The central stars of the planetary nebulae NGC 7027 and NGC 6543

Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45 mu m 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 [Ar VI] 4.53 mu m and [Ne VI] 7.65 mu m lines in the spectrum of NGC 7027. When compared with older observations it is clear that the [Ar VI] 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 [O IV] 25.9 mu m line and the first detection of the [Na III] 7.32 mu m line in the spectrum of NGC 6543. The nan-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 He II limit, and the absence of this line shows that the stellar flux drops at least by a factor 350 at the He II limit. Modeling the [O IV] line may prove to be a valuable test for atmosphere models.