Search Results

Infrared spectroscopic observations of the circumstellar envelopes of AGB and post-AGB stars have identified a variety of inorganic grains including amorphous silicates, crystalline silicates, silicon carbide, carbonates, corundum, spinels and possibly rutiles. Isotopic studies of meteorites have also identified similar species of presolar origin. The existence of aromatic and aliphatic features in the spectra of post-AGB stars suggests that organic compounds in solid-state form are made during the post-AGB phase of stellar evolution. These features show similarity with the IR spectra of kerogen, which is also found in meteorites. These grains therefore represent an important link between stars and the solar system.

Recent developments in infrared spectroscopy have made possible the detection of the stretching and bending modes of organic molecules in space. Infrared emission bands of the C-C and C-H stretching and bending modes of aromatic compounds have been widely observed in carbon-rich planetary nebulae, the descendents of carbon stars. Infrared spectra of the transition objects between carbon stars and planetary nebulae, called proto-planetary nebulae, have shown definite signatures of a variety of aliphatic side-groups attached to aromatic rings. This suggests that molecular synthesis is actively ongoing in the circumstellar environment and the chemical nature of these compounds evolve over time scales as short as several hundred years.

Over the last decade, stellar mass loss has become recognized as an important factor in the evolution of stars. The magnitude of the mass loss rate is found to be greatest amongst high luminosity stars at both the red and blue sides of the HR diagram. Planetary nebulae (PN), which result from an evolutionary phase during which the parent star traverses from the red side to the blue in a relatively short time scale, are likely to be affected by these processes. In this talk, I shall review the mass loss processes relevant to the PN phase and discuss their affects on the formation and evolution of PN.

An unidentified emission feature at 21 μm has been detected in the IRAS Low Resolution Spectra (LRS) of 5 IRAS sources (Kwok, Volk, and Hrivnak 1989, Hrivnak and Kwok 1991). The sources are generally found to be F and G supergiants with cool, detached dust shells. We have searched for additional 21 μm sources in the LRS database and have obtained ground-based UKIRT spectra at 10 and 20 μm in an attempt to confirm the LRS feature.

Several Proto–Planetary Nebulae (PPN) candidates have been observed at the Canada France Hawaii Telescope with the High Resolution Camera (HRCam) (M c Clure et al. 1989, PASP, 101, 1156). Candidates were selected based on their IRAS [12]–[25] and [25]–[60] colors and their lack of variability. With HRCams' small angular pixel size (0.13″/pixel) and active optics we achieved an average angular resolution of 0.65″ in V and I waveband images.

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.

The progress in planetary nebulae (PN) research reported in this symposium is reviewed in the context of our current understanding of the PN phenomenon.

Over the past decade, we have come to realize that mass loss on the asymptotic giant branch (AGB) plays a significant role in the formation of planetary nebulae (PN). Mass ejected during the AGB can now be observed in haloes of PN and we believe that the main shell of PN is formed by the interaction of this material with a later-developed central-star wind. In this review, we show that the evolution from AGB to PN can be traced in a continuous infrared sequence. This sequence predicts properties of proto-PN which allow them to be identified.

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.

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.

Our current state of understanding of the planetary nebulae (PNe) phenomenon is reviewed in the context of modern stellar evolution and dynamical models. Also discussed are a number of new problems that have emerged as the result of recent space (x-ray, UV, optical, and infrared) observations. The roles of PNe in the chemical (atomic, molecular, and solid-state) enrichment of the galaxy and as tracers of the large scale structure of the Universe are also discussed.

Seven compact PNe with high radio surface brightness (and therefore presumably young) were observed with the HST WFPC2 under cycle 8 program 8307 (PI: S. Kwok). Observations were made with three narrow-band filters, F656N (Hα), F658N ([NII]) and F502N ([OIII]). The [NII] images of six of the nebulae are shown in Figure 1.

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.

Planetary nebulae are formed as the result of the interaction between a slow stellar wind from the asymptotic giant branch progenitor and a later-developed fast outflow from the central star. Many of the morphological and kinematic properties of planetary nebulae have been successfully explained by this interacting stellar winds model. The observed diverse morphologies of planetary nebulae can also be understood if the slow wind is not spherically symmetric. However, new observational features such as collimated outflows and multi-polar lobes suggest that the fast wind may be non-isotropic and time variable. The possible roles of magnetic fields and rotation may play in the formation of these features are discussed.

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.

We present an indirect method to probe the dust distribution in PNe. Using the free-free continuum flux density and Hβ recombination line flux relationship and the Case B Hα/Hβ line ratio, we determined the expected Hα flux from the radio continuum maps. The dust optical depth distribution of each planetary nebula was then derived from the expected to observed Hα flux ratio. With HST WFPC2 and VLA A-array observations, dust optical depth maps with resolution as high as ~ 0.1″ can be obtained.

Sub-arcsecond (0.7″) V and I images have been obtained of 13 new proto-planetary nebulae (PPN). The observations were made with the image-stabilization camera (HRCam) on the 3.6 m Canada-France-Hawaii Telescope. The goal of the program is to study the mass-loss history of the stars and to determine when in the evolution the shaping seen in PN occurs.

The unidentified emission feature at 21 μm is now observed in 12 sources, all being objects in transition between the asymptotic giant branch and planetary nebulae phases. The relations between the 21 μm and other emission features, such as the PAH features and the broad 30 μm feature, and the possible origins of the 21 μm feature are discussed.

Planetary nebulae have recently been shown to be useful as standard candles (Ciardullo et al. 1989, ApJ, 339, 53; Jacoby 1989, ApJ, 339, 39). Distances to many galaxies have been determined by fitting a planetary nebula luminosity function (PNLF) to observations of the OIII 5007å line of PNe. Here, the effect of the core mass distribution on the determination of the luminosity function is investigated and a technique for interpolating between model evolutionary tracks is discussed.

A model to simulate the entire spectrum (1000 Å to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ionized, dust, and molecular components of the object are modeled using geometric parameters obtained from visible, radio, infrared, and CO data. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and ff continuum radiations, two-photon radiation, dust continuum radiation, and molecular rotational and vibrational transitions. The dust component is assumed to be heated by a combination of direct starlight and the line and continuum radiation from the ionized nebula. The molecular component of the nebula is coupled to the dust component through the stimulated absorption of the dust continuum radiation. Specifically, we compare the predicted fluxes of the CO rotational lines and the 179.5 μm water rotational line to those observed by the Infrared Space Observatory satellite (Liu et al. 1996, A&A in press).