[Previous] | [Session 82] | [Next]
R. Sankrit, W. P. Blair (JHU), J. C. Raymond (CfA), K. S. Long (STScI), K. R. Sembach, R. L. Shelton (JHU)
We present far ultraviolet spectra of a non-radiative filament in the northeast region of the Cygnus Loop. The spectra were taken at several positions on the filament and allow us to study the spatial distribution of the emission from the shocked gas. Spectra in the 1118Å\ -- 1716Å\ wavelength region were obtained using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). STIS spectra were taken with the 0\farcs5 and 0\farcs2 apertures with spectral resolutions of ~12Å\ and ~5Å, respectively. We detect N~V~\lambda1240, C~IV~\lambda1549 and He~II~\lambda1640 emission in these spectra. Additionally, a higher resolution spectrum was obtained at one position to resolve the N~V doublet into its components at 1238.8Å\ and 1242.8Å\@. Spectra in the 905Å\ -- 1187Å\ wavelength region were obtained using the Far Ultraviolet Spectroscopic Explorer (FUSE). Three FUSE spectra were obtained at positions overlapping with the STIS positions. These spectra were taken through a 4\arcsec~\times~20\arcsec aperture and have a resolution of ~0.05Å\@. We detect O~VI~\lambda\lambda1032, 1038 emission in these spectra, and the flux varies with position behind the shock front. The doublet components are well separated from each other and from the strong Ly\beta airglow line at 1026Å\@.
In the high resolution STIS spectrum the ratio of the N~V doublet lines is very close to 2:1 summed over the entire slit length. This means that the line is close to being optically thin. In the FUSE spectra, the ratio of the O~VI doublet lines varies systematically between 1.6 and 2.0. The higher value, which holds for optically thin emission, is observed at the position furthest behind the shock front. We also calculate a grid of shock models that predict the various line strengths. By comparing these with the observations we constrain the shock velocity and pre-shock density.
This work has been supported in part by Space Telescope Science Institute grant GO-07289.01-96A and in part by NASA Contract NAS5-32985 to The Johns Hopkins University.