We report on the discovery of a new PG 1159 star in the ROSAT XRT all sky survey and give results of a model atmosphere analysis. The X-ray source RX J2117.1+3412 is relatively faint (0.33 cnts−1) and extremely soft. Ground based optical follow-up spectroscopy (OHP, France) proofs its PG 1159 nature: It belongs to the “low gravity emission” spectral subtype. Optically, it is the second brightest PG1159 star. CCD [O III] imagery reveals that the star is surrounded by an old arc-shaped planetary nebula of faint surface brightness. The spectral analysis of the central star was performed with non-LTE line blanketed model atmospheres (Werner 1992). We find a complete agreement between the atmospheric parameters determined at optical wavelengths and in the ROSAT PSPC energy range.

We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.

We report the results of spectroscopic observations of the ultra-high excitation, helium-rich, pre-white dwarf KPD0005+5106 obtained with the Hubble Space Telescope Faint Object Spectrograph (FOS) in 1991 May and July with the red (FOS/RD) and blue (FOS/BL) Digicon detectors. The data reveal a rich line spectrum both in absorption and in emission with ultra-high excitation species present including O VIII, N V, possibly C V, Fe VI, Fe VII and numerous weaker high n, low l, transitions of C IV and O VI as well as the predominant He II (3 → n) and He II (Balmer α) absorption lines. There is a strong emission complex at 2981A which we identify primarily as three transitions of N V commonly seen in WN Wolf-Rayet spectra. We present evidence that high ionization species in emission (O VIII, N V, C IV, Si IV) and in absorption (He II, Fe VI, Fe VII) are longward-shifted relative to the far UV resonance (circumstellar) absorption lines by 25–50 km/s. Based upon the detected species, line velocities, line widths and emission features, we conclude that (1) KPD0005 is the very likely the evolutionary descendant of a WN-WC subluminous Wolf-Rayet progenitor and (2) has ongoing, possibly episodic, mass outflow.

This paper deals with the central stars of two large, low-surface-brightness planetary nebulae: V605 Aquilae, central star of Abell 58, and 0950+139, central star of EGB 6. Both of these nuclei are associated with compact emission-line nebulosities, which are unresolved from the ground. We obtained images with the Faint Object Camera (FOC) on the Hubble Space Telescope of both objects, in order to determine the nature of the compact nebulae.

We present low dispersion UV and optical spectrophotometry of the central star of the PN ES0166–21. The stellar spectrum, from 1200 to 6600 Å, is a featureless continuum. The energy distribution is consistent with a black body of 120, 000 ± 20, 000K. The observed visual magnitude is 18.1.

Luminosity functions of planetary nebulae contain information about the central star mass distributions, nebular, central star, and progenitor evolution, stellar death rates, and a galaxy's star formation and chemical evolution histories. Appropriate observing strategies can be used in combination with various models to extract some of the parameters of these functions. The principal results from these studies are that the central star mass distribution is narrow (σ ∼ 0.02–0.04M⊙), the number of PN in a galaxy depends on galaxy color, and the number of PN in the Galaxy is ∼ 104.

The most extensive application of luminosity function studies has been exploiting the bright end cutoff as a distance indicator. Distances for 25 galaxies have been measured using the methodology outlined by Jacoby, Ciardullo, and collaborators. The PNLF method compares extremely well with other techniques, and is accurate to ∼ 5%. In fact, there is no evidence for systematic effects of any kind, although a small (5–10%) metallicity correction needs to be applied for metal-poor systems.

It is shown from multiple evidence that the λ 5007 Å luminosity of the brightest shells of planetary nebulae has intrinsic scatter, which is well approximated for the brightest objects by an exponential luminosity function, as proposed by Bottinelli et al. (1991). The resulting open-ended luminosity function weakens the usefulness of planetary nebulae as distance indicators.

A review is presented of PN birth rates. The observational determinations for other galaxies are compared with theoretical predictions based on stellar evolution models. The galactic birth rate is derived for different distance scales and is compared with the predicted one from galactic models of the solar neighborhood and with those determined for other galaxies; to reach agreement between observations and predictions long distance scales to PN have to be adopted. The total number of PN is estimated for the Galaxy and other galaxies.

The study of extragalactic planetary nebulae has made rapid progress in recent years with the help of high quantum efficiency detectors. A brief but distinctive phase in the late stage of stellar evolution, planetary nebulae (PNe) are not only interesting objects in their own right, but also are extremely valuable and unique tools for probing the host galaxies. Recent studies have used planetary nebulae as test particles to investigate the dynamics and mass distributions in the halos of early type galaxies.

The available kinematical data on OH/IR stars and PNs are reviewed. Dynamical models for the OH/IR stars are presented.

Developments in the measurement of abundances in planetary nebulae since IAU Symposium 131 are reviewed. Present uncertainties and outstanding problems in the interpretation of emission-line measurements are discussed. Results for different samples of PN are reviewed, in terms of the galactic site. Results for the Galactic disk, Galactic Bulge, Galactic Halo, LMC & SMC, Fornax Dwarf galaxy and M31 are discussed. Attention is drawn to the importance of Magellanic Cloud samples for testing dredge-up theory, and to the strong variation of PN C/O ratios with the local site's metallicity and age. New results for carbon in Cloud PNs are presented. Important problems in the interpretation of the metallicity of Bulge PNs are highlighted. Attention is drawn to the idea that metal-rich stellar populations do not produce PNs, such stars leaving the lower AGB early.

For a first interpretation of the comparison of observational data, the crude “Simple Model” of chemical evolution is quite useful. Since it has well been described in the literature (e.g. Pagel and Patchett 1975, Tinsley 1980), let us here just review the assumptions and whether they are satisfied:

1. 1. The galaxy is a closed system, with no exchange of matter with its surroundings: For the solar neighbourhood this probably is not true (the infamous Gdwarf-“problem”, Pagel 1989b). For the Magellanic Clouds this is most certainly wrong, because of the presence of the Inter-Cloud Region and the Magellanic Stream, and evidence for interaction with each other and the Galaxy as well (cf. e.g. Westerlund 1990).

2. 2. It initially consists entirely of gas (without loss of generality of primordial composition): This is good approximation also for models with gas infall, as long as the infall occurs with a time scale shorter than the star formation time scale.

3. 3. The metal production of the average stellar generation (the yield y) is constant with time: Initially, it is reasonable to make this assumption. For tables of the oxygen yield see Koppen and Arimoto (1991).

4. 4. The metal rich gas ejected by the stars is completely mixed with the ambient gas. To neglect the finite stellar life times (“instantaneous recycling approximation”) is appropriate for elements synthesized in stars whose life time is much shorter than the star formation time scale, such as oxygen, neon, sulphur, and argon.

5. 5. The gas is well mixed at all times: We don't know. The dispersion of H II region abundances may give an indication. In the Magellanic Clouds Dufour (1984) finds quite a low value (±0.08 dex for oyxgen).

From an analytical solution to the Boltzmann-Poisson equations for a thin, vertically isothermal self-gravitating disk with the assumptions that the mass spectrum of stars is expressible as an inverse power-law and the velocity dispersion perpendicular to the plane follows a law of the form we have obtained a vertical height distribution of planetary nebulae: where being the potential at z. Figure 1 shows a normalised height distribution for various values of V0 where we have assumed a Salpeter slope, a ρ(o) = 0.10M⊙pc−3 and set m∗ = ml = 1.0m⊙. Figure 2 shows filled circles obtained from the observational sample of Daub (1982, ApJ 260, 612) superposed on the theoretical distributions. Although there is agreement for large values of z, closer to the plane the observational sample falls below the theoretical curves. Since the observational sample is size-limited, we may be missing a larger fraction of small nebulae closer to the plane. It is also possible a single value of Vo is not a correct representation of reality as these nebulae originate from stars of greatly differing ages and Vo may have changed over the lifetime of the Galactic disk.

In the past few years, it has become clear that planetary nebulae (PN) are a true stage in stellar evolution, especially regarding their chemical composition. This fact led to the introduction of the classification scheme developed by Peimbert, which has been applied to a sample of galactic objects for which a detailed amount of data exists.

Recently a number of new planetary nebulae (PN) have been found in the outer field of the LMC using deep objective–prism J plates taken with a Schmidt Telescope at the AAO. For a number of these PN, we have obtained spectrophotometry on the 2.3m ATT and for a sub–sample, determined the radial velocities using the Echelle Spectrograph of the 2.3m Telescope. All this data is to appear in the ApJ.

Several authors have analyzed the kinematics of elliptical galaxies using surface photometry in combination with absorption line velocity dispersion measurements. However, these analyses never explore the halos of galaxies, since the best absorption line measurements extend only ∼1 re. The only way to extend our knowledge of stellar kinematics to larger radii is to use the emission lines of planetary nebula for radial velocity measurements.

We present abundances for a sample of 57 southern hemisphere galactic planetary nebulae (PN). Optical spectra covering the 3100–7400 Å range were obtained at the AAT. Low resolution UV spectra obtained with the IUE satellite were available for half of these objects and were accessed via the IUE Uniform Low–Dispersion Archive. Additionally, new low resolution IUE SWP observations of Fg 1, M 3-1 and M 3-3 were obtained.

We have selected a sample of planetary nebulae, for which the core masses are determined using distance-independent parameters (Zhang and Kwok 1992). The chemical abundances of He, N, O, and C are taken from the literature for them. Relationships of the ratios of He/H, N/O, and C/O with various stellar parameters of planetary nebulae (PN), such as the core mass, the mass of the core plus the ionized nebular gas, the stellar age and temperature, are examined. It is found that the N/O increases with increasing mass, while the C/O first increases and then decreases with the core mass. No strong correlation seems to exist between the He/H and the core mass. A correlation of the N/O and He/H with the stellar temperature exists. The current dredge-up theory for the progenitor AGB stars cannot satisfactorily account for these patterns of chemical enrichment in PN. Furthermore, the correlations of the N/O and He/H with the stellar age and temperature indicate that besides the dredge-ups in the RG and AGB stages, physical processes that happen in the planetary nebula stage may also play a role in forming the observed patterns of chemical enrichment in the planetary nebulae.

Carbon, nitrogen and oxygen abundances have been found from the recombination lines for 63 planetary nebulae.

The effect of density and temperature fluctuations on the intensity of the recombination and collisionally excited lines is investigated.