THE CLOSEST PLANETARY-NEBULA, SH-2-216, AND ITS INTERACTION WITH THE INTERSTELLAR-MEDIUM

We have obtained mosaics of images of the giant planetary nebula, Sh 2 -216, in H alpha and [N II], covering the entire 1 degrees.6 diameter. These show the interaction with the interstellar medium (ISM) in unpr ecedented detail. While the restriction of the [O III] zone to the reg ion close to the central star is as expected for the O++ Stromgren sph ere in a uniform-density nebula, the lower ionization lines are clearl y strongly influenced by the interaction with the ISM. There is much f ilamentary structure: for example, there are long, thin features in th e obvious interaction region to the east (seen in H alpha) and much wi der structures across the face of the nebula (which are substantially brighter in [N II]. We interpret this as showing that the nebula has a thick outer region which is bright in [N II] and has deep outer corru gations. Over this lies a thin sheet of material that is brighter in H alpha and is also the main location of low-ionization emission such a s [O I]. Thus, the different characteristics of the filaments are due to projection effects of the same basic structure. Two considerations seem to indicate that the general ISM magnetic field is a significant factor in molding the shape of the nebula: first, the corrugations acr oss the face of the nebula are fairly parallel; second, the planetary nebula ram pressure matches the expected ambient ISM magnetic pressure . We discuss the possibilities for using ancient planetary nebulae as probes of the ISM.