1 Introduction

 

Planetary nebulae (PN) and their central stars (CSPN) are ideal tools to check against evolutionary theory: After the ejection of the PN still on the Asymptotic Giant Branch (AGB), the effective temperature ($T_\mathrm{eff}$) of the exciting star increases at almost constant luminosity. The stars can reach very high $T_\mathrm{eff}$ ($\gt 100\,\mathrm{kK}$, e.g. Rauch & Werner 1998). At their hottest stage of evolution, these post-AGB stars are found at the beginning of the white dwarf (WD) cooling sequence where the nuclear burning is close to its end and gravitation is beginning to dominate the further evolution.

Modern spectral analysis of CSPN (e.g. Rauch et al. 1998) provides photospheric parameters which are stringent constraints for evolutionary theory.

The development of both, observational as well as numerical methods, provide now high-resolution images of PN (e.g. Balick et al. 1998) and deprojection techniques (Bremer 1995) which improve the interpretation of the nebula morphology. This will enable us to construct consistent models of PN and central star: Rauch et al. (1994, 1996) have successfully used synthetic ionizing spectra instead of the commonly used blackbodies in order to improve the photoionization model of the nebula. The derived spectroscopic distances allow to calculate the linear dimensions of the PN. Recent improvements of the interacting stellar wind model which take into account time-dependent winds (Dwarkadas & Balick 1998) have shown for the first time the possibility to model small-scale structures which can be observed in high-resolution images. As a by-product, the expansion times of the PN can be determined more reliably and give a much better estimate of the time since the central star has departed from the AGB. Moreover, additional information becomes available because the long recombination time ($\approx 300\,000\,{\rm a} / n_{\rm e}$)in these low density PN ($n_{\rm e} < 100\,\mathrm{cm}^{-3}$) allows a view onto the emission phase of the nebula, e.g. the photospheric abundances at that time providing a second check for theoretical predictions of stellar evolution.

Since the PNe contribute to the metal enrichment of the interstellar medium (ISM), they play also an important role in the galactic chemical evolution. Studies of the interaction of aging PN with the ISM by hydrodynamical simulations (e.g. Soker & Dgani 1997) which aim to describe the complex interaction zone need, as a prerequisite, reliable information about nebula properties and its spatial distribution of matter.

During recent observing campaigns, we took narrow-band images of several PN. In the following sections we present eight of them which exhibit interesting details of nebula morphology and allow to judge the degree of interaction with the ISM.

In the same region of the g - $T_\mathrm{eff}$ plane occupied by the CSPN there are many stars for which no associated PN is known (e.g. Werner et al. 1997). This is still an unsolved problem (Rauch et al. 1998) and an outstanding challenge for evolutionary theory. However, one possible explanation for the "missing'' PN might be that they have already dispersed below the detection limit of the former observations. A search for PN around hot post-AGB stars is described in the following.