We report here a systematic study of the molecular content in the neutral envelope of NGC 7027. The measurements were done in September 1991 with the 30-meter IRAM telescope on Pico Veleta (Spain). The frequencies and transitions covered are given in Table 1. Long integrations were performed in each transitions toward the central position of NGC 7027, and for the strongest lines we obtained detailed maps. The results of the observation are given in Table 1 (in units of main beam temperature).

Aspherical mass loss is likely to be a common feature in the late stages of stellar evolution when the star arrives at the asymptotic giant branch. In the transition phase from the red giant to a proto-planetary nebula (PPN) the star developes an extended circumstellar dust-shell which is in many cases ellipsoid or disk-shaped. An algorithm for treating the radiative transfer problem in dense axisymetric dust shells is presented. The program has been used to determine the temperature distribution in disk-shaped dust envelopes in PNe. The implications of both composition and the spatial structure of the dust shell has been investigated. We confined the viewing angle to the equatorial plane to maximize the effects of the disk geometry. Our computations show that the observed morphology is mainly affected by the geometry of the envelope whereas the integrated spectrum is mainly determined by the characteristics of the grains themselves. The observed morphology in the optical and near infrared wavelength region is determined by scattered starlight in the thin polar region where the optical depth is low both to the the observer and to the central star whereas scattered light in the equatorial plane is blocked by the optical thick disk. This results in the familiar hourglass shaped morphology perpendicular to the orientation of the disk. Towards longer wavelengths the disk becomes more transparent thus the bipolar shape becomes less prominent. At wavelength longer than 5 μm the disk appears as a compact round source produced by scattered light at the inner rim of the disk. In the Mid and Far Infrared thermal emission of dust is the dominant emission process. Observational evidence for dust disks in PNe can be found in several wavelength regions. For example M 2–9 has been observed by Aspin (1984) in the optical region with a CCD Imaging Polarimeter. The polarisation maps show a high degree of polarisation in the lobes whereas emission from the equatorial plane is little or not polarized. In the NIR the change of the appearance from the bipolar to the compact round morphology predicted by our models is confirmed by the presented images in the J (1.2 μm), H(1.6 μm) and K(2.2 μm) filters obtained with the ESO InfraRed Array Camera. This confirms the general picture of M 2-9 as a proto-planetary nebula being surrounded by an extended, optically thick dust disk.

We present recent results from a “search and mapping” program of molecular line emission (mainly CO) from remnant AGB envelopes around planetary nebulae (PNe), using the SEST (La Silla, Chile). New detections in CO J=2−1 include NGC2899 (0.02K), NGC6369 (0.14K) & NGC7009 (0.08K). In many of the detected PNe, notably NGC3132, IC4406, NGC6302, M1-16, and CPD-56°8032, the molecular envelopes contain 2 kinematically distinct outflows. Mapping of the strongest of these shows (1) that the fast (e.g. Vexp≳40–60 km s−1 in NGC3132, IC4406) outflows have bipolar spatial structure, and (2) there exists an equatorial density enhancement in the slower, more massive [· (M⊙yr−1)>5 10−6(NGC3132), >2 10−5(IC4406)] outflows, which presumably collimates the fast outflow (e.g. Sahai et al. 1990, A & A, 234, L1; Sahai et al. 1992, A & A, 251, 560). The fractional CO abundance in the envelope, f(CO), is probably rather low (<10−4), as a result of photodissociation by the stellar and interstellar UV radiation [e.g. f(CO)≲10−5 in IC4406]. HCN, HCO+, and 13CO have also been detected in several PNe, and sensitive upper limits set on CS, C18O, & C17O (in M1-16), and SO (in NGC3132). Some results are tabulated below, and calculations to estimate the molecular masses, mass-loss rates and molecular abundances are in progress.

Infrared array camera images of the planetary nebula NGC7027 at seven wavelenghts between 7.8 and 20μm taken at the IRTF telescope are compared to Hα and Hβ images obtained at the CFHT, and published 2-, 6-, and 20-cm VLA maps, all made with better than ∼ 1 arcsec resolution. The mid-infrared images have also been deconvolved using the Maximum Residual Likelihood algorithm to show structural details on the ∼ 0.3 arcsec scale. The visible, infrared and radio images are aligned with ±0.2 arcsec astrometric precision using common features and visible stars. Image algebra can then be readily performed. The images are analysed to determine the spatial distribution of poly cyclic aromatic hydrocarbon (PAH) grains and ionized gas relative to the continuum dust emission. The results show the extent of the mid-infrared dust continuum, the PAH emission features at 8.65 and 11.25μm, and would reveal any contribution from 9.8 μm “silicate” emission. The optically thicker 20-cm emission more closely resembles the extinction-corrected Hα distribution. The large grains giving rise to the visible extinction (derived from the Hα/Hβ ratio) are distribued differently from the mid-infrared and ionized gaz, indicating the location of the outer cold dust. The 11.6μm image could contain both PAH and continuum grain emission, and we have attempted to separate the two components. It appears that the overall emission distributions are quite similar for all the infrared wavelengths, so the various dust constituents are generally uniformly mixed. However there are indications that the 11.6μm PAH emission distribution is patchy and not identical to the continuum dust, although not distinctly different in radial distance from the center. The ionized gas distribution peaks inside that of the dust continuum. The visible/infrared/radio spatial analysis, and color indices calculated from the infrared images, are used to develop a detailed new morphological model of NGC7027 based on this comprehensive multi-wavelength data set.

There is much direct and indirect evidence for the presence of dust in Planetary Nebulae (PN): variations in extinction across the face of the nebulae; IR emission with strong 25 and 60μm fluxes; broad near-IR emission lines of Silicate, SiC and PAH grains; optically thick lines, such as [C IV]1550Å, have lower strength on account of the increased path length due to dust scattering; a centro-symmetric pattern of polarization vectors in a few PN (Leroy et al., A&A, 160, 171, 1986). An observational programme has begun to study the polarization profiles of bright emission lines in PN arising from dust scattering within the nebulae. The first results are discussed.

Continuum emission has been detected at millimetre and submillimetre wavelengths from a total of thirteen planetary nebulae, all but one for the first time. The observations were obtained with the liquid 3He–cooled UKT14 bolometer on the JCMT and UKIRT telescopes on Mauna Kea, Hawaii.

In this paper I review our understanding of the third dredge-up phenomenon and the nucleosynthesis which occurs during thermally-pulsing AGB evolution.

This review concerns stars on the Asymptotic Giant Branch that are about to develop into planetary nebulae (=PNe). They are still termed “stars”, and properly so, and yet they have already, in statu nascendi, the structure of a PN. There are two different kinds: oxygen-rich stars (spectral class: M) and carbon-rich stars (spectral class: C).

The properties of large amplitude AGB variables (Miras) are reviewed with particular emphasis on their relevance for evolution into PNe. We concentrate on recent observations in the LMC and Galactic Bulge. The available evidence is consistent with the idea that most Miras evolve into PNe and that most old disk stars go through the Mira and PNe phases. However, there are some suggestions that the most metal rich stars in the Galactic Bulge go through neither phase or do so very rapidly. The lifetime of a typical Bulge Mira must be more than 105 yrs and Miras outnumber PNe in the Bulge by about 10 to 1.

Luminous low-mass stars of intermediate Teff – candidates for post-AGB transition objects to PNNi, are all expected to be pulsationally variable. Different modes and their combinations may be involved, leading to a variety of observed behaviour.

The progress in the search for proto-planetary nebulae (PPN) in the last 5 years is reviewed. An observational definition of PPN is developed and a list of current PPN candidates is given. The optical, infrared, and radio properties of PPN are summarized and compared with theoretical models.

A review is given of the properties of post-AGB stars. Methods to find candidate post-AGB stars are briefly discussed, along with some results and selection effects involved. Their photospheric parameters (Teff, log g, abundances), energy distributions and binarity are also discussed. Evidence for mass loss, both past and present, is reviewed.

Planetary nebulae with close binary nuclei are reviewed. It is shown that these systems can be used as a source of information for the physics of the common envelope phase in the evolution of binary systems. Mechanisms for the production of bipolar planetary nebulae are examined and it is concluded that presently the action of binary companions to the central stars appears to provide the most promising mechanism. Other systems (e.g. novae, supernovae) in which similar processes may be operating are discussed.

Thermal pulses in AGB stars cause large luminosity variations at the stellar surface. The role of these luminosity variations in the production of planetary nebulae is discussed. Results of theoretical evolution calculations which include mass loss modulated by thermal pulses are presented.

Planetary Nebulae result from the interaction of fast winds and radiation fields with slow ejecta produced in the late stages of evolution of moderately low mass stars. We review the basic principles of the interaction and describe some of the modifications needed to produce realistic models for comparison with observational data. The present status of numerical modelling is discussed and some important problems outlined.

Interaction of planetary nebulae (PNe) with the interstellar medium (ISM) is quite common, in accord with our understanding of the large-scale structure of the ISM. The characteristic signature of the interaction is a dipole asymmetry resulting from a distortion and compression of the nebula in the direction of stellar motion. Studies of interacting PNe provide us with information on the structure of the ISM, and on the evolution of old PNe and PN halos.

We present the results of accurate numerical radiation-gasdynamic models for spherical (1-d) and aspherical (2-d) planetary nebulae (PNe). The models confirm and surpass previous efforts by recovering the detailed morphologies, kinematics, and ionization structure of real PNe. Our 1-d simulations include the evolution of the central star and a superwind/AGB wind interaction. The importance of the forward wind-blown shock and time-dependant ionization in redistributing mass in the nebulae is demonstrated. Our 2-d simulations, the first of their kind, confirm that aspherical PNe can develop through the interaction of a spherical fast wind with an aspherical superwind. We demonstrate that the full range of nebular morphologies can be recovered by considering the combination of wind/wind interactions, the ionization state of the gas and the projection angle on the sky

Hydrodynamical calculations of Planetary Nebulae (PNe) over 25000 yrs of evolution which include timedependent effects of ionization as well as variable central stars winds and parameters consistent with stellar model calculations show a great variety of velocity and density structures which strongly deviate from the often assumed “homogeneous shells, expanding with constant velocities” (Marten & Schönberner, 1991, hereafter MS). By means of a static photoionization code we calculate the surface brightness in the 10 most prominent nebular emission lines for density structures of the model sequence “VS” as given by MS. The obtained radial emissivities are used together with the velocity and temperature structure of the ionized gas to calculate (thermally broadened) line profiles in order to derive “measured” expansion velocities. We compare the theoretical surface brightnesses with observations and demonstrate some difficulties in the interpretation of nebular expansion velocities, expansion distances and ages.

Asymptotic giant branch stars are the immediate precursors to the planetary nebula stage of stellar evolution. It is clear that the latter stages of a stars life on the AGB are accompanied by either continuous or episodic mass-loss, with the final convulsion being the ejection of the envelope (the future planetary shell), the gradual exposure of the bare CO core and the rapid horizontal evolution to the blue in the H-R diagram. Thus, the structure of the planetary nebula luminosity function, particularly at the higher luminosities (although this phase is extremely rapid), is intimately tied to the luminosity function of the AGB.

We present mid-IR and far-IR images of a variety of AGB stars and red supergiants. The mid-IR images were all taken with the Berkeley/Livermore mid-IR array camera, which employs a 10 × 64 pixel Hughes photoconductor. All the images reported here were taken using a 10% bandpass CVF, at various wavelengths in the 10μm atmospheric window. These were supplemented by IRAS images, some constructed from survey scans, others made as Additional Observations during the pointed phase of the IRAS mission. We have so far observed 11 such sources with our mid-IR camera, and report here that only two of them (R Aql and V Hya) appear to be unresolved.