Some correlations found between different physical characteristics of symbiotic stars and planetary nebulae are discussed from the evolutionary point of view.

Symbiotic stars have sometimes been misidentified as planetary nebulae, because their line spectra are similar to those of planetary nebulae particularly in the ultraviolet L.(Houziaux 1982 in “The Nature of Symbiotic Stars”, M. Friedjung & R. Viotti eds., Reidel. p 229), while the infrared shows the presence of the spectrum of a cool star. The similarity is not complete; when H. E. Schwarz (1988 in “The Symbiotic Phenomenon”, J. Mikolajewska et al eds., Kluwer, p 123) plotted a graph of two ratios derived from emission line fluxes, symbiotic stars did not occupy the regions of the diagram occupied by planetary nebulae and H II regions.

Symbiotic objects are related to planetary nebulae in that they represent late stages of stellar evolution. They are interacting binary systems where a hot companion star ionizes the stellar wind of a red giant. This configuration offers the unique possibility for deriving elemental abundances for cool giants from a nebular spectrum with the diagnostic tools employed for HII regions. The analysis can be applied to different types of symbiotic systems having a G, K or M giant, a Mira variable or a carbon star as cool component. The great advantage of this technique is, that it does not depend on stellar parameters or molecular data, and that it can therefore be used as a test or an alternative for the traditional photospheric abundance determinations.

One of us (Gutiérrez-Moreno 1988) presented some time ago a preliminary version of two diagrams for separating planetary nebulae (PN) from symbiotic stars (SS); tnese diagrams are based on the strength of [O III] lines λλ4363, 5007. Now a new version is presented, which includes more objects and uses measured total intensities corrected by reddening.

In studying low dispersion spectra of symbiotic stars and following the usual dereddening procedure we obtain from different pairs of the Balmer lines Hα, Hβ and Hγ considerably different values for the interstellar absorption C at Hβ. There is a tendency the values to be C αβ > C αγ > C βγ. For one of the objects we obtained C αβ = 2.03, C αγ = 1.51, C βγ = 0. The mean values for a sample of 13 objects with measured H γ are C αβ = 1.82, C αγ = 1.63 and C βγ = 1.13. That could be caused by processes taking place in the emitting gas and neglected in the theory as well as by nonlinearity of the detector system. Selfabsorption in the Balmer lines and collisional excitation could affect in different ways the Balmer decrement.

The German X-ray astronomy satellite ROSAT (launched 1990 June 1) performed an all-sky survey (from 1990 Jul 30 till 1991 Jan 26) with the Position Sensitive Proportional Counter (2 deg FoV, .1–2.4 keV) covering 96% of the sky. Analysis of bright X-ray sources suggests PSPC positions can be accurately determined to within 30 arcsec, and 1‥2 arcmin for soft weak ones. We have examined the survey data for detections from among a list of 189 symbiotic and symbiotic-like objects compiled by Vaidis (1988) with own extensions, chosen for completeness. For 178 objects, all-sky data was available. Standard Analysis Software System (SASS) and EXtended Standard Analysis Software packages (EXSAS) both use a three-step approach to detect sources. After quality screening, a sliding-window algorithm (LD) locates bright sources. These are excluded in the 2D-spline interpolated smoothed background map. A second sliding window MD runs on the background subtracted event files. All possible sources were then tested by maximum likelihood in the background-subtracted, exposure-corrected image. If LD, MD, or ML did not detected a source at the optical position, an upper limit ML program tried to find enhanced emission. For EXSAS, photon event files contain all photons within 20 arcmin radius around a source. Cut radii, background determination, and other parameters can be iteratively improved.

Z Andromeda is considered as the prototype of the symbiotic class. This system behaves essentially as the nearly uncontamined nucleus of a young planetary nebula. The parameters of its hot component (R=0,07 R 0, T > 105 K) show that it lies close to the CSPN in the HR diagram (see Fernández–Castro et al. 1988). During the IUE lifetime (1978 onward), Z And experienced a phase of quiescence followed by an active phase which, starting in April 1984, lasted until about July 1986. Since then the star is recovering its quiescent appearance.

The object BZ Cam = 0623+71 (Krautter et al., 1987) was observed at the telescope AZT-11 of the Crimean Astrophysical Observatory by using the UBVRI photometer-polarimeter of the Helsinki University (Korhonen et al., 1984). No significant linear polarization was found, the mean values have the upper limit of 0.2 per cent.

Theoretical light curves for the decay phase of recurrent novae are obtained using optically thick wind theory. With the OPAL opacity tables the decay time scale is drastically shortened.

A high activity of a precursor during the last years of its existence can be connected with a close binary system orbit changing.

The nebulosities surrounding SN 1987A have a morphological structure strongly resembling some planetary nebulae (e.g. NGC 2392, NGC 3242).

Modelling of PNs from blurred images, as from the Hubble Space Telescope, needs good deconvolution, and the better that is, the more reliable are the results. As an example, we have MIM-deconvolved the HST image of SN 1987A taken in August 1990 by the FOC in [O III] λ5007 Å (F501N). MIM (minimum information method) is a linear deconvolution method with a local smoothness constraint (Pfleiderer 1991). Our results are similar to those already published (Jakobson et al. 1991, Panagia et al. 1991) but we were able to decrease some of the uncertainties. The ring, centered on the SN, has diameter 1680 ± 20 mas (≡ 1.3 ly) and tilt 44.5°±1.5° (Fig 1.). The lumps deviate from the ellipse by up to ≤ 0.05 ly, the ratio of brightest to faintest knots being ≈ 10 (Fig.3). The SN is resolved (Fig.2), being not a uniformly illuminated disk but brighter in the center. Its FWHM is 100 ± 10 mas, i.e. an average expansion velocity since the explosion of v ≈ 3 500 km s−1.

It is now accepted without any doubts that the central stars of Planetary Nebulae (CPN) are rapidly evolving objects in the transition from the asymptotic giant branch (AGB) to the white-dwarf regime. After the pioneering study of Paczyński (1971) it has been demonstrated by Schönberner (1981) that Paczyński's calculations are too crude for the understanding of post-AGB evolution because the latter depends very sensitively on the detailed internal stellar structure, i.e. on the past AGB evolution. More precisely, the evolution of an AGB remnant is a function of the thermal-pulse cycle phase ϕ during which this remnant has been created by the planetary-nebula (PN) formation process. This has been shown by Schönberner (1979, 1983) and later fully been explored by Iben (1984).

Confrontation of theoretical models with observations is the main way of progress of our understanding of the nature of astrophysical objects. This paper discusses different diagrams used for observational testing of theoretical evolutionary models of planetary nebula nuclei (PNNi). Particular attention is paid in indicating and discussing sources of errors and uncertainties which may result in wrong conclusions drawn from the diagrams. Finally, results of recent studies comparing the theory and observations for the Galactic PNNi are critically reviewed.

From self-consistent photoionisation modelling of 147 Magellanic Cloud PN, we have constructed the H-R Diagram for the central stars, and have derived both the chemical abundances and the nebular parameters. We find that the central stars have core masses generally between 0.55 and 0.7 M⊙, and find strong evidence to support the model that they leave the AGB as helium-burning stars following the final shell flash. Type I PN have more massive cores, up to near the Schwartzschild limit, and show clear evidence for the Third dredge-up episode. From HST images, younger PN are very compact. The expansion velocity of the nebula is closely correlated with the position of the central star on the H-R Diagram.

Planetary nebula nuclei on lower luminosity, diagonal tracks in the HR Diagrams are degenerate dwarf stars close to their final radii. For several reasons, these stars have until recently been difficult to identify and study. With the advent of new techniques and technologies, both hydrogen–rich and hydrogen–poor atmospheric sequences have been found.

The attempts in the literature to find a relation between planetary nebula core mass and the nebular abundance ratios N/O and He/H are described. A critical discussion concludes that the evidence for such a relation is weak.

We present a simulation of the population of Galactic bulge planetary nebulae (GBPN), which matches the diagrams obtained from VLA radio observations. This simulation may not be the only one fitting the observed data, but it helps understanding the role of observational uncertainties and selection effects in the interpretation of observational diagrams.

We discuss the effect of metallicity on the luminosities of planetary nebula nuclei (PNNi), and we use theoretical stellar evolution calculations to predict the fraction of PNNi that enter the planetary nebula domain burning helium rather than hydrogen. Both these factors will clearly influence the planetary nebula luminosity function.

Studies of the total spectra of planetary nebulae (PN) show that they usually emit significant fractions of their total energy at infrared wavelengths. Many PN have IRAS observations but we do not understand their broad-band colours. To model these observations requires a combination of a photoionization treatment of the ionized region and a radiative transfer treatment of any neutral dust shell. Including the effects of the central star evolution and an assumed galactic distribution allows many observables to be simulated. I find that the Schönberner 0.64 M⊙ evolutionary track is too slow in the transition to high temperature to match the IRAS colours of PN if a typical AGB wind expansion speed is 10 km s−1. Accelerating the evolution track by a factor of 2 produces reasonable agreement. An equal mixture of carbon-rich and oxygen-rich sources with mass loss rates of 2 × 10−5 M⊙ yr−1 seems to reasonably match the general properties of the PN data from IRAS although the details are still unclear. These models strongly indicate that the post-AGB luminosity decline deduced by Knapp (1986) from radio CO line observations is not correct.