THE CYGNUS LOOP SUPERNOVA REMNANT: NEW OBSERVATIONS AND A FRAMEWORK FOR UNDERSTANDING ITS STRUCTURE AND EVOLUTION (NEBULAE, INTERSTELLAR MEDIUM, X-RAYS)
HESTER, JOHN JEFFREY
Doctor of Philosophy
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 n(,0) (TURNEQ) .1 cm('-3) containing clouds with parsec dimensions and n(,0) (LESSTHEQ) 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. Variations in spectral characteristics of the optical emission are due to a general increase in completeness of the recombination region and decrease in v(,s) with increasing distance away from the edge of a cloud along the face of the front. Models show that the sheet geometries needed to explain the shapes and extents of observed filaments lead to surface brightness and kinematic properties of the emission which are in very good agreement with observation. These geometries require density inhomogeneities within clouds of only (TURN)10% on scales of (TURN)10('17) cm. A thick ((TURN)2") recombination region, resulting from field limited compression, is resolved for the first time. The expected relationship between nonsteady flow and high O III electron temperature is confirmed. Evaporation of clouds cannot account for the X-ray data. The brightest X-ray emission in the Loop comes just interior to young optical emission, and the gas has a pressure of at least six times the average for the Loop. Regions around clouds which should have undergone significant evaporation are not bright in X-rays. The X-ray data can be explained within the context of the large cloud picture by the combined effects of (1) a denser than average intercloud medium near clouds; (2) inertial pressurization of the X-ray emitting shell behind rapidly decelerating portions of the blast wave; and (3) strong compression of the hot gas by reflected and bow shocks around clouds.