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
Abstract:
Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45 
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 m and
[NeVI] 7.65 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 m line and the
first detection of the [NaIII] 7.32 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 
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.
Keywords:
Stars: evolution - Stars: atmospheres -
planetary nebulae: individual - Infrared: ISM: lines and bands