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

Abstract

Line profiles and surface-brightness maps of a bright optical filament in the Cygnus Loop show the filament to be a tangency of our line of sight to a surface bounded by a radiative shock wave. The thickness of the cooling and recombination zones behind the shock are at the limit of optical resolution so that the surface can be thought of as a thin emitting sheet. Optical and ultraviolet spectra, combined with detailed models, show that the shock velocity increases from ~80kms-1 near its northwest end to 140kms _1 at the southeast tip. The cooling and recombination behind the shock are complete (" steady flow ") at its northwest end, but drastically incomplete in the southeast. We discuss the overall geometry which gives rise to the observed feature and the implications of the observations for resonance line scattering in the filament, elemental abundances, and grain destruction in the shock. The high quality and broad wavelength coverage of the data provide very strong tests of the shock models. It is quite satisfying that these spectra are in fact reproducible within a quite reasonable parameter space. By combining line profile information and absolute surface brightness with the shock velocity determined from the ultraviolet spectrum, we determine the preshock density and shock ram pressure. Comparison with the density obtained from the [S n] line ratio determines the nonthermal pressure (magnetic pressure plus cosmic-ray pressure) in the recombination zone, and we find that nonthermal pressure dominates.