The distances, central star parameters Tef, log g, L∗, M∗, and optical thicknesses of the envelopes in the Lyman limits of hydrogen and helium are determined in a self-consistent way for 156 planetary nebulae (PNe) in our Galaxy (129, 22, and 5 PNe in the galactic disk, bulge, and halo respectively). For each nebula, the distance is adjusted in such a way that the theoretical evolutionary age of the central star (CS) should be equal to the dynamical age of the expanding nebular envelope (Malkov, 1994). This method combines the individual approach to each nebula with the possibility of application to a large sample of PNe. Some improvements have been introduced in the method as compared with its original version. The evolutionary tracks and time scales for CS with masses of 0.605, 0.696, 0.836, and 0.940 M⊙ were taken from Blöcker (1995). The change of the nebular expansion velocity during the evolution of a PN was roughly taken into account. The effective temperature of the CS and related parameters were found by the generalized energy balance method (Malkov et al., 1995) or, in appropriate cases, by Zanstra methods. Observational data have been collected from 93 papers published in 1975–1995.

The distance to Planetary Nebulae (PNe) is a most important parameter to study their evolution and the distribution among our Galaxy, but it has been very difficult to determine it precisely. On the other hand, it is well-known that the IRAS colors of PNe are very similar to the blackbody colors. Taking into account of this characteristic, we define a new method to get some information about the distance to PNe by the blackbody fitting of IRAS 4-band fluxes, assuming these fluxes are due to the thermal emission from the nebular dust envelope.

It's a good thing that we're starting to meet every four years rather than every five. There has been tremendous progress in understanding the properties and evolution of central stars of planetary nebulae (CSPN) since the last meeting in Innsbruck in 1992. Much of the credit is due to new spectra and images obtained at observatories that weren't even in existence four years ago. Table 1 lists most of the space observatories used by astronomers reporting at this symposium. The ones marked with the asterisk are new to this conference. The double asterisks for the Hubble Space Telescope instruments remind us that the telescope was repaired in December 1993. The reference gives the identification of the poster paper given at this meeting. We can look forward to similar improvements in ground-based observatories to be reported at the next meeting.

For this review we have chosen to concentrate on recent progress concerning the model atmosphere analysis of stellar winds in central stars of planetary nebulae (CSPNs). Since there is another review in these proceedings (by W.R. Hamann) devoted specifically to Wolf-Rayet type CSPNs, we will not consider them here. The reader interested in recent work on windless model atmospheres applied to CSPNs is referred to reviews by Napiwotzki (1995) and Werner et al. (1996). There is also an interesting paper by Werner (1996) on the Balmer line problem.

High resolution images of planetary nebulae have revealed a variety of non-spherical morphologies. In addition, some planetary nebulae were found to produce highly collimated jets. It is argued that binary central stars may play a crucial role in the production of all of these morphologies. In particular, a specific mechanism is identified for the generation of “point-symmetric” nebulae. It is shown that the study of binary nuclei of planetary nebulae can provide essential information for the understanding of the common envelope phase in the evolution of binary systems.

We analyse the population of PNe and links between binary PNNi and stars which are in the later evolutionary stages. In a model which assumes that all stars are born in binaries, about 16% of PN result from ejection of common envelopes in close binaries, 85% of single PNNi are formed by merger of components of binaries. In the model, 5% of PNNi may be precataclysmic binaries, 5% may be precursors of symbiotic stars, 0.4% may be pre-SN Ia, 0.1% - precursors of hydrogen-deficient giants. About 0.1% of all PNe may be hydrogen-deficient.

Wolf-Rayet (short: WR) stars are characterized by the bright and broad emission lines which dominate their spectra. This class was originally established for Pop. I stars, distinguishing a nitrogen (WN) and carbon (WC) sequence according to the dominating lines. Wolf-Rayet (specifically, WC) type spectra are also shown by a considerable fraction of central stars of planetary nebulae.

The first two pulsating planetary-nebula nuclei (PNNs), those of K 1-16 and Lo 4, were discovered by Grauer & Bond (1984) and Bond & Meakes (1990). They are nonradial multiperiodic g-mode pulsators, with typical periods near 25–31 min and low amplitudes (up to ∼ 0.05–0.1 mag). These PNNs have extremely high temperatures (Teff > 100,000 K), and are hydrogen-deficient with high abundances of C and O. Their spectra and pulsational properties are very similar to those of the pulsating GW Vir (PG 1159–035) white dwarfs.

Sub-arcsecond resolution radio and optical images of the young planetary nebula BD +30 3639 look superficially very similar, showing a pronounced box-shaped bright nebular shell. However, a detailed comparison at high spatial resolution reveals several regions where the optical emission is obscured by localised dusty regions. The highest resolution radio image to date of this PN, a 6 cm map obtained by combining MERLIN and VLA observations (Bryce et al, 1996) has been compared to the highest resolution optical Hβ image from the Hubble Space Telescope (obtained by J.P. Harrington and collaborators) to reveal the differences in emission which are probably due in the main to dust obscuration at optical wavelengths (Bryce et al, 1996 & Arnaud, Borkowski & Harrington, 1996). New 6 km s–1 resolution, spatially resolved spectra of optical emission lines, obtained using the Utrecht echelle spectrometer on the 4.2 m William Herschell Telescope, show that this nebula is bright in the low ionisation emission lines of [N II] 6548 + 6584 å and also appears to be more spatially extended than in the fainter, high ionisation [O III] 5007 å emission. The velocity ellipse observed from a long slit oriented north-south was found to be tilted, indicating an inclined, elliptical morphology rather than a simple, radially expanding spherical shell and the gaps in the velocity ellipse observed with an east-west slit suggest that the true shape is probably an open ring-like structure rather than a closed shell. A deep exposure of the [N II] 6584 å emission line appears to show a collimated, accelerating flow in an eastward direction away from the central star and extending well beyond the main bright nebular ring. It was also observed in the Hydrogen and [Nii] 6548 å emission lines. The H2 1–0 S(1) image of BD +30 3639 by Graham et al (1993) also shows a feature to the east of the main nebular ring. This may well be related to faint halo structures detected by Harrington and collaborators from their HST images (private communication).

[WC] stars are H–deficient central stars of PN which have developed a dense, fast stellar wind. Their spectra can mimic the spectra of massive (Min∼5OM⊙) Wolf-Rayet stars of the carbon sequence. Deriving their parameters is of importance both in understanding the PN and the Wolf-Rayet phenomena. Spectra of both objects were obtained at the AAT in May 1993 with the UCL Echelle Spectrograph (3500–9200 å, R=50,0000). The reddening, determined from comparing the Hβ nebular line fluxes with radio fluxes from Purton et al. (MNRAS 128, 321, 1982), yielded E(B-V)=0.68 (CPD–56° 8032) and 1.00 (He 2–113). Distances are derived using two different methods and they agree within the relative uncertainties. They are 1.35 and 1.50 kpc for CPD–56° 8032 and He 2–113, respectively. The electron temperatures of the C ii line formation region in the wind is derived from a recombination line analysis (20,000 and 17,000 K for CPD–56° 8032 and He 2–113, respectively). From the same stellar wind recombination line analysis, we find C/He=0.12 and 0.16, together with O/He=0.19 and 0.25, for CPD–56° 8032 and He 2–113 respectively.

[WC] stars are H–deficient central stars of PN which can mimic the spectra of massive (Min∼50M⊙) Wolf-Rayet stars of the carbon sequence and can be modelled using the same techniques. Our model calculations for the [WC10] CPD–56° 8032 and He 2–113 are based on the iterative technique of Hillier (A&A 231 111 1990) which solves the transfer equation in the co-moving frame subject to statistical and radiative equilibrium, assuming an expanding, spherically-symmetric, homogeneous and time-independent atmosphere. In extended atmospheres the stellar radius (R∗) is defined as the inner boundary of the model atmosphere at τRoss=20, with the stellar temperature (T∗) defined by the usual Stefan-Boltzmann (T∗ = (L∗/4 π σ R2∗)1/4) relation. For a given mass loss rate (M), the density and velocity field (v(r) = V∞ (1 - R∗/r)β for the supersonic part) are related via the equation of continuity M = 4 π r2 ρ(r) v(r).

The PG 1159 stars represent the hottest stage of post-AGB evolution. Quantitative spectral analyses of most known PG 1159 stars have been carried out by us from optical, UV and EUV observations (see Dreizler et al 1995 for a review). It has been shown that these stars have atmospheres dominated by C and He with a significant admixture of O. These abundances reveal the inter–shell matter of a former AGB star. The four stars, HS 2324+3944, NGC 7094, Abell 43, and Sh 2-68, define a small group of peculiar PG 1159 stars (termed hybrid PG 1159). Unlike all other PG 1159 stars hydrogen is detected in their spectra. Three of them are CSPNe. Our Non-LTE analyses (Dreizler et al 1995; Dreizler et al 1996) show that these stars have typical PG 1159 Teff, log g as well as C and He abundances (Table 1). In contrast, the O abundance is lower than in PG 1159 stars. N is probably present but near the detection limit of the currently available spectra. Hybrid PG 1159 stars tend to have lower masses/luminosities than ordinary PG 1159 stars. A reduced mass-loss in their post-AGB evolution might be responsible for the incomplete removal of the H rich envelope. However, peeling of a post-AGB star alone can not produce the observed abundance pattern. In addition, mixing is required. A first evolution calculation with time dependent mixing of Iben & MacDonald (1995) shows some qualitative agreement in the abundance pattern like the C/He ratio it is, however, not able to explain the overall abundances.

Two multisite campaigns devoted to HD 35914, the variable central star of IC 418, have been performed (Handler et al., 1996). In the second one, many spectra of the star were obtained during five nights on November 1994. The Echelle REOSC spectrograph and a 1024 × 1024 Tektronix CCD detector, attached to the 2.1m telescope at the Observatorio Astronómico Nacional in San Pedro Mártir, México, was used then. The north-south diameter of the nebula was included in the 20 spectra taken the night of November 18 (184–minute total exposure). The spectral coverage and reciprocal resolution was 4300 å to 7350 å and about 0.16å/pix, respectively. The projected slit size was 1″ × 26″, approximately.

Close binary systems may undergo the “Common Envelope” (CE) phase when the primary star expands on the red giant branch or the asymptotic giant branch. Filling its Roche Lobe, the primary transfers mass to the companion driving it out of thermal equilibrium and causing it to expand as well. The giant core and the companion star become surrounded by a CE. When sufficient energy is deposited in the circumstellar material this will be ejected and the binary orbit will shrink further (see review by Iben 1995). Planetary nebulae (PNe) with short-period binary nuclei are considered the most probable post-CE candidates. Abell 35, Lotr 1 and Lotr 5 (the Abell 35-like objects) are the only three PNe with binary nuclei known to contain a very hot UV-bright primary and a chromospherically active, rapidly rotating, G-K companion that dominates the optical spectrum. The origin of these unusual systems is unclear and hence presents a challenge to theories of binary star evolution. Identified in 1966 by Abell, Abell 35 is possibly the largest PN known (D=1.6 pc at a distance of 360 pc, Jacoby 1981) and also the oldest (the kinematical age is 185.000 years from the small expansion velocity of 4.2 km/s, Bohuski 1974). The bright giant star BD −22° 3467 (mv = 9.6mag) lies off-center within the nebula. A white dwarf was detected at the same location in 1988 in IUE spectra obtained by Grewing and Bianchi. BD − 22° 3467 has a vsin i of 90 km/s (Vilhu et al. 1991), variable Hα and Ca II emission lines associated with chromospheric activity, and a variable light curve (P=0.76 days, Jasniewicz and Acker 1988) probably produced by the rotation of the giant star. All attempts to determine the orbital period have failed, raising doubts as to whether the nucleus of Abell 35 is a close binary at all. In pursuit of this point, we have started a radial velocity study of the giant companion.

The aim of this study is to derive the masses of the central stars (CSPN) for a large sample of the planetary nebulae (PN). These masses, M∗, are derived from the observed PN positions in three diagnostic diagrams and their comparison with evolutionary tracks of model PN. Two of the diagrams, namely LZan(H) versus TZan(H) and Mv versus Rneb, have already been used in numerous studies. The third one, SHβ versus SV, has recently been introduced by Górny, Stasińska & Tylenda (1996, hereinafter GST96). Here SHβ is the nebular surface brightness in Hβ and Sv is defined as Fv/(πθ2), where Fv is the stellar flux in the V band and θ is the observed nebular angular radius.

In the framework of the extensive study of [WC]-type nuclei of planetary nebulae undertaken since 1993 by Acker and collaborators, we report wind fluctuations for the two brightest late-type [WC] stars in the sky, observed at Observatoire de Haute-Provence (OHP, France) and Observatoire du mont Mégantic (OMM, Canada). In particular we confirm Balick et al.'s (1996, AJ, 111, 834) CIIIλ5696 line profile variations for the [WC8] nucleus of NGC 40. Moving features seen on the top of this line are interpreted as outflowing “blobs” which are accelerated in the WR wind. Kinematic parameters have been derived and compared to those observed for massive WR stars. This will be discussed in a forthcoming paper (Grosdidier et al., 1996, in preparation). Assuming the average motion of the blobs reflects the global wind expansion and adopting the ad hoc β velocity law, we find that the exponent β must be larger than 3 in order to fit the measured radial accelerations (up to ∼ 0.08 km s–2). Evidence for clumping in the wind of the [WC9] nucleus BD+30°3639 is shown for the first time and presented in Acker et al. (1996, A&ALetters, in press). In contrast to NGC 40, the clumps in this star show very low acceleration.

Three different kinds of variable stars can be found among central stars of Planetary Nebulae. Close binary central stars change their light level because of eclipses and due to the reflection effect; some hot variables have been identified as multiperiodic pulsators (e.g. see Ciardullo & Bond 1996).

HS 2324+3944 is one of only four “hybrid” PG 1159 stars. The latter objects are a subgroup of DO white dwarfs. The spectra of “hybrids” show an He ii/C iv absorption trough similar to the “classical” PG 1159 stars, but also strong Balmer lines (Napiwotzki & Schönberner 1991).

Jasniewicz et al. (1994) have announced and started in 1994 a program of chemical analysis of the G-stars at the centre of Abell 35-type planetary nebulae. First results concerning BD-22° 3467 and HD 112313 located respectively at the centre of Abell 35 and LoTr5 have been published by Jasniewicz et al. (1996) and Thévenin & Jasniewicz (1996).

We present a preliminary abundance analysis, utilising recombination theory, for the [WC] Wolf-Rayet central stars of the planetary nebulae NGC 6751 and NGC 6905. This analysis is based on optical spectrophotometry of a sample of [WC] stars which show a strong O vi 3811, 34 å feature in emission. We have performed a recombination line analysis of stellar wind emission lines which are judged to be optically thin, in order to derive relative C, O and He abundances. We also present a comparison of the derived wind abundances with those of Population I WO stars. For the [WC 4] central star of NGC 6751, we have derived C/He = 0.35 and C/O = 4.2, by number. And for the [WC 3] central star of NGC 6905, we have derived C/He = 0.36 and C/O< 12. These surface abundances are comparable to the abundances that have been derived for early type [WC] stars via more complex NLTE modelling (e.g. Koesterke & Hamann 1996).