SNR boundary. But as have been pointed out in previous studies [e.g., @gre88], the radio continuum boundary is not sharp and the ambient shock is thought to have propagated beyond the apparent radio boundary. It therefore seems to be reasonable to consider that the -NE filament is excited by the SNR shock too, although we need spectroscopic observations to understand the nature of the -NE filament.

### Circumstellar Origin of filaments

The filaments are more likely of circumstellar origin than interstellar. If interstellar, they must be dense clumps originally in an ambient or parental molecular cloud. We do not expect to observe molecular material around small, young core-collapse SNe in general because massive stars clear out the surrounding medium with their strong UV radiation and strong stellar winds during their lifetime. Some molecular material may survive if the progenitor star is an early B-type (B1–B3) star, which does not have strong UV radiation nor strong stellar winds [@mck84b; @che99]. A difficulty with this scenario, however, is that then the swept up mass at the current radius (3 pc) is likely to be much greater than the ejecta mass, so that the remnant should have been already in Sedov stage where 