Spatial and spectral interpretation of a bright filament in the Cygnus Loop

A comparison is made of optical and UV line intensities and spatial and spectral optical line profiles of a well-defined Cygnus Loop filament with theoretical models. It is found that the sharp filament is due to the tangency to the line of sight of a large, thick sheet of emitting gas. The emitting region associated with the spur is very deep, and there is substantial gradient in shock velocity along the filament. Severe incompleteness of the recombination zone is found at the high-velocity end, and resonance scattering in the emitting region attenuates C IV and other resonance lines, as expected. There is evidence for depletion of Si and Fe relative to other elements. Nonthermal pressure apparently dominates the recombination zone of the filament. New techniques are introduced for determining the completeness of a shock and for determining the preshock density without recourse to the standard density diangostic line ratios.