Statistics of planetary nebulae with [WR] central stars

The status of planetary nebulae with Wolf-Rayet type central stars([WR] PN) remains one of the most important problems inthe investigation of planetary nebulae. We cannot claim to understand theevolution of low and intermediate-mass stars without answering the questionhow [WR] PN are created.Analyzing the statistical properties of the whole population of [WR] PNand comparing them to other planetary nebulae (non-[WR] PN) bringsimportant information on their origin and evolutionary status. In thisarticle I will summarize our results of this type of studies and show whatlimits they put on the possible evolutionary routes of [WR] PN creation.     

[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).     

Shaping planetary nebulae: is it different for [WR] stars?

This review discusses the physics of the formation of planetarynebulae around low mass WR stars, or [WR] stars. It especially focuseson the differences which can be expected due to the differentcharacter of the fast winds from these [WR] stars. Their fast windsare more massive and are highly H deficient and metal enrichedcompared to the winds of normal central stars of planetarynebulae. This is expected to lead to faster expansion velocities forthe nebulae and a longer momentum-driven phase in the evolution of thewind-driven bubble, leading to more turbulent nebulae. Theobservational evidence also shows that the process which produces the[WR] stars is unlikely to influence the onset of aspherical mass loss,something which can be used as a test for models for aspherical massloss from AGB and post-AGB stars. Finally it is shown that thenebular characteristics rule out a very late He shell flash as theorigin of most [WR] stars.     

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Brc/HI(14-6) line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot he used to judge whether a star is a Be star or not.     

Properties of planetary nebulae with WR central stars

A review of the observational properties of the Wolf-Rayet central stars is given.     

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory （ISO） Short-Wavelength Spectrometer （SWS） spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Brα/HI（14-6） line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot be used to judge whether a star is a Be star or not.     

[OIII] Electron temperatures in planetary nebulae

Relative level populations for O III, derived using electron impact excitation rates calculated with the R-matrix code, are used to deduce the electron-temperature-sensitive emission-line ratioR=I(2s ²2p^{2 1}D–2s²2p21S)/I(2s² 2p23P_1,2–2s²2p^{2 1}D) =I(4363 Å)/I(4959 + 5007 Å) for a range ofTe = (7500–20000 K) applicable to planetary nebulae. Electron temperatures deduced from the observed values ofR in several planetary nebulae are in excellent agreement with those determined fromTe-sensitive line ratios in other species, including CIII]/C [II], [NII] and [ArIII], which provides support for the accuracy of the atomic data adopted in the level population calculations.     

ISO SWS Spectroscopy of WR 146 (WC6+O)

We have obtained ISO SWS spectroscopy of WR 146 (WC6+O) covering the wavelength range 2.6-20 μm. WC6 wind emission is observed in numerous lines of He II and C IV, as well as in the [Ne III] 15.5 μm line, but not in [Ne II] 12.81 or [Ne V] 14.32 μm. An analysis of these spectra (and complementary radio and optical data) yields for the WC6 star: v∞ = 2700 km s-1; M=2.6×10-5M⊙yr-1; C/He = 0.15; and a neon abundance bound of 3.4×10-3≤Ne/He≤6.8×10-3. The neon abundance is close to that predicted in stellar evolution models of WC stars. This revised version was published online in August 2006 with corrections to the Cover Date.     

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 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.     

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.     

Integrated fluxes for emission lines in the ultraviolet spectra of several planetary nebulae

The IUE satellite observatory has been used to obtain absolutely-calibrated emission line fluxes for diagnostic lines of multiply-ionized C, N, O, Si, Ne, and Ar which occur in the ultraviolet spectral region of planetary nebulae. These data, when combined with data from the blue, visual, and near infrared, will provide improved estimates of ionic concentrations, plasma temperatures and densities, and elemental abundances.     

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.     

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'.     

Ionization stratification in [WR] winds

We summarize currently available evidence for ionization stratification in the winds of [WR] stars and point out that ionization stratification may be important to explain with radiation-driven wind theory the winds of [WR] stars, while also providing an empirical tool to investigate [WR] wind structures.     

Orientation of planetary nebulae within the Galaxy

Narrow-band CCD images of 209 axially symmetrical planetary nebulae (PNe) have been examined in order to determine the orientation of their axes within the disc of the Galaxy. The nebulae have been divided into the bipolar (B) and elliptical (E) PNe morphological types, according to the scheme of Corradi &38; Schwarz. In both classes, contrary to the results of Melnick &38; Harwit and Phillips we do not find any strong evidence for non-random orientations of the nebulae in the Galaxy. Compared with previous work in this field, the present study takes advantage of the use of larger and morphologically more homogeneous samples and offers a more rigorous statistical analysis.