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Narrow and broad-band HST WFPC-2 images of a portion of the NW quadrant of the Crab Nebula capture features representative of the broad range of filament properties within the nebula. Structure is observed down to the resolution limit of the images. Filament morphologies vary widely, and include isolated knots, large diffuse patches, long tube-like structures, and cometary features. This morphological diversity is also reflected in the spectral properties and ionization profiles of filaments. The dominant effect is the concentration of low ionization emission into small, sharp features, embedded within more extended and diffuse high ionization emission. This is an oversimplification, however. The scale for this spatial stratification can range from $< 0.1$" to several arcseconds. Other filaments show isolated knots of high or (occassionally) low ionization emission, or sharp features which are spatially coincident in a range of lines. Small dust absorption features seen in the continuum image are generally coincident with low ionization filament cores, and trace the locations of self-shielding regions where molecules and dust can form and survive.
The relative volumes of regions responsible for emission in different lines can vary by orders of magnitude from location to location within the nebula. Hester et al. (1990 ApJ, 350, 149) suggested that this should be the case, and pointed out that this greatly complicates even relative comparison of different lines in ground based spectra. More generally, it is clear from the data that ground-based spectrophotometry of the Crab filaments integrates over physical conditions that are far more diverse and complex than generally assumed.
It has been understood for some time that the thermal filaments in the Crab confine the synchrotron nebula. The morphology of the filaments, in particular structures reminiscent of salt-finger instabilities pointing inward into the synchrotron nebula, shows clear signs of this interaction.
