Conclusions

Armed with these numerical simulations, perhaps we can make some sense of some observations. The Cygnus Loop is one of the largest (apparent) SNRs in the sky and presents many fascinating morphological variations for comparison to theory. Of particular interest is the region in the NE quadrant known as XA (Hester & Cox, 1986), a cloud in the early stages of being shocked (Danforth et al.1999); and the Southeast Cloud, a fine example of a post-shock cloud.

Fesen, Kwitter and Downes (1992, hereafter FKD) observed several areas of the Cygnus Loop including the Southeast Cloud (Figure 6). The similarity with the theories is striking. On the left we see a sharp, dimpled blast wave (visible in H$\alpha$ but no other bands). The cloud is in the center right and shows evidence of crushing and instabilities. FKD interpret the sharp filament on the right as the reverse shock propagating back into the remnant, but recent analysis (Levenson, Blair, priv. comm.) suggests that this is simply another portion of the blast wave oriented edge-on to our line of sight and not related to the clod system.

A more complicated system is XA, also in the Cygnus Loop but located in the east-northeastern region at the southern end of a large complex of radiative filaments (Danforth et al.1999). To first inspection, Figure 7 appears similar to that investigated by FKD; we see a bright knot of emission surrounded by a series of sharp, curving filaments. The similarity to Figure 4a is striking. However, we see that the sharp curved filaments on the east (left) are emitting strongly in [OIII], a sure sign of a radiative shock. According to our theories, SN blast waves in free space are supposed to emit weakly in the Balmer lines only. Shock velocity diagnostics give conflicting signals and there appears to be far too much column depth of high-ionization ions.

The conclusion drawn by Danforth et al.is that this cloud is a dense region in an ambient medium which is less diffuse than the typical ISM in the region. What we are seeing could be the shock wave encountering a the wall of a cavity blown by the SN precursor.

Clearly, the whole topic of cloud-shock interactions bears a great deal more investigation. Simulations keep improving with more advanced code and faster computing time allowing better resolution (both in time and space) and more accurate physics. Observationally, we need to keep investigating regions of the Cygnus Loop, the Crab Nebula, Vela, Puppis-A and the rest of the large SNRs looking for examples of fragmentation, evaporation and clouds in different stages of iacceleratednteraction with shocks.