GLOBAL FAR-ULTRAVIOLET PROPERTIES OF THE CYGNUS LOOP

We present the C III lambda 977, O VI lambda lambda 1032, 1038 and NI] lambda 1486 emission line maps of the Cygnus Loop, obtained with the newly processed data of the Spectroscopy of Plasma Evolution from Astrophysical Radiation ( SPEAR; also known as FIMS) mission. In addition, the Si iv+O IV] line complexes around 1400 angstrom are resolved into two separate emission lines whose intensity demonstrates a relatively high Si IV region that was predicted in the previous study. The morphological similarity between the Ovi and X-ray images, as well as a comparison of the Ovi intensity with the value expected from the X-ray results, indicates that large portions of the observed O VIemissions could be produced from X-ray emitting gas. Comparisons of the far-ultraviolet (FUV) images with the optical and HI 21 cm images reveal spatial variations of shock-velocity populations and high FUV extinction in the direction of a previously identified Hi cloud. By calculating the FUV line ratios for several subregions of the Cygnus Loop, we investigate the spatial variation of the population of radiative shock velocities as well as the effects of resonance scattering, X-ray emitting gas, and nonradiative shocks. The FUV and X-ray luminosity comparisons between the Cygnus Loop and the Vela supernova remnant suggest that the fraction of shocks in the early evolutionary stages is much larger in the Cygnus Loop.