Abundances of [WC] central stars and their planetary nebulaee

We review elemental abundances derived for planetary nebula (PN) WCcentral stars and for their nebulae. Uncertainties in the abundances of[WC] stars are still too large to enable an abundance sequenceto be constructed. In particular it is not clear why the hotter [WCE]stars have C and O abundances which are systematically lower than those oftheir supposed precursors, the [WCL] stars. This abundance differencecould be real or it may be due to unaccounted-for systematic effects inthe analyses. Hydrogen might not be present in [WC] star winds asoriginallysuggested, since broad pedestals observed at the base of nebular lines canplausibly be attributed to high velocity nebular components. It isrecommended that stellar abundance analyses should be carried out withnon-LTE model codes, although recombination line analyses can provideuseful insights. In particular, C II dielectronic recombinationlines provide a unique means to determine electron temperatures in cool[WC] star winds. We then compare the abundances found for PNe which have [WC] central starswith those that do not. Numerous abundance analyses of PNe have beenpublished, but comparisons based on non-uniform samples and methods arelikely to lack reliability. Nebular C/H ratios, which might be expected todistinguish between PNe around H-poor and H-rich stars, are rather similarfor the two groups, with only a small tendency towards larger values fornebulae around H-deficient stars. Nebular abundances should be obtainedwith photoionization models using the best-fitting non-LTE modelatmosphere for the central star as the input. Heavy-metal line blanketingstill needs to be taken into consideration when modeling the central star,as its omission can significantly affect the ionizing fluxes as well asthe abundance determinations. We discuss the discrepancies between nebularabundances derived from collisionally excited lines and thosederived from optical recombination lines, a phenomenon that may havelinks with the presence of H-deficient central stars.     

[WC] central stars of planetary nebula and the born-again phenomenon

[WC] central stars of planetary nebulae are members of the larger class of hydrogen-deficient central stars. The whole class constitutes about20% of all spectroscopically-known central stars. Observational connections between [WC] central stars and the born-again phenomenon show that at least a fraction of the [WC] stars can be createdthrough this scenario. However, it is unlikely that the class as a wholeevolved through this channel.In this paper the arguments against a born-again origin for the whole class of [WC] central stars of planetary nebula are outlined. It is suggested that the roleof the H-deficient weak emission lines stars might be crucial in explaining the origin of [WC]stars. It is also demonstrated how difficult it isto pin down the exact stellar parameters of these objects (which help toposition them on the HR diagram). This is due to the largely unknown distancesand to the fact that small changesin the model assumptions can have large repercussions on the derived parameters.This difficultyhampers our efforts to determine the true evolutionary position of individual [WC] central stars, as well as their relationship to one another, andtherefore pin down their origin.     

The infrared properties of barium stars

The infrared properties of barium stars are studied using published data in the K band and from IRAS . At 12 and 25 μm the emission from barium stars shows no excess over photospheric emission. Thus the claim made by Hakkila that some barium stars show evidence of the presence of warm (∼300  K ) circumstellar material is not supported. The 60-μm properties of barium stars are studied using survival analysis methods, and it is found that very few (3.7 ± 2.6 per cent) barium stars exhibit far-infrared excesses. Furthermore, it is found that the incidence of excess emission at 60 μm is lower in barium stars than for normal G and K giants. This may indicate that the mass-transfer event that is assumed to have taken place in barium stars has removed any cool circumstellar material that may have existed in these systems. Alternatively, it is suggested that the incidence of infrared excesses in normal G and K giants may have been over-estimated as a result of not fully accounting for foreground contamination by interstellar cirrus.     

Photometric variability of WC9 stars

Do some Wolf–Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf–Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf–Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during three consecutive weeks of 19 Galactic WC9 stars and one WC8 star for photometric variability in two optical bands, V and I . Of particular interest are the correlated variations in brightness and colour index in the context of carbon dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of  ∼ 0.1 μm  . However, most photometric variations occur at surprisingly low levels, and in fact almost half of our sample shows no significant variability at all above the instrumental level (  σ∼ 0.005– 0.01  mag).     

The winds of WC10 central stars of planetary nebulae

We present the results of an analysis of the winds of two WC10 central stars of planetary nebulae, CPD-56°8032 and He 2-113. These two stars have remarkably similar spectra, although the former exhibits somewhat broader emission line widths. High resolution spectra (up to R=50 000) were obtained in May 1993 for both objects at the 3.9 m AAT, using the UCL Echelle Spectrograph. The fluxes in individual Cii auto-ionising multiplet components, many of which were blended, were derived. Lines originating from auto-ionising resonance states situated in the C²⁺ continuum are very sensitive to the electron temperature, since the population of the these levels is close to LTE. The measured widths and profile shapes of these transitions are presented and are in excellent agreement with those predicted on the basis of their calculated auto-ionising lifetimes. The wind electron temperature is derived for both stars from the ratio of the fluxes in four such transitions (T _e =18 500 K±1 500 K for CPD-56° 8032 andT _e =13 600 K±800 K for He 2-113). Optical depth effects are investigated using normal recombination lines to obtain an independent wind electron temperature estimate in excellent agreement with the dielectronic line analysis.     

The variability of Magellanic cluster infrared stars

We report on the light variations of the infrared stars that were discovered recently in the Magellanic clusters NGC 419, 1783 and 1978. Their periods, of 528, 458 and 491 days, are among the longest known for carbon-rich Mira variables in the Clouds. All three IR stars were found to lie on the extension of the period– M _bol relation derived from the shorter-period C-rich Miras while they were 0.45–0.70 mag fainter than the extension of the period– M _K relation. Their main sequence masses were determined by isochrone fitting to be 1.5–1.6 M_⊙, consistent with the prediction of the evolutionary models of Vassiliadis & Wood.     

Spectral analyses of [WCL]-type central stars

The optical spectra of the seven late-type Wolf-Rayet central stars NGC-40 (PNG 120.0+09.8, subtype [WC8]), He 2-99 (PNG 309.0–04.2, [WC9]), BD+30°3639 (PN G 064.7+05.0, [WC9]), CPD-56°8032 (PNG 332.9–09.9, [WC11]), He2-113 (PNG 321.0+03.9, [WC11]), M4-18 (PNG 146.7+07.6, [WC11]) and K2-16 (PNG 352.9+11.4, [WC11]) are analyzed by means of spherically expanding model atmospheres. The NLTE simulations account for the elements hydrogen, helium, carbon and oxygen. As main results effective temperature, element abundances and final velocity of the wind are determined for each star. Assuming distances or luminosities for the objects, also the stellar radii and the mass-loss rates can be fixed. The results of these analyses establish empirical constraints for the evolutionary status of WC-type central stars as post-AGB objects and provide input for modeling their planetary nebulae.     

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.     

Carbon stars in infrared surveys

In order to prepare the analysis of the forthcoming near-IR surveys (DENIS, 2MASS) and to interpret them in terms of galactic structure and AGB evolution, we present model predictions of AGB distributions in K, in 12m and in (J-K, [12–25]) space, as they will be obtained from these surveys, cross-correlated with IRAS Catalog. The simulations have been made by adding the AGB to the Besançon model.     

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.     

The evolution from visible to infrared variable AGB stars

Results of the Vassiliadis and Wood (1993) model for the evolution on the AGB are used to study how DENIS can complement an optical and IRAS based study of Long Period Variable AGB stars in a field towards the galactic bulge.     

Circumstellar dust model for infrared stars

The expected thermal emission from the shell around stars has been calculated on the graphite model. The calculated spectral distribution is in reasonable agreement with the observed infrared emission from some of the infrared stars.     

The evolved B[e] star HD 87643: observations and a radiation-driven disc wind model for B[e] stars

New high-resolution spectroscopic and medium-resolution spectropolarimetric data of the B[e] star HD 87643 are presented, complemented with optical broad- and narrow-band imaging. The spectrum of HD 87643 exhibits the hybrid characteristics well known to be representative of the group of B[e] stars; a fast wind with an expansion velocity in excess of 1000 km s⁻¹ is measured in the hydrogen and helium lines, while a slower component is traced by lower excitation lines and forbidden lines. Clues to the geometry of the rapidly expanding circumstellar shell are provided by the startling polarization changes across Hα. Comparison with published schematic calculations indicates that the polarizing material is located in a slowly rotating, expanding disc structure. A hydrodynamical model is then presented, the results of which are consistent with the original two-wind concept for B[e] stars, and which exhibits kinematic properties that may well explain the observed spectral features in HD 87643. The model calculations use as input a B star undergoing mass loss, surrounded by an optically thick disc. The resulting configuration consists of a fast polar wind from the star and a slowly expanding disc wind. The model also predicts that the stellar wind at intermediate latitudes is slower and denser than in the polar region.     

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.