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4 MWP1

  MWP1 has been discovered by Motch et al. (1993) around one of the hottest stars ever analyzed by means of NLTE stellar atmospheres: RXJ2117.1+3412, which has been discovered by ROSAT[*] (Motch et al. 1991). RXJ2117.1+3412 is a helium- and carbon-rich PG1159 star with $\mbox{$T_\mathrm{eff}$}\hspace{-0.5mm} =\hspace{-0.5mm} 170~\mathrm{kK}$(Werner et al. 1996). The enormous size of this PN was first noted by Appleton et al. (1993) who found an apparent diameter of 13$\hbox{$^\prime$}$ and discuss its morphology in detail. Narrow-band images are also presented in an atlas of ancient PNe which are interacting with the ISM by Tweedy & Kwitter (1996).

The images presented here are much deeper and have a higher resolution. We show mosaics of four CAFOS images (exposure time 20min each, field size $\approx\!16\hbox{$^\prime$}$). The resulting mosaic field size is $\approx\!28\hbox{$^\prime$}$(Fig.1).

\includegraphics [width=8.8cm]{8208fig1.eps}\end{figure} Figure 1: MWP1. The H$\alpha$ image is shown left, [O III] $\lambda$5007Å right

On our images MWP1 appears to be an elliptical PN with axis lengths of $14\hbox{$^\prime$}45\hbox{$^{\prime\prime}$}$ and $8\hbox{$^\prime$}36\hbox{$^{\prime\prime}$}$ (Fig.1). At a distance of 1.4+0.7-0.5kpc (Motch et al. 1993) its linear diameter is 6+3-2pc.

With its tremendous size, MWP1 is one of the largest known PN: Two other PN, IsWe2 (PNG107.7+07.8, d=6.5pc) and Sh2-176 (PNG120.2-17.6, >7pc), have similar extensions but their diameters have been calculated using the strength of the interstellar NaD lines in order to determine their distance (Napiwotzki & Schönberner 1995) which entails a relatively large error limit.

The large PN MWP1, however, is a challenge for evolutionary theory because its expansion time (about 150000 years if we assume $v_{\rm exp} = 20\,{\rm
km~s^{-1}}$) is at least two orders of magnitude longer than the time since the central star's departure from the AGB (about 1000 years, Blöcker 1995). One possible explanation for this discrepancy may be found in the born-again post-AGB nature (Iben et al. 1983) of the central star: A late He-shell flash at already declining luminosity brings back the star to the AGB and it experiences a second, He-burning, post-AGB phase on a three times longer time scale. However, a "normal'' PN with $M_{\rm PN} = 0.2\,M_\odot$ and an expansion velocity of $v_{\rm exp} = 20\,{\rm
km~s^{-1}}$ will disperse in about 20000 years below the detection limit and hence, MWP1 has to have extraordinary parameters. Nebula spectra with a high S/N ratio as well as measurements of the expansion velocity are required in order to investigate this problem.

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