Introduction - The Observational Basis

Much of our basic understanding of conditions in the ISM comes from observations of shock waves and their interaction with it. Supernova Remnants (SNRs) are prime examples of ISM interactions and, while most observed SNRs are too distant to resolve individual details, a handful are near enough and large enough to show clearly different morphologies at different locations along their rims. These morphologies are indicative of different types of shock interactions with the ISM.

Roughly we see two different types of filaments in SNRs (Figure 1). Non-radiative filaments are visible primarily in Balmer transitions and in the X-ray. They are smooth in appearance and very thin and may stretch for several parsecs or more. The physical interpretation is that a fast shock is moving through very homogeneous, tenuous medium. The shock is fast enough that the post-shock gas is heated to X-ray temperatures and cools very slowly. The northern rim of the Cygnus Loop is a perfect example of this. In the optical, we see a very thin, smooth ribbon of H$\alpha$ emission. In the X-ray, we see a sharp edge dividing a bright, hot, shocked region from a dark pre-shocked region.

The other general class is the clumpy, knotted morphology characterized by short, complicated filaments and bright line emission. This is generally believed to be indicative of cloud shock interaction which bends and warps the planar symmetry of the blast wave. The SN blast wave has traveled through a denser medium which cools more rapidly and is thus brighter. Cooling is typically partly or wholly radiative and many heavy ion lines are seen (OVI, [OIII], CIV, [CIII], NV, etc). It is these interactions which we wish to explore further in this paper.