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
Received 4 July 1996 / Accepted 5 September 1996
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 [Ar VI] 4.53 m and [Ne VI] 7.65 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 m line and the first detection of the [Na III] 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 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
Last change: December 16, 1996