8–13 μm dust emission features in Galactic bulge planetary nebulae

A sample of 25 infrared-bright planetary nebulae (PNe) towards the Galactic bulge is analysed through 8–13 μm spectroscopy. The classification of the warm dust emission features provides a measure of the C/O chemical balance, and represents the first C/O estimates for bulge PNe. Out of 13 PNe with identified dust types, four PNe have emission features associated with C-based grains, while the remaining 9 have O-rich dust signatures. The low fraction of C-rich PNe, ≲ 30 per cent, contrasts with that for local PNe, around ∼ 80 per cent, although it follows the trend for a decreasing frequency of C-rich PNe with galactocentric radius (Paper I). We investigate whether the PNe discussed here are linked to the bulge stellar population (similar to type IV, or halo, PNe) or the inner Galactic disc (a young and super-metal-rich population). Although 60 per cent of the PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfies our criteria, and they are probably linked to the inner Galactic disc. In the framework of single star evolution, the available information on bulge PNe points towards a progenitor population similar in age to that of local PNe (type I PNe are found in similar proportions), but super-metal-rich (to account for the scarcity of C-rich objects). Yet the metallicities of bulge PNe, as inferred from [O/H], fail to reach the required values – except for the C-rich objects. It is likely that the sample discussed here is derived from a mixed disc/bulge progenitor population and dominated by type IV PNe, as suggested by Peimbert. The much higher fraction of O-rich PNe in this sample than in the solar neighbourhood should result in a proportionally greater injection of silicate grains into the inner Galactic medium.     

Hyperfine splitting of [Al vi] 3.66 μm and the Al isotopic ratio in NGC 6302

The core of planetary nebula NGC 6302 is filled with high-excitation photoionized gas at low expansion velocities. It represents a unique astrophysical situation in which to search for hyperfine structure (HFS) in coronal emission lines from highly ionized species. HFS is otherwise blended by thermal or velocity broadening. Spectra containing  [Al  vi ] 3.66 μm ³P₂←³P₁  , obtained with Phoenix on Gemini South at resolving powers of up to 75 000, resolve the line into five hyperfine components separated by 20–60 km s⁻¹ as a result of the coupling of the   I = 5/2  nuclear spin of ²⁷Al with the total electronic angular momentum J . The isotope ²⁶Al has a different nuclear spin of   I = 5  , and a different HFS, which allows us to place a 3σ upper limit on the ²⁶Al/²⁷Al abundance ratio of 1/33. We measure the HFS magnetic dipole coupling constants for [Al  vi ], and provide the first estimates of the electric quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition.     

The optical spectrum of the planetary nebula NGC 6543

With the Hamilton echelle spectrograph at the Lick Observatory, emission-rich spectral lines of the planetary nebula NGC 6543 were secured in the wavelength range from 3550 to 10 100 Å. We chose two bright regions, ∼8 arcsec east and ∼13 arcsec north of the central star, the physical conditions and chemical abundances of which may differ as a result of the different physical characteristics involving the mass ejection of different epochs. By combining Hamilton echelle observations with archive UV data secured with the International Ultraviolet Explorer ( IUE ), we obtain improved diagnostics and chemical compositions for the two observed regions. The diagnostic diagram gives the average value of T _e=8000∼8300 K, and the electron number density near N _e∼5000 cm⁻³ for most ions, while some low-excitation lines indicate much higher temperatures, i.e. T _e∼10 000 K. With the construction of a photoionization model, we try to fit the observed spectra in a self-consistent way: thus, for most elements, we employ the same chemical abundances in the nebular shell; and we adopt an improved Sobolev approximation model atmosphere for the hydrogen-deficient Wolf–Rayet type central star. Within the observational errors, the chemical abundances do not seem to show any positional variation except for helium. The chemical abundances of NGC 6543 appear to be the same as in average planetary nebulae. The progenitor star may have been an object of one solar mass, most of the heavier elements of which were less plentiful than in the Sun.     

The kinematics of NGC 4361, a Population II planetary nebula with a bipolar outflow

High-resolution, spatially-resolved profiles of H α , He  ii λ 6560 and [O  iii ] λ 5007 and deep narrow-band CCD images in the H α and [O  iii ] λ 5007 emission lines have been obtained of the planetary nebula (PN) NGC 4361. In addition, VLA-DnC λ 3.6-cm continuum observations are presented. This material allows one to explore in unprecedented detail the morphology and kinematics of this PN. The morphology of this object is complex given the highly filamentary structure of the envelope, which is confirmed to possess a low mass. The halo has a high expansion velocity that yields incompatible kinematic and evolutionary ages, unless previous acceleration of the nebular expansion is considered. However, the most remarkable result from the present observations is the detection of a bipolar outflow in NGC 4361, which is unexpected in a PN with a Population II low-mass-core progenitor. It is shown that shocks resulting from the interaction of the bipolar outflow with the outer shell are able to provide an additional heating source in this nebula.     

The Galactic disc distribution of planetary nebulae with warm dust emission features – I

We investigate the Galactic disc distribution of a sample of planetary nebulae characterized in terms of their mid-infrared spectral features. The total number of Galactic disc PNe with 8–13 μm spectra is brought up to 74 with the inclusion of 24 new objects, the spectra of which we present for the first time. 54 PNe have clearly identified warm dust emission features, and form a sample that we use to construct the distribution of the C/O chemical balance in Galactic disc PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified infrared emission bands have the highest N/O ratios, while PNe with the silicate signature have either very high N enrichment or close to none. We find a trend for a decreasing proportion of O-rich PNe towards the third and fourth Galactic quadrants. Two independent distance scales confirm that the proportion of O-rich PNe decreases from     per cent inside the solar circle to     per cent outside. PNe with warm dust are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to     They also have higher 6-cm fluxes, as expected from more evolved PNe. We show that the IRAS fluxes are a good representation of the bolometric flux for compact and IR-bright PNe, which are probably optically thick. Selection of objects with     should probe a good portion of the Galactic disc for these young, dense and compact nebulae, and the dominant selection effects are rooted in the PN catalogues.     

Scaling for the intensity of radiation in spherical and aspherical planetary nebulae

Planetary nebulae are imaged using three different physical processes. The first process is the expansion of the shell, which can be modelled by the canonical laws of motion in the spherical case and by momentum conservation when gradients of density are present in the interstellar medium. The second process is the diffusion of particles that radiate from the advancing layer. Three-dimensional diffusion from a sphere and one-dimensional diffusion with drift are analysed. The third process is the composition of an image through an integral operation along the line of sight. The framework developed is applied to A39, the Ring nebula and the etched hourglass nebula MyCn 18.     

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.     

A GLIMPSE of new and possible planetary nebulae at mid-infrared wavelengths

We have undertaken a mid-infrared (MIR) search for new planetary nebulae (PNe) using the Spitzer Space Telescope GLIMPSE Galactic plane survey. This has involved searching extant GLIMPSE data products for morphologically appropriate structures, and investigating sources having IRAS colours similar to those of Galactic PNe. We have found 12 sources which have a high probability of being high-extinction PNe, and which possess MIR and IRAS colours, and shell morphologies similar to those of previously identified Galactic nebulae. Calibrated mapping of these structures and profiles in all four of the IRAC bands (3.6, 4.5, 5.8 and  8.0 μm  ) suggests that many (if not all) of the nebulae possess at least two primary structures: an interior high surface brightness shell, corresponding to what is probably the primary ionized zone, and a much weaker halo extending to very much greater distances from the nucleus. These latter regimes are particularly evident at longer MIR wavelengths (5.8 and  8.0 μm  ), and it is probable that they trace the nebular photodissociative regimes, where emission derives from small-grain continua and/or polycyclic aromatic hydrocarbon molecular bands. This latter behaviour has also been noted in previous analyses of Galactic PNe.