Hubble Space Telescope STIS observations of the Cygnus Loop: Spatial structure of a nonradiative shock

We present a spatially resolved ultraviolet spectrum of a nonradiative shoc k front in the Cygnus Loop, obtained with the Space Telescope Imaging Spect rograph on board the Hubble Space Telescope. The spectrum covers the wavele ngth range 1118-1716 Angstrom with an effective spectral resolution of simi lar to 12 Angstrom. The 0." 1 spatial resolution of these data provides a h uge improvement over earlier ultraviolet spectra, allowing us to study the spatial distribution of high-ionization line emissions directly behind the shock front. We are able to isolate individual shock features in our spectr um by comparing the STIS spectrum with a Wide Field Planetary Camera 2 H al pha image of the region. Isolating the brightest shock tangency, we identif y lines of N v lambda 1240, C Iv lambda 1549, He II lambda 1640, O v lambda 1371, O IV], Si Iv lambda 1400, and N IV] lambda 1486, as well as the hydr ogen two-photon continuum. The N v line peaks similar to 0." 3 behind the C Iv and He II emission and is spatially broader than the other emissions. A lso, the observed line ratios of C Iv and He II to N v are higher in our br ight-shock spectrum than in previous observations of the same filament obta ined through much larger apertures land little or no spatial resolution), i ndicating that there must be a more widely distributed component of the N v emission. We calculate shock models and show that the observed separation between the C Iv and N v emission zones and observed line intensities const rain the combinations of shock velocity and preshock density that are allow ed.