1996A&A...315L.253B

Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45µm were obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. A first analysis of these spectra, with the aid of photo-ionization models, is presented. We report the first detection of the [ArVI] 4.53µm and [NeVI] 7.65µm lines in the spectrum of NGC 7027. When compared with older observations it is clear that the [ArVI] line and possibly also other lines have increased in strength since 1981. We argue that a likely explanation for this variability is a change in the spectral energy distribution of the central star, possibly an increase in effective temperature. However, this result needs to be confirmed by further observations. We also report a non-detection of the [OIV] 25.9µm line and the first detection of the [NaIII] 7.32µm line in the spectrum of NGC 6543. The non-detection is not expected based on a blackbody approximation for the spectrum of the central star. The ionization threshold for O³⁺ is just beyond the HeII limit, and the absence of this line shows that the stellar flux drops at least by a factor 350 at the HeII limit. Modeling the [OIV] line may prove to be a valuable test for atmosphere models.