In our paper (Sharova, 1995) a method of determining the distances to galactic planetary nebulae has been based on the recently established (Sharova, 1992) regression relationship between the central stars radius and the temperature. Now our scale is compared to 18 other scales, in four of which the central stars parameters also are used, instead of nebulae ones. The scale distance estimations DS are compared with independently known individual distance D. The values log(DS/D) and their dependence on radius of the nebulae are analysed. Each scale is described by two parameters: a geometrical average value < log DS/D > and the relation ρ/ρc, where ρ is the correlation coefficient for log DS/D — log RN, and ρc is its critical value. If ρ/ρc > 1, the correlation is significant with the confidence probability 95%. For example we obtained the following results: ρ/ρc = 0.153, 0.494, 2.126, 2.228; < DS/D >=0.999, 2.145, 2.126, 1.414 for our scale and for widely known MKH, CKL, VdSZ scales respectively. By both criteria our scale achieves better agreements, on average, with individual estimations of distances than others.

While the basic properties of AGB stellar evolution are well established, comprehensive observational studies of late phases of intermediate mass stars continue to generate puzzles for current stellar models. Here, I review current techniques to model AGB stars, and I discuss important aspects of current research of AGB (and post-AGB) stellar evolution with a particular focus on how these interrelate.

We have searched 44 optical carbon stars for circumstellar C2 Swan (0,0) band absorption lines using the high dispersion echelle spectrograph for the 188 cm reflector at Okayama Astrophysical Observatory. We have detected the circumsteller C2 lines definitely in 18 stars, possibly in 6 stars, and not detected in 20 stars. We discuss the observed properties of the circumstellar C2 lines. We infer the C2 line-forming region is located at the innermost part of the circumstellar envelope.

Four dead OH/IR stars are identified in a complete Arecibo sample of OH/IR stars, together with one in the process of dying.

We review the basics of AGB nucleosynthesis in thermal pulses, in the hydrogen burning shell, and by Hot Bottom Burning.

We present new evolutionary sequences for low and intermediate mass stars (1M⊙ to 6M⊙) for three different metallicities, z = 0.02, 0.008 and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch (AGB) phase. We have two sequences of models for each mass, one which includes mass-loss and one without mass-loss. For an overview of AGB evolution and nucleosynthesis, see Herwig (2002) and Lattanzio (2002).

The carbon star IRC +10 216 is a long-period Asymptotic Giant Branch (AGB) star suffering from strong stellar winds (several 10-5 M⊙/yr; Loup et al. 1993) which have led to an almost complete obscuration of the star by dust. Due to the high mass-loss rate, long period of P = 649 d (Le Bertre 1992), and carbon-rich chemistry of the dust-shell, IRC +10 216 is obviously in a very advanced stage of its AGB evolution. High-resolution near-infrared imaging of IRC +10 216 has revealed that on sub-arcsecond scales (100 mas) its dust shell is clumpy, bipolar, and changing on a time scale of only ~1 yr (Weigelt et al. 1997, 1998, Haniff & Buscher 1998, Osterbart et al. 2000, Tuthill et al. 2000). Since most dust shells around AGB stars are known to be spherically symmetric, whereas most proto-planetary nebulae (PPN) show an axisymmetric geometry (Olofsson 1996), it appears likely that IRC +10 216 has already entered the transition phase to the PPN stage. This suggests that the break of the dust-shell symmetry between the AGB and post-AGB phase already takes place at the end of the AGB evolution.

H-deficient post-AGB objects, e.g. PG1159 type star K1-16 and born-again AGB star Sakurai's object, have been reported to be significantly iron-deficient. We find that the iron deficiencies expected due to neutron-capture nucleosynthesis during either the progenitor AGB evolution and/or the neutron burst that occurs as a result of the rapid burning of protons during a post-AGB He-flash are generally in line with observations.

We identify H2O, CO, SiO and CO2 bands in near-infrared ISO/SWS spectra of the RV Tau star R Sct. These molecules originate from an atmosphere around the photosphere. RV Tau stars are commonly considered as post-AGB stars. We argue that R Sct may be a thermal-pulsing AGB star, observed in a helium-burning phase.

We present the first speckle interferometric observations of R CrB, the prototype of a class of peculiar stars which undergo irregular declines in their visible light curves. The observations were carried out with the 6 m telescope at the Special Astrophysical Observatory near maximum light (V = 7, 1996 Oct. 1) and at minimum light (V = 10.61, 1999 Sep. 28). A spatial resolution of 75 mas was achieved in the K-band. The dust shell around R CrB is partially resolved, and the visibility is approximately 0.8 at a spatial frequency of 10 cycles/arcsec. The two-dimensional power spectra obtained at both epochs do not show any significant deviation from circular symmetry. The visibility function and spectral energy distribution obtained near maximum light can be simultaneously fitted with a model consisting of the central star and an optically thin dust shell with density proportional to r−2 and amorphous carbon as its constituent. The inner boundary of the shell is found to be 82 R* (19 mas) with a temperature of 920 K near maximum light. However, this simple model fails to simultaneously reproduce the visibility and spectral energy distribution obtained at minimum light. We show that this discrepancy can be attributed to thermal emission from a newly formed optically thick dust cloud. Simultaneous fits of the observed SED and visibility with models including a thermally emitting dust cloud suggest the presence of a newly formed dust cloud as hot as 1200 K with a radius of 4 – 5R*, in addition to an optically thin dust shell whose inner boundary is ~ 170 R* with a temperature of ~ 690 K. Alternatively the discrepancy can be attributed to an unusual extinction curve of dust grains in the obscuring cloud which was present in front of the star at minimum light. The details of the observations and modeling are described in Ohnaka et al. (2001, A&A 380, 212).

We discuss the period evolution of the Mira variable R Hya. We show, using observations dating back to 1662, that it has evolved from a stable 500-day period to a stable 380-day period. The onset of the period evolution coincided with a sharp decline in mass-loss rate. We suggest that mass-loss fluctuations over a period of a few hundred years may be common in Miras, and relate to a non-linear pulsation instability.

In this contribution an overview is given of recent accurate chemical abundance studies in post-AGB stars. The intrinsic nature of the enrichement and the spread in metallicity together with the absence of strong molecular veiling make post-AGB stars very useful to constrain AGB (chemical) evolutionary models. S-process enrichment is, however, not a general characteristic of post-AGB stars and a photospheric chemical study is not always a good tracer for the evolved status of an object.

We discuss the occurrence of the s-process during the radiative interpulse phase of rotating AGB stars. Due to differential rotation, protons are mixed into 12C-rich layers after thermal pulses, in the course of the so called third dredge up episode. We follow the time evolution of key isotope abundances in the relevant layers with a post-processing code which includes time dependant mixing and nucleosynthesis. In rotating AGB models, the mixing persists during the entire interpulse phase due to the steep gradient of angular velocity at the envelope-core interface. As the layers containing protons and 12C, which are formed this way, become hotter, a 13C-pocket is formed in a natural way. However, in this situation also 14N is formed and spread over the entire 13C-pocket. We include the neutron consuming 14N(n,p) reaction in our network and determine to what extent it reduces the production of trans-iron elements. We propose that rotation may be responsible for the spread of efficiencies of the 13C neutron source as required by observations.

The structure and evolution of central stars of planetary nebulae (CSPNe) is reviewed. CSPNe represent the rapid transitional stage between the Asymptotic Giant Branch (AGB) and the white-dwarf domain. It is shown that the whole evolution off the AGB through the central-star regime depends on the evolutionary history. The detailed evolution into a white dwarf is controlled by the internal stellar structure which, in turn, is determined by the duration of the preceding AGB evolution and therefore by the AGB mass-loss history. The evolution of hydrogen-deficient central stars has been a matter of debate since many years. Convective overshoot appears to be a key ingredient to model these objects. Various thermal-pulse scenarios with inclusion of overshoot are discussed, leading to surface abundances in general agreement with those observed for Wolf-Rayet central stars.

A synthetic method for tracing the evolution of the post-AGB stars is presented. The main advantage of this procedure, based on the published numerical evolutionary sequences, is that it is simple, fast, and allows to treat post-AGB objects of arbitrary mass.

We present a stellar evolution model sequence of the past and future evolution of the post-AGB born again star Sakurai's object (V 4334 Sgr). In order to match the short evolutionary time scale we have to assume that the convective ingestion of hydrogen-rich envelope material into the He-flash convection zone proceeds slower than predicted by the mixing length theory. For the future we predict a swift second evolution through the central star region before a second born-again evolution occurs.

The study of proto-planetary nebulae (PPNs) leads to a better understanding of both the preceding asymptotic giant branch and the succeeding planetary nebula phases of stellar evolution. Recent results are reviewed, emphasizing the properties of the central stars and the shape and chemistry of the nebulae. The study of PPNs is seen to be important in its own right.

CIT 3 is an oxygen-rich long-period variable evolving along the Asymptotic Giant Branch and is one of the most extreme infrared AGB objects. Due to substantial mass loss it is surrounded by an optically thick dust shell which absorbs almost all visible light radiated by the star and finally re-emits it in the infrared regime. We present the first near infrared bispectrum speckle-interferometry observations of CIT 3 in the J-, H-, and K′-band (resolution: 48 mas, 56 mas, and 73 mas). The interferograms were obtained with the Russian SAO 6 m telescope. While CIT 3 appears almost spherically symmetric in the H- and K′-band it is clearly elongated in the J-band along a symmetry axis of position angle −28°. Two structures can be identified: a compact elliptical core (eccentricity ~0.8) and a fainter north-western fan-like structure (full opening angle ~ 40°). The development of such asphericities close to the central star suggests that CIT 3 is in the very end of its AGB evolution or even in transition to the proto-planetary nebula phase where most objects are observed in axisymmetric geometry.

We present high-sensitivity near-IR images of a carbon-rich proto-planetary nebula, AFGL 618, obtained with the Infrared Camera and Spectrograph (IRCS) mounted on the 8.2m Subaru Telescope. The deep near-IR images have revealed “bullets” and “horns” extending farther out from the edges of the previously known bipolar nebulosities that consist of dust-scattered star light component and shock-excited line emission component. That these bullets and horns represent the positions from which [Fe II] IR lines arise is strongly suggested from the spatial coincidence between these near-IR microstructures and the optical collimated outflow structure observed by the recent HST/WFPC2 imaging, together with the previous detection of shock-excited, forbidden IR lines of atomic species at those locations. At these positions of the near-IR mincrostructures, we have also discovered CO clumps moving at > 200 km s-1 from our re-analysis of the existing 12CO J = 1 – 0 data obtained with the Berkeley-Illinois-Maryland Association (BIMA) interferometer array. These findings indicate that fast-moving CO clumps seem to be impinging upon the surrounding ambient circumstellar shell, thereby causing shocked emission regions which manifest themselves as the near-IR microstructures at the shock interface. To deepen our understanding of the connection between the near-IR microstructures and the CO outflow structure, we are currently conducting higher resolution observation in CO lines with the BIMA array.

In a previous paper (Simis, Icke, & Dominik 2001, hereafter SID2001), it was shown that the concentric, quasi-periodic shells around IRC +10216 seem to originate on the AGB, as an instability in the wind. In this contribution a grid of model calculations is used to investigate the formation mechanism further. Comparing the calculations leads to the conclusion that mass loss variability on the AGB, and hence the formation of shells, is more likely to occur for relatively high stellar mass and temperature, and low luminosity than for lower mass and temperature and higher luminosity.