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 shock front in the Cygnus Loop, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The spectrum covers the wavelength range 1118–1716 Å with an effective spectral resolution of ∼12 Å. The 0.″1 spatial resolution of these data provides a huge 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 spectrum by comparing the STIS spectrum with a Wide Field Planetary Camera 2 Hα image of the region. Isolating the brightest shock tangency, we identify lines of N V λ1240, C IV λ1549, He II λ1640, O V λ1371, O IV], Si IV λ1400, and N IV] λ1486, as well as the hydrogen two-photon continuum. The N V line peaks ∼0.″3 behind the C IV and He II emission and is spatially broader than the other emissions. Also, the observed line ratios of C IV and He II to N V are higher in our bright-shock spectrum than in previous observations of the same filament obtained through much larger apertures (and little or no spatial resolution), indicating 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 constrain the combinations of shock velocity and preshock density that are allowed.