Cygnus Loop supernova remnant: new observations and a framework for understanding its structure and evolution

New observational data on the Cygnus Loop supernova remnant (SNR) include: (1) a detailed high resolution comparison of x-ray and optical emission for a field in the SE; (2) a map of the [O III] electron temperature for the field previously studied by Hester, Parker, and Dufour (1983); and (3) CCD imagery of the NE limb in the light of four emission lines. A wide range of new and existing observations of the Loop are for the first time interpreted within the context of a single physical description. The Cygnus Loop is not an evaporative SNR evolving into the McKee and Ostriker (1977) ISM, nor are tiny cloudlets necessary to explain its morphology. The data show the Cygnus Loop to be evolving into a medium consisting primarily of an intercloud phase with N0 approx. 0.1 cm-3 containing clouds with parsec dimensions and N0 less than or equal to 10 cm-3. The optical emission arises from extensive sheet like radiative shock fronts driven into the clouds. These fronts locally form the outer boundary of the remnant. The appearance of x-ray emission outside the optical emission on the limbs is due solely to projection effects. The distorted and bumpy shock front is shown to give rise in projection to the filamentary morphology of the remnant