We present VLBA observations of the spatial and velocity distributions of 22 GHz H2O and 1612 MHz OH masers in the OH/IR star W 43A. These masers have the same systemic velocity and are, therefore, likely to be associated with the common stellar object. However, the kinematical structures of them are quite different and independent. Most of the H2O masers are extremely collimated spatially and kinematically. The H2O maser jet also seems to be precessing. On the other hand, the OH masers exhibit clear arc-shaped structures indicating a spherically-expanding shell with weak collimation. The W 43A jet is very likely to be predominantly composed of hot molecules traced by H2O maser emission and formed in the immediate vicinity of an unknown star next to another OH/IR star. Such a “molecular jet” is likely to appear only during the short period before a star forms an elongated planetary nebula.

We measured 2.1 – 2.3 μm spectra for a fairly complete sample of known proto-planetary nebulae (PPN) at declinations greater than -30°. This spectral range includes the H2 emission lines 1-0 S(1), 1-0 S(0), 2-1 S(1), 2-1 S(2), and 3-2 S(3). We detected H2 emission from 16 of the 51 nebulae in our survey, including radiatively-excited H2 from several non-bipolar PPN.

The relative morphologies and structures of molecular and ionized gas emission from planetary nebulae (PNe) allow a better understanding of the nature and evolution of these objects. The classical paradigm for the structure of PNe is that of an ionized gas bubble bounded by neutral gas and molecules. However, it has been shown that molecular gas exists within ionized regions, leading to a re-evaluation of the classic structure. In the Helix Nebula (NGC 7293) dense condensations known as cometary knots are known to exist in the main ionized nebula. The molecules in these knots are shielded from the ionizing radiation and thus survive within the ionized zone. Another PN in which H2 emission is seen to originate from within the ionized nebula in NGC 6720 (the Ring Nebula).

NGC 7027 has been observed in eleven molecular species (in seventeen transitions) in the 200 and 300 GHz bands with the James Clarke Maxwell Telescope (Hasegawa & Kwok 2001). The results include a first detection of C2H in this source. The observed spectra of HCO+, H13CO+, HCN, CN, C2H, and CO+ show line widths larger than that of bulk CO emission but coincident with the full width at detection limit of weak wings in CO spectra. The sizes of the HCO+, HCN, and CN emitting regions are 13″ in diameter at half-peak level, significantly smaller than that (60″) of the CO emitting region. The emission of all the observed molecules other than CO and 13CO must originate from a very small volume compared with the entire CO envelope of NGC 7027. Since the central 10″ region is an ionized region, the molecular emission region (except CO) must be geometrically thin (ΔR = 1″ – 2″) and must be close to the ionization front.

We present 12CO J=1-0 observations of a small sample of evolved stars to investigate the evolution of the molecular envelope from the AGB phase to the formation of the PN.

The discovery of many dense, dusty condensations in the Helix Nebula, NGC 7293, by Meaburn et al. (1992) was the first direct evidence of the real inhomogeneity of the medium of a planetary nebula. On account of the small distance of the Helix nebula from the Sun (~200pc), the knots (cometary globules) can be resolved from the ground and studied in detail from HST imaging (O'Dell & Handron, 1996). The condensations typically have a projected diameter of <2″ and hence sizes of <6 x 1015 cm. The condensations consist of a dusty core, visible as absorption against the background high ionization central region of the nebula for the foreground globules, and with a bow-shaped ionization front, strong in low ionization emission. The emission is displaced in the direction towards the central star, often with an outwardly-directed radial tail.

We report high resolution CO (1–0) line imaging of a prominent cometary globule in the Helix Nebula. The results confirm earlier observations (Huggins et al. 1992) that globules consist of dense condensations of neutral gas embedded in the ionized nebula, and they provide new constraints on the structure and kinematics of the gas.

We present a chemical model of the central torus around CRL 618 in order to confirm that long-chained hydrocarbons and benzene are formed at detectable abundances. We reproduce the observed abundances of these molecules comparatively well. We infer from our calculations that CRL 618 must be a young PPN, with a post-AGB age of approximately 600 yr.

I review some aspects of the infrared continuum emission of planetary nebulae, with particular emphasis on the various dust features. The original presentation with colour figures can be found at http://www.iras.ucalgary.ca/~volk/iau209.html.

Shells of planetary nebulae (PNe) are usually modeled applying photoionization only although there is a significant fraction of dust embedded in the nebular shells. The calculated temperature and dust-to-gas ratio from IRAS colors is “faked” by the IR flux continuum contribution from the gas and by transiently heated dust grains. Also the effects of grain growth time and thus maximum grain size lead to strong deviations from the model input values, adopting the IRAS data fits.

Silicates are the most abundant dust component in our Universe. For a long time it was generally assumed that around evolved stars only amorphous silicates were present. The infrared space observatory (ISO; Kessler et al. 1996) discovered that the abundance of crystalline silicates can be quite significant. Thanks to ISO it is now for the first time possible to determine the exact chemical composition of the silicates, and in contrast to the amorphous silicates, the crystalline silicates turned out to be very Fe-poor (i.e. forsterite and enstatite). Careful investigations of the spectra of crystalline silicates in several conditions, both in laboratory and in space, have taken place. It was found that not only the abundance of the crystalline silicates is related to the spatial distribution of the circumstellar dust, but also the shape of the features differs between sources with and without a disk-like dust distribution. Because of the many spectral features of the crystalline silicates one can easily determine temperatures and mass ratios for enstatite and forsterite. But even single features contain interesting information. The position and FWHM of the 69.0 micron forsterite feature, might be used as a new diagnostic of characteristic temperatures of (crystalline) silicate dust, while the 33.6 micron forsterite feature bears evidence for its formation history.

We present mid infrared spectra of two planetary nebulae with H poor central stars that exhibit the “21” and “30” μm emission features. These features are found in carbon rich dust surroundings. The presence of these features shows that these sources have suffered an extended period of carbon rich mass loss before becoming H poor. We discuss the occurrence of both O-rich and C-rich dust in the sample of planetary nebulae observed with the Infrared Space Observatory.

We have analyzed Infrared Space Observatory Short Wavelength Spectrometer (ISO–SWS) observations for 16 Wolf–Rayet ([WR]) planetary nebulae (PNe) with the aim of identifying the dust features present in this group of objects. We have found that Polycyclic Aromatic Hydrocarbon (PAH) molecular bands (which are present in most of the observed [WR] PNe) are more frequent than crystalline silicate features.

It has been well known since the IRAS mission that dust emission represents a significant fraction of the energy output from PNe (Zhang & Kwok 1991). Although the dust component in PNe was long thought to be due to the remnants of the envelopes of AGB stars (Kwok 1982), we now know that dust in PNe has a much richer chemical composition. In addition to amorphous silicates and SiC features commonly seen in AGB stars, PNe have been found to have strong aromatic infrared features (Russell et al. 1977), crystalline silicate features (Waters et al. 1997), and an unidentified emission feature at 30 μm (Forrest et al. 1981). In this paper, we show the ISO spectra of a number of PNe illustrating the diverse dust chemistry in PNe.

Carbon-rich AGB stars, protoplanetary nebulae (PPNe), and PNe show a variety of dust features attributed to carbon-based compounds. Of particular interest are the three dust features peaking near 21, 26 and 33 μm. The 26 and 33 μm features (collectively the “30 μm feature”; see Volk et al. 2002) are always observed together. The 21 μm feature is weak in the AGB phase and becomes more prominent in the PPN phase, which implies carrier evolution. We wish to see how this evolution continues into the PN phase from detailed modelling of the infrared spectra of PNe.

We report the discovery of carbonates in the Planetary Nebulae NGC 6302 and NGC 6537 (Kemper et al. 2002). In the ISO LWS spectra far-infrared features have been identified with calcite and dolomite by comparison with laboratory spectra of these minerals. This is the first time that carbonates have been identified outside the solar system. In a follow-up study (Kemper et al., in prep.) a detailed analysis of the mineral composition of the dust in NGC 6302 is presented.

We have computed photoionization models of planetary nebulae containing dust. Photoelectric heating by small grains has an important effects on the temperature structure of the nebulae and could solve a number of problems that have found no satisfactory solution so far.

The late stages of stellar evolution of stars with low or intermediate mass (1 M⊙ to 8 M⊙) are characterized by extensive mass loss, which can be traced in their circumstellar environment. Here we consider the case of IRAS 16342-3814, a bipolar proto planetary nebula which shows very high velocity (~ 50 km s-1) OH maser emission (Sahai et al. 1999). We present the complete Infrared Space Observatory (ISO) spectrum and an infrared TIMMI2 Q-band image of this source. Based on these data we discuss the composition and geometry of the circumstellar dust in IRAS 16342-3814.

The carbon and nitrogen isotopic ratios of pre-solar SiC grains of type A+B suggest a proton-limited nucleosynthetic process as encountered, for instance, during the very late thermal pulse of post-AGB stars. We study the nuclear processes during this phase and find carbon and nitrogen isotopic ratios which can reproduce those of A+B grains. These results are still preliminary because they depend on uncertain factors such as the details of mixing during the post-AGB thermal pulse, the rates of some nuclear reactions, and the assumptions on mixing during the progenitor AGB phase.

Many carbon-rich PNe show strong emission features due to various stretching and bending modes of aromatic hydrocarbon bonds (AHB). Comparisons between ISO spectra of evolved carbon stars, proto-planetary nebulae (PPNe), and PNe have shown that the AHB features develop late in the PPN phase (Kwok et al. 1999). In order to test the formation history of this carbonaceous component, it would be desirable to image the spatial distributions of the dust components. The Gemini Telescopes, optimized for infrared observations with adaptive optics capabilities, offer the unique opportunity for such observations. In this paper, we present the preliminary results of imaging of NGC 7027 and BD+30°3639 with the OSCIR mid-infrared camera at Gemini North.