An abundance analysis of the photosphere of the F-type Post-AGB candidate IRAS 05341-+0852 is presented. It shows that the star is metal-poor ([Fe/H]=–1.0) and carbon-rich (C/0≈2.2). Carbon, nitrogen, oxygen, silicon, and possibly lithium and aluminum are found to be over abundant. Most importantly this star has large overabundance of s-process elements which are as follows: [Y/Fe]= 1.80, [Ba/Fe]= 2.58, [La/Fe]= 2.86, [Ce/Fe]= 2.95, [Pr/Fe]= 2.27, [Nd/Fe]= 1.97, and [Sm/Fe]= 0.86. The overabundances of s-process elements and carbon in IRAS 05341+0852 is direct evidence for the association of s-process enhancements with shell-flashes and dredge-up. These are likely responsible for the increase in C/O. The possible overabundance of Li ([Li/Fe]≤2.5) and Al ([Al/Fe]≤ 1.1) in IRAS 05341+0852 could indicate that there was Hot Bottom Burning (HBB), where the base of the convective envelope is hot enough for nucleosynthesis to occur (Lattanzio 1993). HBB has been suggested as the mechanism responsible for the production of Li in the Li-rich AGB stars discovered by Smith and Lambert (1989). In fact these are bright AGB stars which are oxygen-rich rather than carbon-rich. Recent calculations by Sackmann and Boothroyd (1992) showed that Li-rich and O-rich AGB stars are the result of HBB. In the HBB models temperatures of the order of 0.5–1 × 108 K are encountered at the base of the convective envelope (Blöcker and Schönberner 1991). This is hot enough for the reaction 25Mg(p,γ) 26 Al resulting in the production of Al.

Stellar binary companions account for bipolar PNe (∼ 11% of all PNe1), and some ellipticalls (22%2). The rest of axisymmetrical PNe (40% to 60% of all PNe) are due to substellar objects (planets and brown dwarfs)3. This classification of axi symmetrical PNe suggests that substellar objects are commonly present within several × AU around main sequence stars, and that several substellar objects must be present around most main sequence stars3. Some substellar and low mass stellar companions will enter the primary envelope only as the primary reaches the upper AGB. Thus, the early mass loss episode will be spherical, leading to the formation of a spherical halo around an elliptical inner region. Gas giant planets and brown dwarfs close to the primary, will not allow Earth-like planets to have stable orbits. Systems with no Jupiter-like planets will allow Earth-like planets to be present. These stars will form spherical PNe (10%-20% of all PNe, including spherically ejected PNe that have been deformed by the ISM through which they move4). Systems with substellar objects at large separation, as Jupiter in the solar system, will also allow Earth-like planets to be present. These systems will form PNe with spherical halo. Therefore, life may have been present in planets around the central stars of round PNe and elliptical PNe with round halos.

Based on the mass-loss description developed by Blöcker (1995, A&A, 297, 727), we present first exploratory computations of the dynamical evolution of a dusty stellar wind envelope around an intermediate mass star during the last 300 000 years on the AGB and its transformation into a planetary nebula during the following 5 000 years of post-AGB evolution. To model the dynamics of the cool dusty envelope, we used a two-component (gas/dust) 1D radiation hydrodynamics code which computes the radiation pressure on dust grains and the structure of the envelope in a self-consistent way, including the variable frictional coupling between dust and gas. The grains are either carbon or oxygen based and of single size and spherical shape.

We present HST and ground-based images of the bipolar proto-planetary nebula M1-92. Trammell et al. (1993) studied Ml-92 using spectropolarimetry and concluded that the continuum emission is reflected from the inner regions of the object, the forbidden lines are formed locally in the bipolar lobes, and the permitted lines are partially reflected and partially formed in the bipolar lobes. The emission produced in the lobes of Ml-92 is indicative of shock heating, Vs = 60–100 km s–1. Shock emission dominates the optical line spectra of the lobes of M1-92 and is a powerful probe into the properties of the mass loss occurring between the AGB and PN phases of evolution.

Some observed parameters of planetary nebulae with H–deficient (mostly WR–type) nuclei (PNN) are compared to those of H–rich ones and to theoretical predictions from the born again scenario. An example is shown in Fig. 1

In our search for new obscured PN, candidates were selected from the IRAS PSC based on their infrared colors that are typical for PN. These PN candidates were observed in the radio. On average 20 % of the objects were detected (Van de Steene & Pottasch: 1993, A&A 274, 895; 1995, A&A 299, 238). Optical spectroscopy showed that the PN candidates detected in the radio have emission line spectra typical of PN (Van de Steene et al. 1996a A&AS 118, 243; 1996b A&AS in press).

We have started phase referenced monitoring of the 1667 MHz masers in the Mira variable U Her. Ultimately we hope to determine the annual parallax for this star. Three sets of VLB A observations have been performed so far. The first on July 22 1994, the second on March 6 1995 and a third on August 14 1995. Observations included bright extragalactic sources to calibrate instrumental effects and cycles of calibrator–source–calibrator (≈ 2° away) of typically 6 minutes. Without any special astrometric analysis software we detect the proper motion of U Her with an accuracy of several milli–arcseconds.

Systematic radial velocity monitoring programs revealed that all optically bright, high-latitude post-AGB stars that show a near-IR excess in their energy distribution are binaries (see the contribution of R. Waters et al. in these proceedings). A standard AGB evolution for the shorter period objects seems, however, impossible since the orbits are too small to accommodate an AGB star! Non-standard phenomena connected to the specific binary nature have to be invoked in order to understand these systems.

We have observed CRL 2688 (the Egg nebula) in the 13 CO J = 1–0 and CS J = 1–0 and 2–1 lines by the Nobeyama Millimeter Array with a resolution of about 4″. The 13 CO velocity channel maps show that emission consists of three components; a spherical core, an extended envelope, and a bipolar high-velocity component (Yamamura et al. 1995).A spherical shape of the core despite of the maximum optical depth of about the unity indicates that the disk-like structure, expected from the shape of the bipolar nebula, is smaller than the present beam size (corresponding to about 6 × 1016cm at 1kpc). The combined maps made from the data by the NMA and the Nobeyama 45-m telescope show that emission spreads towards the south of the center more then the opposite direction.

In this review a critical examination of our present knowledge about the structure and evolution of central stars of planetary nebulae is presented. In particular, the still enigmatic origin of the Wolf-Rayet central stars is addressed, too. Some directions for future work are given at the end.

It is well known that the evolution of white dwarfs (WDs) depends sensitively on the question whether they have “thin” or “thick” envelopes of H and He (see Wood 1995). Standard evolutionary caluclations (e.g. Paczynksi 1971) show that at the tip of the Asymptotic Giant Branch the envelope masses are tightly correlated with the mass of the hydrogen exhausted core (≈ total mass). Accordingly, the masses of hydrogen, M H, and helium, M He, on top of the degenerate C/O interiors decrease by orders of magnitudes with increasing stellar mass. In contrast, many applications of WD calculations consider only single values of q H,He = log(M H,He/M ∗) asuming either “thick” or “thin” envelopes.

The peculiar new object (α2000 = 17h52m32.7s, δ2000 = −17°41′08″) in Sagittarius discovered by Y. Sakurai (IAU Circ. 6322 (1996)) first classified as a slow nova turned out to be a star experiencing a late He-flash. The only other examples in historical times are V605 Aql, the central star of A 58 in 1919 and possibly FG Sge. The extremely H-poor central nebulae in A 30 and A 78 are considered the remainder of late He-flashes that happened thousands of years ago.

Ever since the pioneering work of Schönberner (1979, A&A, 79, 108) and Iben (1984, ApJ, 277, 333) who showed that the evolution of post-AGB remnants might be affected by late thermal pulses of the helium-burning shell, resulting in a temporary growth of these objects to red giant dimensions, many attempts were made to explain a number of puzzling objects, among them the well-known variable central star FG Sge as well as the R CrB and PG 1159 stars, by this so-called “born-again AGB” scenario (Iben et al. 1983, ApJ, 264, 605; Iben & MacDonald 1995, in: White Dwarfs, Springer, p. 48). However, it is still not clear if the frequency of occurrence of such events is high enough as to be consistent with the number of born-again candidates. This is due to the very short evolutionary timescales during the pulse and the character of the post-pulse evolution which resembles the first post-AGB phase and makes it difficult for an observer to distinguish such objects from “normal” central stars.

Sakurai's object is thought to be undergoing a final thermal pulse or shell flash. It is the first example of this type of object to be studied using modern instrumentation and will be useful and unique for the time being for placing theoretical models of these transient events on a firm observational footing. The only other definite candidate is V605 Aql – now the WC central star of the planetary nebula Abell 58 – which is thought to have undergone a final thermal pulse around 1918.

Time scales for post-AGB stellar evolution are still unclear. In order to study them it is useful to look for old PNe and compare nebula expansion and stellar evolutionary ages. We have selected very hot members from different white dwarf sub-classes. Many of them are objects recently discovered in the Hamburg-Schmidt Survey (Hagen et al. 1995). The sample includes three peculiar objects whose spectra show signatures of an extremely hot wind (≈ 106 K), namely absorption lines of ultra-high ionized metals, e.g. O VIII (Werner et al. 1995). A complete list is given in the table below. The search was performed by direct Hα narrow band imaging using a wide angle (20′ × 20′) CCD camera (WWFPP) attached to the Calar Alto 1.23m telescope (28.9.-3.10.1995).

Radial abundance gradients in the disks of spiral galaxies are made evident from observations of ionized nebulae (HII regions, planetary nebulae, and SNRs) and stars. They have been observed for several chemical elements, and their presence is an established fact, despite some inconsistencies in the results. Therefore, these gradients can be considered as an additional constraint to the chemical evolution models, as the age-metallicity relation or the metallicity distribution of the G-dwarfs.

We present the results of a deep and uniform narrow-band Hα imaging survey for planetary nebulae (PNe) in the southern galactic bulge. The goal of this survey was to obtain a sample of bulge tracers which is being used to study the dynamics of the Milky Way bulge (Beaulieu, 1996).

The vertical structure of the Galaxy is stratified in different populations. Gas and young stars have a low velocity dispersion (about 10 km/s) and thus are confined in a very thin disk (100 pc of scale height). Old stars have larger velocity dispersions, and are thus encountered up to a few kpc.

Planetary Nebulae (PNe) are transient objects of considerable astrophysical interest: there are thought to descend from low and intermediate initial mass stars and therefore cover a large range of ages. On the other hand there are easily identified thanks to their bright emission-line spectra. In view of this it is perhaps surprising that little use has been made of PNe for kinematical studies of the Milky Way.

We present a database on the chemical composition of 200 planetary nebulae (PNe) in our Galaxy. Abundances of He, C, N, O, Ne, Mg, Si, S, and Ar are found for 175, 64, 190, 200, 170, 13, 40, 173, and 187 PNe respectively. The abundances for all PNe are obtained with the same atomic data and by the same method of abundance determination, so the database is highly homogeneous. The method of the determination of nebular abundances based on a photoionization model grid is described by Golovaty et al. (1993). Observational data have been compiled from 105 papers published in 1976–1994. For most of PNe in our sample, the distances and central star masses are determined as well (Malkov, this volume).