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[110.01] ISO Far-Infrared Spectroscopy of Several Planetary Nebulae

R.H. Rubin, S.W.J. Colgan, A. Liao (NASA/ARC), R.J. Dufour (Rice University), J.P. Harrington (U. Maryland), D.A. Levine, S.D. Lord (IPAC/Caltech)

We report observations with the Infrared Space Observatory's Short Wavelength Spectrometer (SWS) of the planetary nebulae (PNs) NGC~2022, NGC~6210, NGC~6818, NGC~7009, and IC~2165. We have measured line fluxes and upper limits for many ionic fine-structure transitions as well as hydrogen and helium emission lines. One of our goals is to derive electron density (N_e) from several diagnostic line pairs in order to address density variations within these objects. The N_e values are obtained from the flux ratios of the following: [Ne~III] (15.6/36.0\mum), [Ne~V] (14.3/24.3), [Mg~V] (5.61/13.5), [S~III] (18.7/33.5), [Ar~III] (8.99/21.8), and [Ar~V] (7.90/13.1). Because the ISO/SWS aperture size (14''~\times~20'' to 20'' \times 33'') depends on wavelength, the above line fluxes are not always directly comparable. The best object for our analysis in this sense is IC~2165, which is smaller than the smallest aperture. For the other PNs, the use of line flux ratios may still be valid as long as the specific ionic emitting zone is circumscribed by the smaller of the two apertures. The safest line pair is [Ne~V] (14.3/24.3) because both of these lines are observed in the same aperture. We will present the results of our analysis. One preliminary conclusion is that there are some instances of the observed line flux ratio being clearly out of range of the theoretical predictions using current atomic data. For instance, the observed [Ne~V] (14.3/24.3) ratio for NGC~6818 is less than the predicted ratio in the low N_e limit, which may point toward a need to reexamine the collision strengths.

This research was supported by NASA through data analysis grants to the ISO General Observer program.