Axisymmetry in protoplanetary nebulae – II. A near-infrared imaging polarimetric survey

In an imaging polarimetry survey of candidate post-AGB stars, scattering envelopes are detected around 20 objects. These envelopes represent the final mass-loss phases at the end of the AGB. In all cases, evidence for axisymmetry in the dust density is seen, suggesting that the presence of an axisymmetric outflow may be a ubiquitous phenomenon of the AGB to post-AGB transition. We use the polarized flux images to classify the objects into detached shell, bipolar and unresolved targets. Modelling based on a simple axisymmetric shell geometry supports the contention that post-AGB objects fall into one of two classes that differ in the amount of dust in the circumstellar environment: the detached shells correspond to stars with an optically thin expanding circumstellar envelope (CSE) whereas the bipolar and unresolved targets have optically thick dust structures, probably in the form of discs, which remain bound to the star, rather than partaking in the expansion of the AGB CSE. It is suggested that this bifurcation in morphology is rooted in the presence or absence of a binary companion, which determines whether or not a disc forms. Because the detached shell objects also appear axisymmetric, an additional mechanism for generating the axisymmetry, such as a magnetically shaped outflow, is needed if they do indeed have single star progenitors.     

The dual dust chemistries of planetary nebulae with [WCL] central stars

The rather rare class of central stars of planetary nebulae that show very low-excitation Wolf–Rayet spectra has been a subject of great interest, particularly in the infrared, since its discovery in the late 1960s. Further peculiarities have been found with the advent of infrared spectroscopy from ISO . Notably, these objects simultaneously betray the presence of regions of carbon-rich and oxygen-rich dust chemistry. We compare and contrast complete ISO spectra between 2 and 200 μm of a sample of six [WC8] to [WC11] central stars, finding many similarities. Among this sample, one star provides strong evidence of quasi-periodic light variations, suggestive of a dust cloud orbiting in a plane from which we view the system.     

Symbiotic stars with Wolf-Rayet spectra as protoplanetary nebulae?

Symbiotic systems, in particular symbiotic novae, have been suggested to be very early stages of planetary nebulae. Some of them have been described as going through a Wolf-Rayet phase. We argue that there may be a direct relation between symbiotic objects and planetary nebulae, and that the Wolf-Rayet phase is connected to an active spell of the hot companion. Symbiotic stars could lead to planetary nebulae with two central stars with different radiation temperatures and luminosities, where each has the power to ionize a planetary nebula on its own.     

[WR] nuclei of planetary nebulae: Inferences from statistical studies of the nebulae

There are about 50 galactic planetary nebulae know to have [WR] type nuclei. We have compared their nebular properties with those of the other planetary nebulae in the Galaxy. We have found that the nebular morphological types are similarly distributed in the two groups. Bipolar nebulae constitute only 20% of the total in each group. The distribution of the nebular electron densities and abundance ratios N/O, He/H and C/O are the same in the two groups. The only marked difference is that nebular expansion velocities are larger in the group of planetary nebulae with [WR] central stars. We argue that the WR phenomenon does not preferentially occur in more massive central stars of planetary nebulae, contrary to what has been suggested in some former studies. We demonstrate that, for most of the observed [WR] type objects, the WR phenomenon cannot be triggered by a late helium shell flash event.The results of our investigation are published inAstronomy & Astrophysics 303, 893 (1995) and in the proceedings of the 2nd International Colloquium on Hydrogen-deficient Stars, C.S. Jeffery & U. Heber (eds), Astronomical Society of the Pacific Conference Series, Vol. 96, p. 209 (1996).     

Infrared photometry of eight planetary nebulae

We discuss the infrared (IR) (1.25–5 µm) photometry of eight planetary nebulae performed in 1999–2006. For all of the nebulae under study, we have firmly established IR brightness and color variations on time scales shorter than one year and up to 6–8 years. The greatest IR brightness variations were observed in IC 2149, IC 4997, and NGC 7662. Their J magnitudes varied within 0 _. ^m 2–0 _. ^m 25. In the remaining objects, the J magnitude variations did not exceed 0 _. ^m 15. All of the planetary nebulae under study exhibited IR color variations. Based on the IR photometry, we have classified the central regions of the planetary nebula NGC 1514 and of the northern part of NGC 7635 seen through a 12″ aperture as a B(3–7) main-sequence star (NGC 1514) and a ～O9.5 upper-main-sequence star (NGC 7635). The nebulae IC 4997 and NGC 7027 exhibited an excess emission (with respect to the emission from a hot source) at λ > 2.5 µm.     

Asymmetry and inhomogeneity in proto- and young planetary nebulae

I study some effects of aspherical mass loss during the last stages of the asymptotic giant branch (AGB) on the appearance of proto-planetary nebulae (proto-PNs) and young PNs. The aspherical mass loss can be small-scale inhomogeneities, and/or axially symmetric mass-loss geometry. I first examine the role of the dust opacity in the optical band on the appearance of proto-PNs. I conclude that large optical depths will be found in proto-PNs that are post-AGB stars having high equatorial mass-loss rates, which require a stellar binary companion for their existence. In these cases light from the central star will reach larger distances along and near the polar directions, leading to the appearance of an elongated reflection nebula. These proto-PNs will become bipolar PNs, i.e., PNs with two lobes and an equatorial waist between them, or extreme ellipticals, e.g., a ring but no lobes on the two sides of the equatorial plane. I then derive the conditions for the enhancement of non-radial density inhomogeneity by the propagation of the ionization front at the early PN stages. The ionization will proceed faster in the radial direction along low-density regions. The low-density regions will be heated earlier, and they will expand as a result of their higher pressures, reducing further their densities. The opposite occurs in high-density regions. The condition for this ionization instability to develop is that the ionization time difference between two directions at the same radius is longer than the sound crossing time between these two regions. This condition for the ionization front instability can be expressed as a condition on the mass-loss rate inhomogeneity, i.e., its dependence on direction.     

The infrared [WC] stars

A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class of IR-[WC]stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation.Most or all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulae indicaterecent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP)scenario. Although both the colours and the stellar characteristicspredict fast evolution, it is shown that this phase must last for10⁴ yr. The morphologies of the nebulae are discussed. Forone object in Sgr, the progenitor mass (1.3 M_⊙) is known.The stellar temperatures of the IR-[WC] stars appear much higher inlow metallicity systems (LMC, Sgr). This may be indicative of anextended `pseudo' photosphere. It is proposed that re-accretion ofejected gas may slow down the post-AGB evolution and so extend the lifetime of the IR-[WC] stars.     

Thermal emission spectra of silicates from planetary nebulae

Calculations of the grain equilibrium temperature and of the expected infrared spectra of IC 418, BD+30° 3639, NGC 6572 and NGC 7027 have been performed using dielectric constants of lunar silicates. The results have been compared with previous work on pure graphite and ice-mantle grains. Lα heating of dust followed by thermal re-emission is consistent with the large infrared excesses detected in planetary nebulae. An extra source of heating is, nevertheless, necessary to fit correctly the experimental results. It appears from the calculations that, for each object, it is possible to define theoretically the most probable nature of the emitting dust.     

The Magnetic Field Effect on Planetary Nebulae

In our previous work on the 3-dimensional dynamical structure of planetary nebulae the effect of magnetic field was not considered. Recently Jordan et al. have directly detected magnetic fields in the central stars of some planetary nebulae. This discovery supports the hypothesis that the non-spherical shape of most planetary nebulae is caused by magnetic fields in AGB stars. In this study we focus on the role of initially weak toroidal magnetic fields embedded in a stellar wind in altering the shape of the PN. We found that magnetic pressure is probably influential on the observed shape of most PNe.     

X-ray emission by a shocked fast wind from the central stars of planetary nebulae

We calculate the X-ray emission from the shocked fast wind blown by the central stars of planetary nebulae (PNe) and compare with observations. Using spherically symmetric self-similar solutions, we calculate the flow structure and X-ray temperature for a fast wind slamming into a previously ejected slow wind. We find that the observed X-ray emission of six PNe can be accounted for by shocked wind segments that were expelled during the early-PN phase, if the fast wind speed is moderate,   v ₂∼ 400–600 km s⁻¹  , and the mass-loss rate is a few times  10⁻⁷ M_⊙ yr⁻¹  . We find, as proposed previously, that the morphology of the X-ray emission is in the form of a narrow ring inner to the optical bright part of the nebula. The bipolar X-ray morphology of several observed PNe, which indicates an important role of jets, rather than a spherical fast wind, cannot be explained by the flow studied here.     

Near-infrared spectroscopy of (proto)-planetary nebulae: molecular hydrogen excitation as an evolutionary tracer

We present an in-depth analysis of molecular excitation in 11 H₂-bright planetary and protoplanetary nebulae (PN and PPN). From newly acquired K -band observations, we extract a number of spectra at positions across each source. H₂ line intensities are plotted on 'column density ratio' diagrams so that we may examine the excitation in and across each region. To achieve this, we combine the shock models of Smith, Khanzadyan & Davis with the photodissociation region (PDR) models of Black & van Dishoeck to yield a shock-plus-fluorescence fit to each data set.
Although the combined shock + fluorescence model is needed to explain the low- and high-energy H₂ lines in most of the sources observed (fluorescence accounts for much of the emission from the higher-energy H₂ lines), the relative importance of shocks over fluorescence does seem to change with evolutionary status. We find that shock excitation may well be the dominant excitation mechanism in the least evolved PPN (CRL 2688 – in both the bipolar lobes and in the equatorial plane) and in the most evolved PN considered (NGC 7048). Fluorescence, on the other hand, becomes more important at intermediate evolutionary stages (i.e. in 'young' PN), particularly in the inner core regions and along the inner edges of the expanding post-asymptotic giant branch (AGB) envelope. Since H₂ line emission seems to be produced in almost all stages of post-AGB evolution, H₂ excitation may prove to be a useful probe of the evolutionary status of PPN and PN alike. Moreover, shocks may play an important role in the molecular gas excitation in (P)PN, in addition to the low- and/or high-density fluorescence usually attributed to the excitation in these sources.     

PAH's and Crystalline Silicates in Planetary Nebulae

An overview is given of the PAH and crystalline silicate emissions seen in the SWS guaranteed-time programme on planetary nebulae. Of the 9 objects on which good continuum measurements were obtained above 29 μm, 7 show evidence of olivine emission at 33.5 μm. PAH emission is seen in 5 of these objects, 3 objects show both PAH's and olivines. The presence of both types of features in a single object points to separate carbon-rich and oxygen-rich episodes of mass ejection. The spectrum of the nebula NGC 6302 shows a wealth of features in the range between 20 and 45 μm, many of which can be identified with olivines or pyroxenes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Detecting planets in planetary nebulae

We examine the possibility of detecting signatures of surviving Uranus/Neptune-like planets inside planetary nebulae. Planets that are not too close to the stars (orbital separation larger than ∼5 au) are likely to survive the entire evolution of the star. As the star turns into a planetary nebula, it has a fast wind and strong ionizing radiation. The interaction of the radiation and wind with a planet may lead to the formation of a compact condensation or tail inside the planetary nebula, which emits strongly in H α , but not in [O  iii ]. The position of the condensation (or tail) will change over a time-scale of ∼10 yr. Such condensations might be detected with currently existing telescopes.     

Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (10⁴–10⁵ cm⁻³) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC₃N and SiC₂ are in good agreement with observations, while those of CN, HCO⁺, CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH₂, CH₂⁺ , HCl, OH and H₂O are anticipated to be highly abundant in these objects.     

Bipolar jets in planetary nebulae: An analytical model

The various and hitherto partially unsolved problems relative to the origin of bipolar jets or highly collimated fast outflows in planetary nebulae are reviewed within the framework of a stationary magnetohydrodynamic model. In order to explain the observations of high polar velocities and the presence of polar blobs or knots in planetary nebulae, theoretical models are proposed taking into account both a large scale azimuthal magnetic field and an anisotropic turbulent velocity field. The models predict equatorial-to-polar density ratios which are rather small, in the range 2 to 3. Conversely, the polar-to-equatorial velocity contrasts are higher, with typical values upto 10. Thus thead hoc hypothesis implicit in the literature that the density contrast is varying in inverse ratio to the velocity one, does not seem well adapted to the bipolar jet phenomenon in planetary nebulae. We point out, therefore, that the bipolar jets have to be considered as a transient aspect of a very complex phenomenon. The model can be applied to objects such as He 2–104 or Mz3, M2–9.     

The dynamics of double-envelope planetary nebulae

A number of criteria are elaborated based on the careful analysis of nebulae images which confirm the version of the origin of double-envelope planetary nebulae by means of dynamical separation but not by multiple ejection. The importance of stellar winds in the origin of the gigantic halos around double envelope nebulae is outlined. The problem concerning the existence of two types of filamentary planetary nebulae (Figure 8) is raised: type A, filaments are the fragmentations of the Rayleigh-Taylor instability, the result of dynamical interaction of the envelope with the outer interstellar matter (NGC 6543), and type B, the origin of filaments is connected with the Magnetic field fluctuations (A 43, A 72).The possibility of the origin of three-envelope nebulae in the framework of the dynamic separation version is discussed (NGC 7293). Attention is paid to the impossibility of outbursts with low velocities (20–30 km s^–1) from hot stars, i.e., the nuclei of nebulae.     

Bubbles in planetary nebulae and clusters of galaxies: jet bending

We study the bending of jets in binary stellar systems. A compact companion accretes mass from the slow wind of the mass-losing primary star, forms an accretion disc and blows two opposite jets. These fast jets are bent by the slow wind. Disregarding the orbital motion, we find the dependence of the bending angle on the properties of the slow wind and the jets. Bending of jets is observed in planetary nebulae which are thought to be the descendants of interacting binary stars. For example, in some of these planetary nebulae, the two bubbles (lobes) which are inflated by the two opposite jets are displaced to the same side of the symmetry axis of the nebula. Similar displacements are observed in bubble pairs in the centre of some clusters and groups of galaxies. We compare the bending of jets in binary stellar systems with that in clusters of galaxies.     

A search for CO+ in planetary nebulae

We have carried out a systematic search for the molecular ion CO⁺ in a sample of eight protoplanetary and planetary nebulae in order to determine the origin of the unexpectedly strong HCO⁺ emission previously detected in these sources. An understanding of the HCO⁺ chemistry may provide direct clues for the physical and chemical evolution of planetary nebulae. We find that the integrated intensity of the CO⁺ line may be correlated with that of HCO⁺, suggesting that the reaction of CO⁺ with molecular hydrogen may be an important formation route for HCO⁺ in these planetary nebulae.     

Submillimetre photometry of post-asymptotic giant branch stars

Stars in the post-asymptotic giant branch (post-AGB) phase of evolution are surrounded by detached circumstellar envelopes containing dust which emits thermally in the mid- and far-infrared. Here we present 850-μm SCUBA photometry of nine candidate post-AGB stars. All targets are detected at 850 μm and we use these fluxes to estimate the envelope dust masses and, by comparison with the 100-μm IRAS fluxes, the dust emissivity index.     

Radio planetary nebulae in the Magellanic Clouds

We report the extragalactic radio-continuum detection of 15 planetary nebulae (PNe) in the Magellanic Clouds (MCs) from recent Australia Telescope Compact Array+Parkes mosaic surveys. These detections were supplemented by new and high-resolution radio, optical and infrared observations which helped to resolve the true nature of the objects. Four of the PNe are located in the Small Magellanic Cloud (SMC) and 11 are located in the Large Magellanic Cloud (LMC). Based on Galactic PNe the expected radio flux densities at the distance of the LMC/SMC are up to ∼2.5 and ∼2.0 mJy at 1.4 GHz, respectively. We find that one of our new radio PNe in the SMC has a flux density of 5.1 mJy at 1.4 GHz, several times higher than expected. We suggest that the most luminous radio PN in the SMC (N S68) may represent the upper limit to radio-peak luminosity because it is approximately three times more luminous than NGC 7027, the most luminous known Galactic PN. We note that the optical diameters of these 15 Magellanic Clouds (MCs) PNe vary from very small (∼0.08 pc or 0.32 arcsec; SMP L47) to very large (∼1 pc or 4 arcsec; SMP L83). Their flux densities peak at different frequencies, suggesting that they may be in different stages of evolution. We briefly discuss mechanisms that may explain their unusually high radio-continuum flux densities. We argue that these detections may help solve the 'missing mass problem' in PNe whose central stars were originally  1–8 M_⊙  . We explore the possible link between ionized haloes ejected by the central stars in their late evolution and extended radio emission. Because of their higher than expected flux densities, we tentatively call this PNe (sub)sample –'Super PNe'.