THE IONIZATION STRUCTURE OF OLD PLANETARY-NEBULAE WHICH INTERACT WITHTHE INTERSTELLAR-MEDIUM

Four old planetary nebulae have been observed, and are found to be int eracting with the interstellar medium (ISM). This appears to have comm enced fairly recently in A34 and A61, since the tell-tale asymmetric b rightness enhancements are present in images taken in both [N II] and [O III]. However, the [N II]/[O III] ratio is much higher in the inter action region than elsewhere, probably due to the increased densities and complicated motions expected there. By contrast, the regions of A3 1 and A62 which are bright in [O III] are considerably smaller than th e size of the nebula visible in [N II]. We interpret this as being due to a more advanced interaction than with A34 and A62. This ionization stratification may provide a simpler approach for probing the surroun ding ISM than using hydrodynamic models.