FAR-INFRARED SPECTROSCOPY OF PLANETARY-NEBULAE WITH THE KUIPER-AIRBORNE-OBSERVATORY

We present new far-infrared line observations of the planetary nebulae (PNs) NGC 7027, NGC 7009, and NGC 6210 obtained with the Kuiper Airbo rne Observatory (KAO). The bulk of our data are for NGC 7027 and NGC 7 009, including [Ne V] 24 mu m, [O IV] 26 mu m, [O III] (52, 88) mu m, and [N III] 57 mu m. Our data for [O III] (52, 88) and [N III] 57 in N GC 7027 represent the first measurements of these Lines in this source . The large [O III] 52/88 mu m flux ratio implies an electron density (cm(-3)) of log N-e[O III] = 4.19, the largest N-e ever inferred from these lines. We derive N++/O++ = 0.394 +/- 0.062 for NGC 7027 and 0.17 9 +/- 0.043 for NGC 6210. We are able to infer the O+3/O++ ionic ratio from our data. As gauged by this ionic ratio, NGC 7027 is substantial ly higher ionization than is NGC 7009, consistent with our observation that the former produces copious [Ne V] emission while the latter doe s not. These data help characterize the stellar ionizing radiation fie ld. From our [O IV] and [O III] fluxes, we are able to show that O++ i s by far the dominant oxygen ion in NGC 7009. As a result, the O/H abu ndance inferred using these data tends to corroborate the value found from UV/optical, collisionally excited lines. We determined accurate r est wavelengths for the [Ne V] 2s(2)2p(23)P(1)-->2s(2)2p(23)P(0) (lamb da(rest) = 24.316 +/- 0.008 mu m) and [O IV] 2s(2)2p(2)P(3/2)(0)-->2s( 2)2p(2)P(1/2)(0) (lambda(rest) = 25.887 +/- 0.007 mu m) transitions fr om observations of one or both of the bright PNs NGC 7027 and NGC 7009 . Our [O IV] value, to the best of our knowledge, is the most accurate direct determination of this lambda(rest) prior to the Infrared Space Observatory (ISO). These new KAO data will be beneficial for comparis on with the ISO observations of these PNs.