Among the Central Stars of Planetary Nebulae (CSPN) there are several stars which show Wolf-Rayet-type spectra resembling those of Pop. I Wolf-Rayet (WR) stars of the carbon sequence (WC). Due to progress in computer technology and new solution techniques it became possible ten years ago to calculate models which account for the very complex physical conditions in Wolf-Rayet atmospheres (Hillier 1989, Hamann et al. 1992). These models have been successfully applied to the vast majority of Pop. IWR stars in the Galaxy and the Magellanic Clouds (Hamann et al. 1995) and, in the last three years, to an increasing sample of CSPNs of [WC] type (Koesterke & Hamann 1996, Leuenhagen & Hamann 1996, Leuenhagen this meeting). Here we present the analyses of ten CSPNs of early [WC] type, i.e. from [WC 2] to [WC 4].

A flexible and extensible software package for analyzing two-dimensional and multi-object spectra of nebulae has been implemented within Microsoft Excel (version 5). From lists of calibrated fluxes (and errors) for lines in a series of spectra, the program first identifies transitions by referring to wavelengths generated from atomic data. Colour excesses can be computed from a reddening law and ratio of total to selective extinction of the user's choice. Line fluxes can be corrected for stellar absorption and reddening and reported in a format suitable for publication. Temperatures, densities, ionic abundances, ionization correction factors, and total abundances can be computed. (The interface for the temperture/density calculation is shown below.) Emission coefficients for forbidden transitions are calculated using an adaptation of the FIVEL five-level atom routine (DeRobertis, 1987). As well, emission coefficients are supplied for an extensive set of recombination lines. The user has complete control over which spectral lines are used in an analysis – there are no pre-ordained methods. Abundance analyses can be performed using the physical conditions either measured or specified for each spectrum. Uncertainties are propagated throughout. The atomic data used in all of the calculations are easily updated or augmented by the user. Since the software is embedded in Microsoft Excel, graphical representations of the data are easily created, and extended analyses using the full functionality of the spreadsheet are easily implemented.

The model atmospheres of central stars of planetary nebulae are often calculated under the assumption of the plane-parallel geometry. This assumption seems to be reasonable for stars with relatively thin atmospheres like white dwarfs, where the thickness of the atmosphere is only few kilometers while the corresponding radius is about several thousands of kilometers. Nevertheless, calculations of a grid of pure hydrogen model atmospheres of hot white dwarfs demonstrated that this assumption may fail even for thin atmospheres (Kubát 1995). We found small differences between line profiles (about 1%) calculated under the assumption of plane-parallel and spherically symmetric atmospheres. Such differences are detectable by contemporary observational technique, and, consequently, may not be neglected. In addition, sphericity effects are important also in stars of other types (Kubát 1996). We decided to test the assumption of the plane parallel geometry also for sample model atmospheres of central stars of planetary nebulae. We assumed static atmospheres in radiative, hydrostatic, and statistical equilibrium (NLTE) consisting of hydrogen and helium. We calculated spherically symmetric and plane parallel model atmospheres of two central stars, namely LoTr4 and K1-27. Basic atmospheric parameters of these stars were adopted from Rauch et al. (1996, 1994), respectively. The atmospheric parameters for LoTr4(T eff = 120000K, log g = 5.5, M = 0.65M⊙, n(H)/n(He) = 0.5 yield a model with an extension (r(τR = 10–5)/r(τR = 2/3)) of 1.028. For K1-27, the atmospheric parameters (T eff = 100000K, log g = 6.5, M = 0.52M⊙, n(H)/n(He) = 0.2 produce only a small extension of 1.0035. The differences in temperature structure of our model atmospheres are more pronounced for a star with lower gravity and, consequently, larger extension. Continuum flux is lower for spherical atmospheres. This difference is larger for LoTr4, i.e. a star with more extended atmosphere, and almost negligible for K1-27. Paschen and Pickering He ii lines also show differences. These differences are quite large (several per cent) for α lines, and they decrease towards higher series members. Using plane-parallel model atmospheres instead of spherically symmetric ones introduces a systematic error into results. This error is present also in highly sophisticated NLTE line blanketed models. Due to the above mentioned differences one should avoid using lower series members (e.g. He ii 4686 å) for determination of atmospheric parameters (T eff, log g, abundances,…) from plane-parallel atmospheres. Details of our calculations will be presented in Astronomy and Astrophysics.

In this work recent results of spectral analyses of five late–type [WC] central stars are presented. We use spherically expanding model atmospheres to simulate non–LTE radiation transfer in mass–loosing envelopes. A detailed description of the model calculations and the stellar parameters of further eight [WCL]–CSPN can be found in Leuenhagen & Hamann 1994, A&A 283, 567, and Leuenhagen et al. 1996, A&A 312, 167.

The optical spectra of [WC] central stars of the latest subtypes ([WC11], [WC12]) exhibit not only stellar emission lines of hydrogen (in some cases), helium, carbon and oxygen, but also features of nitrogen (N ii, N iii), neon (Ne i), magnesium (Mg ii), aluminium (Al iii), silicon (Si iii, Si iv) and iron (Fe iii). Some of these features are also visible in the peculiar [WC9]–type object SwSt 1 which has a lower final wind velocity and a smaller wind density than normal [WC9] stars.

We are engaged in using the HIRES echelle spectrograph (Vogt et al. 1994) on the 10 m Keck I Telescope to significantly increase the number of central stars of planetary nebulae (CSPN) studied spectroscopically at high resolution and signal-to-noise ratio. With Keck we are able to extend our previous work (Méndez et al. 1988, 1992; McCarthy 1988) to much fainter magnitudes. In short, comparisons of the observed HI Balmer, HeI, and He II line profiles to the Munich grid of plane-parallel non-LTE model atmosphere line profiles provide distance- and nebula-independent determinations of CSPN effective temperature, surface gravity, and helium abundance. For CSPN showing wind emission, the comparisons are made to new “unified” models (reviewed by Kudritzki et al., this meeting) which include radiation-driven winds. The first results of this on-going program are shown below.

Küstner (1921) catalogued K648 in his photographic survey of M15, but it was not recognized as a PN central star until Pease (1928) discovered the nebula, denoted Ps1. As one of very few PN known in globular clusters — it was the only known until Gillet et al. (1989) reported the discovery of a second in M22 and Jacoby et al. (this meeting) announced two new but very faint objects — K648 offers one of the better opportunities to study the post-AGB evolution of extreme Pop. II stars. Previous investigations of the nebula and star (Adams et al. 1984; Peña, Torres-Peimbert, & Ruiz 1992; Heber, Dreizler, & Werner 1993) all concluded that the stellar temperature is slightly less than 40000 K. Heber et al. also concluded the photospheric He and C abundances were 3 × and 5 × higher than solar.

SAO 244567 (= Hen 1357) is a very young planetary nebula. It was discovered by Parthasarathy et al. (1993, A and A 267, L19). Based on the spectrum obtained around 1950 Henize (1976, ApJ Suppl 30, 491) classified it as a B or A type H-alpha emission line star. The optical spectrum of SAO 244567 obtained in 1971 shows that it was a post-AGB B1 supergiant at that time. It has turned into a planetary nebula within the last 20 years (Parthasarathy et al. 1995, A and A 300, L25).

The variability of P Cygni profiles is important because of its connection with the mechanism of wind production and with the behaviour of the associated mass loss rates.

N66 (WS 35, SMP 83) is a Type I (He-N rich) PN in the LMC with a high ionization degree. It shows a bipolar morphology with a filamentary structure (Dopita et al. 1993). Its central star has shown very impressive changes, in short time scale, that have been investigated. Here we describe the history of these changes:

An important aim of nebular spectroscopy is the identification of new elements. Since the pioneering work of Bowen (1928), who identified the “nebulium” lines with forbidden lines of abundant elements, most elements in nebulae have been discovered thanks to forbidden lines. The number of elements known in planetary nebulae and H II regions was 2, 5, 15, 17, and 20 in 1896, 1927, 1938, 1960, and 1975 respectively.

Broad emission lines λλ 682.5, 708.2 nm, observed for years in the spectrum of symbiotic stars, have been identified by Schmid (1989) as due to Raman scattering of O VI 103.2, 103.6 nm by H0. In one symbiotic star, van Groningen (1993) detected Raman scattered He II.

Recently we have started a systematic reevaluation of the existing observational methods of analysis hitherto applied to PCB-s. Here we report the first results of our investigations. The improved Napier's algorithm (Napier, 1968) to model the light curves of PCB (with the aid of the set of our computer programs in Turbo Pascal) is used. The source function is taken either from Sobieski (1965) or Strittmatter (1974). The entire luminosity received from the cool companion is calculated by integration of the emerging radiation over its disk; contributions from the illuminated and unilluminated portions of the disk are accounted for. We assume: a) the validity of the LTE in the photosphere of cool component, b) constancy of the monochromatic to mean absorption coefficient ratio within the photosphere, c) the hot star radiates as the absolutely black body, d) there are no other effects influencing the light curve except for the reflection effect. We have modelled the light curves of EC11575-1845 (Chen et al (1995). The analysis of the temperature distributions in the heated photosphere indicates the occurence of the temperature inversion. To make a rough estimate of physical conditions which can induce generation of evaporative wind we have used two criteria: i) the temperature inversion, ii) relation between radiative pressure and the effective gravitational acceleration. We assumed the density varies with the height in the atmosphere of the illuminated star as ρ ≃ exp(−Φ/R gT), Φ - being the Roche potential. The integral equation following from the definition of the mean optical depth was solved numerically to establish the relation between the mean optical depth and the distance in the atmosphere. We find the characteristic height scale for X-ray and EUV radiation is ∼ 106 − 107 cm (for concentration of particles ∼ 1013 cm −3). To check the validity of the ii) criterium we used a simplified model of radially expanding evaporative stellar wind and mass flux J conservation condition along the stream tube of the form J ∼ ρ s v s r s 2 exp −(Φ/R g T) (Pustylnik (1995)) and found M ∼ 10−12–10−11M/y. Although mass loss of such a rate cannot compete with the effects of the angular momentum loss which is responsible for a secular orbital shrinkage, the evaporative wind should significantly alter the structure of the the cool irradiated components.

Very hot central stars (CSPN) of highly excited planetary nebulae (PN) display directly the formation of white dwarfs. Only a few of these CSPN have been analyzed so far due to their low brightness and thus, the interpretation of their evolutionary status is hampered by statistical incompleteness. In the last decade many spectral analyses of very hot post-AGB stars by means of state-of-the-art NLTE model atmospheres have been performed (e.g. Rauch et al. 1996; Werner & Rauch 1994; Rauch & Werner 1995) and our picture of post-AGB evolution has been improved.

The knowledge of the planetary nebula central star - white dwarf transition region has dramatically increased during the most recent years and dozens of stars now populate this former gap in the HRD. Each addition to the limited sample of central stars that has been studied spectroscopically in detail is, however, of value.

Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 μm to 45 μm 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 in Beintema et al. (1996).

Emergent fluxes from a non-LTE model atmosphere code designed to calculate realistic O star atmospheres are applied to nebular calculations. The stellar models include wind and blocking effects due to several million doppler-shifted lines in the radiation driven atmosphere in a full NLTE treatment. These lines, which arise from heavy elements, reduce the stellar flux drastically between 228 and 912 å (Pauldrach et al. 1994) — the most important frequency regime for ionization in gaseous nebulae. We have three models with effective temperatures of 50000, 40000 and 35 000 K and log g values of 3.9, 3.8 and 3.5 respectively. They have been used to model Hii regions and their use has led to significant differences in the ionization structure of the nebular models.

During the ORFEUS-SPAS (Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer on the Shuttle Pallet Satellite) mission STS-51, flown in September 1993, we observed the central star of the planetary nebula NGC 6543 in the far ultraviolet (90 nm to 115 nm) wavelength region using the University of California, Berkeley spectrometer with a spectral resolution of 0.03 nm.

We present results obtained on well-known AGB and post-AGB sources with the Long Wavelength Spectrometer (LWS) and ISOCAM on board the Infrared Space Observatory (ISO). The main findings of these studies are briefly summarized.

First results of the ISO1 Short-Wavelength Spectrometer on three bright planetary nebulae NGC 6543, NGC 7027 and NGC 6302 provide a wealth of information on solid-state features and spectral lines. Spectra from 2.4 to 45 μm are presented, at a reduced resolution of about 600.