Voyager spectral observations of the Cygnus Loop nebula, 600-1700 A

Observations of the Cygnus Loop objects NGC 6995 and NGC 6992, which are defined by bright filamentary structure in visible radiation, are reported which were made with the Voyager 2 UV spectrometer in the wavelength range from 600 to 1700 A. The results are compared with far-UV observations performed during the Apollo 16 mission. It is found that measurable emission in the region from 912 to 1050 A is sharply confined to the location of the optical filaments, that emissions in the range from 1200 to 1680 A exhibit a sharp decline 0.5 deg of arc west of the filaments toward the center of the Loop, and that the most likely emissions in the 912-1050-A region are C II at 1037 A, C III at 977 A, and O I at 936 and 999.5 A. The results suggest that the radiation in the 912-1050-A range is associated with cooler postshock regions and that the shock velocities range from 70 to 100 km/s