Interpretation of IUE and other spectra of planetary nebulae
The current status of observations of planetary nebulae and their central stars is presented. Particular attention is given to ultraviolet observations secured with the IUE satellite. The physical processes occurring in these objects are discussed, with emphasis on the atomic data. The planetary nebulae SwSt 1, IC 2501, and NGC 4361 have been observed with the IUE satellite. These observations have been combined with observations made by other workers at optical, infrared, and radio wavelengths in order to determine physical conditions in these objects. The electron density in the compact nebula SwSt 1, determined from the C III λλ1907/1909 line ratio, is high (~10(^5) cm(^-3)), consistent with the high emission measure determined from radio observations. The C/0 abundance ratio in this nebula is found to be < 1 , i.e. the envelope is oxygen rich, as suggested by the identification of the silicate feature in the 8-13 µm infrared spectrum. The ultraviolet spectrum of the central star of SwSt 1 shows a few P Cygni Profiles due to a 2000 km s(^-1) stellar wind. The analysis of observations of IC 2501 suggests that there are two main regions in the nebula: one emitting the [0 III] lines, with T(_e) = 9400 K, the other emitting the [0 II] and [N II] lines, with T(_e) = 9000 K. IC 2501has a marked SiC feature in its 8-13 µm infrared spectrum and has C/0 > 1. The ultraviolet observations are therefore consistent with the interpretation of the infrared observations, namely, that objects with SiC are carbon rich. An ionization model of NGC 4361 is presented which fits the observed ultraviolet and optical line fluxes well. It is found that the inclusion in the model nebula of charge exchange and dielectronic recombination processes greatly improves the computed ionization and thermal structure of the nebula.