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Session 55 - Supernova Remnants.
Oral session, Tuesday, January 14
The Cygnus Loop presents an outstanding case study of a supernova remnant, being nearby, bright, and relatively unobscured by dust. At both optical and X-ray wavelengths, it appears nearly circular, yet it also exhibits complex structure on smaller scales. High-resolution soft X-ray observations obtained with the ROSAT-HRI and complete optical images in H\alpha, [S II], and [O III] help resolve this apparent paradox. Rather than restrict investigation to regions that are are exceptionally bright or otherwise peculiar, such an unbiased multi-wavelength survey allows a global view of this supernova remnant and its environment.
The dominant features in X-ray and optical emission are the result of blast wave interactions with large inhomogeneities in the interstellar medium. X-ray emission is enhanced both in high-density regions and as the result of secondary shocks that are reflected off of clouds. High density clouds also slow shocks, allowing radiative cooling zones to develop. These are detected in optical line emission. Non-radiative shocks, observed as Balmer-dominated filaments, directly trace the shock front in atomic gas over half of the perimeter. The blast wave has encountered density enhancements over more than 80% of its projected edge.
These data provide evidence that the Cygnus Loop is the result of a cavity explosion. The stellar supernova progenitor homogenized the cavity interior and modified clouds on the periphery. Currently the blast wave propagates through the walls of the cavity, which is comprised of an atomic shell and denser clouds. The surrounding interstellar medium, which the progenitor has processed, fundamentally determines the remnant's appearance at X-ray and optical wavelengths. The observed features are expected rather than surprising in the context of stellar evolution that preceded the supernova.
Program listing for Tuesday