The prototypical dust-enshrouded carbon Mira IRC +10216 is known to exhibit intrinsic changes on a time scale of the order of only a few years as revealed, e.g., by CO infrared line profiles (Winters et al. 2000a), its infrared light curves, and by high spatial resolution monitoring in the infrared (Osterbart et al. 2000; Tuthill et al. 2000). In particular, the infrared light curves indicate a possible periodicity on a ≈ 20 yr time scale, i.e. that a recurrent phenomenon might lead to the observed variations. Such multi-periodicity time scales of several (≈ 10) stellar pulsation periods are predicted by consistent hydrodynamical models which include a proper treatment of dust formation (e.g. Winters et al. 2000b). In these models discrete dust layers form in time intervals which are several times longer than the typical pulsation period of an AGB star (Fleischer, Gauger, & Sedlmayr 1995; Höfner, Feuchtinger, & Dorfi 1995).

IRAS 16115-5044 has been identified as a possible post-AGB star on the basis of its position in the IRAS 2-colour diagram (van der Veen et al. 1989). A study by Van de Steene and Pottasch (1993) failed to detect any radio-continuum flux from the object, suggesting it is still in the pre-planetary nebula phase, that is, the central star is not yet hot enough to ionize the circumstellar shell.

The central star is bright in the infrared (K=4.9) but faint in the visible which we attribute to an optically thick circumstellar shell (J-K=2.61, van der Veen et al. 1989). Oudmaijer et al. (1995) obtained a K-band spectrum of lRAS 16115-5044 longward of 2.2 microns, and did not detect any discernable features.

We have completed a polarimetric survey of 46 post-AGB stars which indicates that where an extended circumstellar envelope is seen then it is axisymmetric to some degree. This suggests that all post-AGB envelopes are axisymmetric and that the responsible mechanism must be ubiquitous.

We show how the different assumptions on the transition time (the time lag between the superwind quenching at the AGB and the illumination of the Planetary Nebula [PN]) reflect into very different theoretical outcomes, both in the characteristics of the stellar remnants, and in the evolution of the nebular shells.

We use a Monte Carlo simulation of post-AGB stars with a set of assumptions on the transition time, to show the effect on the resulting location of the stars on the HR diagram, and on the derived core mass distributions.

We have also performed numerical simulations of the PN formation process, and investigated the effects of the transition time on the resulting PN structure. We found that the transition time determines not only the size of the PN shell, but also its dynamical evolution.

We show the important implications that the transition time has on the observable parameters during the PN stage.

We present near-IR spectroscopy of a sample of 30 IRAS sources recently identified as late AGB stars, post-AGB stars or early PNe. The spectra obtained are centred at various wavelengths covering the molecular hydrogen v=1→0 S(1) 2.122 μm and v=2→1 S(1) 2.248 μm emission lines, the recombination lines of hydrogen Brγ 2.166 μm, Pfγ 3.741 μm and Brα 4.052 μm, and the CO[v=2→0] first overtone bandhead at 2.294 μm. As a result of these observations we have increased from 4 to 13 the total number of proto-PNe detected in H2 and we have confirmed that the onset of H2 emission takes place in the post-AGB phase. When the molecular hydrogen is fluorescence-excited the detection rate is found to be directly correlated with the evolutionary stage of the central star, rather than with the nebular morphology. In contrast, shocked-excited H2 is detected only in strongly bipolar proto-PNe, sometimes even at an early stage in the post-AGB phase. The strong correlation of shocked-excited H2 emission with bipolarity found confirms the result previously reported by Kastner et al. (1996) in evolved PNe. However, our results show that this correlation does not exist in the case of fluorescence-excited molecular hydrogen. (to be published in A&A).

We have selected a PPN candidates sample from IRAS color-color diagram. The selected criteria, the objects list of whole sample and photometric and spectroscopic observational results for southern objects have been published (paper I, Hu et al, 1993), and CO and OH maser observations for the total sample are published in 1994 (paper II, Hu et al, 1994). The 20 northern objects were observed. The optical photometric observations were taken at the J. Kapteyn telescope with CCD camera, infrared photometric observations were obtained with 1.26 meter infrared telescope at Xinglong station, Beijing Observatory, and spectroscopic observational observations were carried out using the FOS attached on the 2.5 meter Issac Newton telescope in 1989; in 1996 and 2001 we re-observed these objects using 2.16 m telescope and OMR spectroscopy at Xinglong station, Beijing Observatory. We have not found any variations between two epochs for most objects in their optical spectra except IRAS 19367+2458. The spectra we obtained in 1989 IRAS 19367+2458 is a planetary nebula with [WC] core star (Hu and Dong, 1992), but in our spectra obtained in 1996 and 2001 it is a M-type star. There are two possibilities: 1. We made mistakes of identification, if so, where is the planetary nebula? 2. It varies, but we can not find any difference in the spectra obtained in 1996 and 2001. The question is still open, more observations are needed in the future.

Hu et al (1993) have selected a sample of proto-planetary nebula candidates based on the IRAS color-color diagram. IRAS 01005+7910 is one object of this sample. We have observed this object photometrically (Table 1) and spectroscopically (Fig. 1). From its spectral type of B2I and optical color of B - V = 0.23, we can derive the reddening as E(B - V) = 0.39 and interstellar/circumstellar absorption Av = 1.20. If it is a normal B-type supergiant, the distance module will be m - M = 16.05 and distance d = 16.2 kpc. Due to the galactic latitude b = 16.6, it should be located at about 4.6 kpcabove the galactic plane. This does not fit with normal B-type supergiant and we considered that it is a post-AGB star located at high galactic latitude. Recently Hrivnak et al (2000) found that this object shows carbon-rich features in the infrared. A paper on observations, reduction and discussions of this object has been submitted to the Chinese Journal of Astronomy and Astrophysics.

Resent high-resolution optical imaging has directly revealed reflection nebulosity around proto-planetary nebulae (PPNs), the transition objects between asymptotic giant branch (AGB) stars and planetary nebulae (Sahai et al. 1998, Su et al. 1998, Ueta et al. 2000, Su et al. 2001). The existence of bipolar nebulae observed in the PPN phase suggests the presence of asymmetry in the AGB circumstellar dust shell. In order to model these objects, a self-consistent radiation transfer model is necessary. As a first attempt, we construct an approximate two-dimensional dust radiative transfer model to simultaneously fit the spectral energy distribution (SED) and images of a centrally-heated dust envelope.

Our observing campaign of proto-planetary nebula (PPN) dust shells has shown that the structure formation in planetary nebulae (PNs) begins as early as the late asymptotic giant branch (AGB) phase due to intrinsically axisymmetric superwind. We describe our latest numerical efforts with a multi-dimensional dust radiative transfer code, 2-DUST, in order to explain the apparent dichotomy of the PPN morphology at the mid-IR and optical, which originates most likely from the optical depth of the shell combined with the effect of inclination angle.

We have used radio observations of OH masers in proto-planetary nebula (PPN) candidates to probe the magnetic field structure of these objects. Here we present the first results of our study, concerning the PPN OH17.7–2.0.

Photometric and polarimetric observations of Long-Period Variables (LPVs), carried out at Byurakan Observatory over the last few years, have yielded some interesting results. In particular, one of the important results obtained for Mira type variables is that there is a certain correlation between brightness and polarization: the maximum degree of polarization is midway on the increasing branch of the light curve and in all probability its change has a periodic character. More recently an important result has been obtained for a post AGB star SAO 124414. The polarimetric and photometric observations of this star were carried out over a period of 3 years. At an almost constant brightness, strong, irregular changes of light polarization were observed in three bands (BVR) of the spectrum. The analysis of the observations does not show periodicity in changes of polarization. In this report the observational results for SAO 124414 are presented. In the study of the evolution of red giants and supergiants, future observations of this star taken simultaneously in optical and radio wavelengths, could be very significant.

We review our present knowledge about the formation and evolution of planetary nebulae and discuss the relevant processes responsible in creating and shaping planetary nebulae out of a cool AGB wind envelope. Based on 1D simulations we show that a hydrodynamical treatment along the upper AGB leads quite naturally to more realistic starting configurations for planetaries with density slopes steeper than r−2. Taking into account photoionization and wind interaction in a realistic manner, the hydrodynamics of post-AGB wind envelopes leads to density structures and velocity fields in close resemblance to observations of spherical or elliptical planetary nebulae.

Planetary nebula (PN) formation is intimately linked to stellar evolution. The stellar wind during the AGB and post-AGB stages depends on the progenitor mass and determines, together with the radiation field of the star, the structure and evolution of PNe. We summarize the results of the PhD thesis of E. Villaver, where we combined the results of stellar evolution with a numerical code to study the PN formation process.

The formation and evolution of a planetary nebula is based on the occurrence of a strong AGB wind and the rapid evolution of the central star with corresponding changes of its ionizing flux and wind power. We have studied the influence of different mass-loss histories in combination with various central-star properties.

Several points involved in observational studies of evolution of planetary nebulae and their central stars are discussed. It concerns observed masses of nebulae and central stars, nature of H-deficient central stars, role of time-dependent ionization in the nebular haloes, and a recent discovery of strong enrichements in some planetary nebulae.

The determination of effective temperatures of very hot central stars (Teff > 70 000 K) by model atmosphere analyses of optical H and He line profiles is afflicted with considerable uncertainty, primarily due to the lack of neutral helium lines. Ionization balances of metals, accessible only with UV lines, allow more precise temperature estimates. The potential of iron lines is pointed out. At the same time iron and other metal abundances, hardly investigated until today, may be derived from UV spectra. We describe recent HST spectroscopy performed for this purpose.

A search for iron lines in FUV spectra of the hottest H-deficient central stars (PG1159-type, Teff >100000 K) taken with FUSE was unsuccessful. The derived deficiency is interpreted in terms of iron depletion due to n-capture nucleosynthesis in intershell matter, which is now exposed at the stellar surface as a consequence of a late He shell flash.

Recent advances in the modelling of stellar winds driven by radiation pressure make it possible to fit many wind-sensitive features in the UV spectra of hot stars, opening the way for a hydrodynamically consistent determination of stellar radii, masses, and luminosities from the UV spectrum alone. It is thus no longer necessary to assume a theoretical mass–luminosity relation. As the method has been shown to work for massive O stars, we are now able to test predictions from the post-AGB evolutionary calculations quantitatively for the first time. Here we present the first rather surprising consequences of using the new generation of model atmospheres for the analysis of a sample of central stars of planetary nebulae.

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite observed several high and low excitation planetary nebulae in the wavelength range 905–1187 Å at a spectral resolution of λ/Δλ ~ 15,000.

We present fully consistent models for O-type stars that reproduce the observed UV spectra simultaneously with the observed terminal velocities and mass loss rates. We demonstrate how, as a consequence, the models can be used to determine the stellar parameters from the observed UV spectra alone.

We present a new grid (solar and halo abundance ratios) of state-of-the-art fully line-blanketed NLTE model atmospheres which covers the parameter range of central stars of planetary nebulae. The grid is available at the WWW.