Astronomy and Astrophysics Letters dedicated to ISO

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

D.A. Beintema - P.A.M. van Hoof - F. Lahuis - S.R. Pottasch - L.B.F.M. Waters - Th. de Graauw - D.R. Boxhoorn - H. Feuchtgruber - P.W. Morris


Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45  tex2html_wrap_inline375 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 [ArVI] 4.53  tex2html_wrap_inline375 m and [NeVI] 7.65  tex2html_wrap_inline375 m 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] 25.9  tex2html_wrap_inline375 m line and the first detection of the [NaIII] 7.32  tex2html_wrap_inline375 m 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 O tex2html_wrap_inline385 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.

Stars: evolution - Stars: atmospheres - planetary nebulae: individual - Infrared: ISM: lines and bands

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