AN ATLAS OF ANCIENT PLANETARY-NEBULAE AND THEIR INTERACTION WITH THE INTERSTELLAR-MEDIUM

We present an imaging atlas of the largest planetary nebulae (>8'), ta ken at H alpha, [N II], and [O III]. Using this data, we have develope d a set of simple criteria for determining whether a planetary nebula (PN) is indeed interacting with the interstellar medium (ISM). On this basis, we conclude that most in our sample reveal significant interac tions with the ISM. We discuss also how a large sample of ancient plan etary nebulae can be used to derive a filling factor for coronal gas i n the ISM.