Hubble space telescope wide field planetary camera 2 imaging of the crab nebula. I. Observational overview

We present Hubble Space Telescope WFPC2 images of a portion of the Crab Nebula supernova remnant obtained using narrow passband filters. These images show a wide range of filament morphologies including small individual isolated knots, long smooth arcs, and chaotic filament complexes, with evidence that some filament cores are far denser and cooler than had been expected. Although several filaments show resolved ionization structures, in general such structure is not seen on a filament-by-filament basis. In areas where the filaments are mainly on the remnant's near side, "shadowing" features are seen against the bright synchrotron emission background because of absorption by dust in the filaments. While most of these dust features appear to be the cores of knotty-type filaments, we identify a remarkable sinuous dust shadow that maintains a coherent structure over ∼25″ (0.24 pc, assuming d = 2 kpc). The obscuration caused by this narrow structure requires surprisingly dense dust concentrations. The HST imaging data reveal relatively few emission-line structures that are unresolved at WFPC2's 0″.10 resolution, with characteristic angular knot sizes of 0″.4 to 0″.8, corresponding to physical scales of 1.2-2.4 × 10¹⁶ cm at the assumed distance of 2 kpc. Such sizes are roughly in accord with expectations from theoretical models of the filaments. Future papers in this series will concentrate on specific aspects of the data set in more detail.