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.     

Broad absorption lines in the spectra of planetary nebulae as indicators of fast flows

The possibility of using broad absorption lines (on the order of 100 km/sec or more) of the Mg II 2800 doublet, formed in regions of planetary nebulae with fast collimated flows and, probably, manifested against the background of the short-wavelength wing of the Balmer continuum of the spectrum as indicators of such flows, is discussed. For this purpose, using existing analytical (dynamical) models of mass-loaded isothermal flows and numerical (photoionization) models of planetary nebulae, theoretical line profiles are calculated in the Sobolev approximation and their dependence on certain parameters is shown. An advantage of the proposed method consists in the possibility of revealing fast flows in faint, star-like planetary nebulae. Translated from Astrofizika, Vol. 42, No. 4, pp. 519–530, October–December, 1999.     

On the pseudo-resonance absorption lines in planetary nebulae

The possiblity of the excitation of a new class of spectral lines, so calledpseudo-resonance absorption lines in the conditions of planetary nebulae, is predicted. These lines are formed by permitted atomic transitions from the metastable levels to the upper subordinate, but not metastable, levels. To observe pseudo-resonance lines of nebular origin is possible only on the spectra of the nuclei of planetary and diffuse nebulae in the form of absorption lines.The preliminary list of pseudo-resonance lines is given in Table I; all of them are located in the region of vacuum ultra-violet (1000–3000 Å), therefore, their observation is possible only under space conditions.In stellar photospheres as well as in the interstellar medium the formation of pseudo-resonance lines is impossible.     

Helium recombination spectra as temperature diagnostics for planetary nebulae

Electron temperatures derived from the He  i recombination line ratios, designated T _e(He  i ), are presented for 48 planetary nebulae (PNe). We study the effect that temperature fluctuations inside nebulae have on the T _e(He  i ) value. We show that a comparison between T _e(He  i ) and the electron temperature derived from the Balmer jump of the H  i recombination spectrum, designated T _e(H  i ), provides an opportunity to discriminate between the paradigms of a chemically homogeneous plasma with temperature and density variations, and a two-abundance nebular model with hydrogen-deficient material embedded in diffuse gas of a 'normal' chemical composition (i.e. ∼solar), as the possible causes of the dichotomy between the abundances that are deduced from collisionally excited lines and those deduced from recombination lines. We find that T _e(He  i ) values are significantly lower than T _e(H  i ) values, with an average difference of  〈 T _e(H  i ) − T _e(He  i )〉= 4000 K  . The result is consistent with the expectation of the two-abundance nebular model but is opposite to the prediction of the scenarios of temperature fluctuations and/or density inhomogeneities. From the observed difference between T _e(He  i ) and T _e(H  i ), we estimate that the filling factor of hydrogen-deficient components has a typical value of 10⁻⁴. In spite of its small mass, the existence of hydrogen-deficient inclusions may potentially have a profound effect in enhancing the intensities of He  i recombination lines and thereby lead to apparently overestimated helium abundances for PNe.     

Far infrared emission from three new planetary nebulae

As dust emission in the far infrared (FIR) is a characteristic property of planetary nebulae we searched the Infrared Astronomical Satellite (IRAS) point-source catalogue for confirmatory evidence on the two new possible planetary nebulae S 68 and 248 - 5 identified by Fesen, Gull & Heckathorn (1983) and the high-excitation planetary nebula 76 + 36 detected by Sanduleak (1983). We identify the nebulae 248 - 5 and 76 + 36 with IRAS sources 07404 - 3240 and 17125 + 4919, respectively and have determined their dust temperature, total FIR emission and optical depth. We also set a lower limit ranging in value from 1.2 × 10^-6 to 3.7 × 10^-5 forM _dust /M _bd of the nebula 248 - 5 depending on whether its grain material is silicate or graphite. S 68 could not be identified with an IRAS source.     

The features of infra-red emission of planetary nebulae in the inner galaxy

414 galactic PN with measured infra-red luminosities (302 of which are located within the sector of galactic longitudes from 345° to 15°) are investigated. All nebulae are subdivided into four mass classes and different distance scales for each class are used. Planetary nebulae located in the inner Galaxy are selected. The galactic radial gradients of infra-red luminosities and excesses are obtained for each nebula mass class. Possible explanations of the effect are discussed.     

Magnetic fields in planetary nebulae and post-AGB nebulae

Magnetic fields are an important but largely unknown ingredient of planetary nebulae. They have been detected in oxygen-rich asymptotic giant branch (AGB) and post-AGB stars, and may play a role in the shaping of their nebulae. Here we present SCUBA submillimetre polarimetric observations of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two carbon-rich nebulae, to determine the geometry of the magnetic field by dust alignment. Three of the four sources (NGC 7027, 6537 and 6302) present a well-defined toroidal magnetic field oriented along their equatorial torus or disc. NGC 6302 may also show field lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the field aligns with the torus, whilst an other part approximately aligns with the polar outflow. It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for a disorganized field in the south-west corner, where the SCUBA shows an indication for an outflow. The findings show a clear correlation between field orientation and nebular structure.     

PAH features in the ISO SWS01 spectra of [WR] planetary nebulae

We present Infrared Space Observatory (ISO)Short Wavelength Spectrometer (SWS) observations for 16Wolf–Rayet ([WR]) planetary nebulae (PNe) in the range from 2.4 to16.5 m with the aim of analyzing the dust features present inthis group of objects. We have found that Policyclic AromaticHydrocarbon (PAH) molecular bands are present in most of the observed[WR] planetary nebulae with clear exception for K 2–16 among latetype [WC] stars.     

Flows in clumpy planetary nebulae

The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed.     

Filamentary structures in planetary nebulae

We have studied small-scale, filamentary features in 14 planetary nebulae and found that some structures are recurrent and shaped like the letters V and Y, with the apex or stem pointing toward the central parts of the nebula. Two such filaments containing dust, one in NGC 3132 and one in NGC 7293, were investigated in more detail. The mass and density of the filaments were obtained from extinction measurements, and their physical properties were derived. We propose that the structures are confined by magnetic fields, and derive magnetic field strengths of about 10⁻⁸ T, in line with earlier estimates. We also estimate the magnitude of the electric currents that we expect are generated in these dynamic systems. We propose a theory where the magnetic fields control the sculpting and evolution of small-scale filaments. This theory demonstrates how the substructures may form magnetized flux ropes that are twisted around each other, in the shape of double helices. Similar structures, and with similar origin, are found in many other astrophysical environments.     

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.     

A mechanism for the production of planetary nebulae

C¹² stars in the range 1.04–1.55M are evolved to simulate the core evolution of the possible precursors of planetary nebulae. The nuclear shell burning in stars above 1.2M advances to within about 0.2M of the surface, where the intense radiation interacts with the surface matter and causes mass loss. Comparison between our theoretical results and observations suggests that this may be a mechanism by which planetary nebulae are formed.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Emission lines due to interstellar dust in the visible spectra of nebulae

It is suggested that certain sharp lines in the visible and near ultraviolet spectra of nebule may be due to fluorescence from grains. Evidence is presented that some of these lines could arise from MgO or CaO grains. Some sharp diffuse interstellar bands may also appear in the spectra of nebulae.