widespread.