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W.P. Blair, R. Sankrit, K.R. Sembach, R.L. Shelton (JHU)
We have observed bright radiative shocks in the Cygnus Loop supernova remnant (SNR) in the far ultraviolet 912 - 1187 Å\ region using the Far Ultraviolet Spectroscopic Explorer (FUSE). At a distance of only ~440 pc, the Cygnus Loop suffers little foreground extinction and the bright radiative filaments are excellent far-UV targets. FUSE provides an extended source velocity resolution of 20 - 100 \rm km ~ s-1, depending on the spectrograph aperture used and aperture filling factor. Hence, we have been able to separate source emission lines from stronger nearby atmospheric lines, resolve closely spaced lines, and observe structure within the strongest emission lines. Present are the resonance emissions of O~VI \lambda\lambda1031.9,1037.6, C~III \lambda977.0, and N~III \lambda991.0, expected from earlier work with the Hopkins Ultraviolet Telescope (HUT). Also present are lines of S~IV, S~VI, [Ne~V], Ne~VI, Si~III, Si~IV, and the excited state C~III \lambda1176 multiplet (not previously detected in the Cygnus Loop).
The O~VI doublet is particularly interesting since the ratio of the two lines provides information on the optical depth of the emitting regions. In general, the FUSE observations show \lambda1032:\lambda1038 ratios between 1.5 and 2, indicating partially optically thick emission. The strong resonance lines like O~VI and C~III \lambda977 also show central absorption reversals due to overlying gas either from within the Cygnus Loop itself or from the intervening ISM. These effects have long been suspected, but are invisible at the low resolution available previously with HUT and IUE, and have affected observed line ratios in these earlier analyses.
The observations used three instrumental apertures, spanning about 3\arcmin\ of the remnant in any given pointing. O~VI emission was seen in the spectrum from each aperture, indicating that O~VI is not confined to the bright optical filaments but is widespread.
This work is supported by NASA Contract NAS5-32985 to the Johns Hopkins University.
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