it would appear as a thin, limb-brightened shell. The thick-shell morphology of 11p2, however, indicates that it is not yet in Sedov stage. We therefore consider that the filaments are of circumstellar origin which fits well into the SN IIL/b scenario.

It is plausible that the progenitor of 11p2 had a strong wind which contains dense clumps. Numerous such clumps have been observed in Cas A, e.g., “Quasi-stationary flocculi (QSF)", which are slowly moving, dense optical clumps immersed within a smoother wind [@van71; @van85]. In the 320-yr old Cas A, the shock propagating into the clump is fast [100–200 ; @che03] while in the 1620-yr old 11p2 it is slow (30–50 ). Their velocity ratio is comparable to the ratio ($\sim 1/5$) of SNR expansion velocities, which suggests that the winds in 11p2 and Cas A have similar properties. We may estimate the density contrast between the clump and the smoother wind from the ratio of the shock speed into the clump ($v_c=30-50$ ) to the SNR forward shock speed $\vexp$. If we adopt the result of the radio (20 cm) expansion studies by [@tam03], $\vexp=1350\pm280$  so that the density contrast would be $(\vexp/v_c)^2=700-3,000$. For comparison, [@che03] estimated a density 