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Astron. Astrophys. 315, L253-L256 (1996)

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

D.A. Beintema1, P.A.M. van Hoof2, F. Lahuis3, S.R. Pottasch2, L.B.F.M. Waters4,1, Th. de Graauw1, D.R. Boxhoorn1, H. Feuchtgruber5,3, and P.W. Morris3,6

1 SRON Laboratory for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands
2 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
3 Agencia Espacial Europea, Apartado de Correos 50727, Villafranca del Castillo, E-28080 Madrid, Spain
4 Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
5 Max-Planck-Institut für extraterrestische Physik, Postfach 1603, D-85740 Garching, Germany
6 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received 4 July 1996 / Accepted 5 September 1996

Abstract. Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45  img1.gifm 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  img1.gifm and [Ne VI] 7.65  img1.gifm 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  img1.gifm line and the first detection of the [Na III] 7.32  img1.gifm 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 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.

Key words: stars: evolution - stars: atmospheres - planetary nebulae: individual - infrared: ISM: lines and bands

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

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Last change: December 16, 1996
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