Baryonic fraction in the cold plus hot dark matter universe

The observational infrared spectra of a number of Wolf–Rayet stars of WC8–9 spectral classes are shown to be quite satisfactorily explained by making use of the detailed theoretical model of a dust shell made up of spherical amorphous carbon grains, the dynamics, growth–destruction, thermal and electrical charge balance of which are taken into account. The dust grains acquire mainly positive electrical charge, move with suprathermal drift velocities and may grow up to 100–200 Å as a result of implantation of impinging carbon ions. For most of the stars the fraction of condensed carbon does not exceed 1 per cent. While the nature of the grain nucleation remains unknown, the condensation distances and the grain seed production can be estimated by fitting the observational spectra with theoretical ones.     

Five WC9 stars discovered in the AAO/UKST Hα survey

We report the discovery of five massive Wolf–Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission-line stars in the Anglo-Australian Observatory United Kingdom Schmidt Telescope (AAO/UKST) Southern Galactic Plane Hα survey. The 6195–6775 Å spectra of the stars are presented and discussed. A WC9 class is assigned to all five stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near-infrared (NIR) excesses characteristic of hot dust emission (as seen in the great majority of known WC9 stars), we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to seven. Reddenings and distances for all five stars are estimated.     

2MASS and WISE data analysis of Galactic Wolf-Rayet stars

We collected almost all Galactic Wolf-Rayet (hereafter WR) stars found so far from the literature. 578 WR stars are gathered in this paper. 2MASS counterparts with good quality magnitudes in all JHK bands are listed for 364 WR stars. In addition, WISE counterparts for these sources are also identified. It is found that free-free emission is the main dominant source for the infrared excess in most WR stars up to 3.4 μm. However at the longer wavelengths the thermal radiation is dominant. In addition, WR stars in Clusters of the Galactic center region have the strong infrared excess in the near infrared due to the dust thermal emission from the strong star forming activity in the Galactic center region. For some WR stars with the WC spectral type, in particular, with WCd type, the dust thermal emission is important radiation source while many WR stars with the WC spectral type have the near infrared flux enhancement from the broad line emission in the K band. It is also shown that many single WC stars with different spectral sub-types have different locations in the near infrared two-color diagram, in particular, WC6 and WC9d stars can be separated respectively from other spectral type stars while single WN stars with different spectral sub-types can not be separated in the near infrared two-color diagram.     

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.     

Dust Formation around Wolf-Rayet Stars

We discuss the observational evidence in favour of dust formation around massive, evolved carbon-rich Wolf-Rayet stars. Large dust shells containing (amorphous carbon) grains are observed in persistent dust makers, such as the pinwheel system WR104 (WC9+OB) and periodic/episodic dust makers, such asWR140 (WC7+O) close to periastron. Problems with the nucleation and growth of dust in such harsh environments are discussed. It is probable that mixing of carbon-rich material from the WC star plus hydrogen from an OB companion, together with efficient shielding via the shocked region of their colliding winds, provides the necessary ingredients. Many problems with the nucleation and growth remain to be addressed. This revised version was published online in July 2006 with corrections to the Cover Date.     

[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 [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.     

Spectroscopy of WC9 Wolf–Rayet stars: a search for companions

A spectroscopic search for luminous companions to WC9-type Wolf–Rayet stars making circumstellar dust reveals the presence of absorption lines attributable to companions in the blue spectra of WR 69 (HD 136488) and WR 104 (Ve2–45). Comparison of spectra of WR 104 observed in 1995 and 1997 showed the absorption lines to be more conspicuous in the latter observation and the emission lines weaker, suggesting a selective eclipse of the WC9 star similar to that observed by Crowther in 1996. The WC9 emission-line spectra are shown to be less uniform than previously thought, showing a significant range of O  ii line strengths. The only two WC9 stars in the observed sample that do not make circumstellar dust, WR 81 (He3–1316) and WR 92 (HD 157451), are found to have anomalously weak O  ii and strong He  ii lines. We suggest that these spectroscopic differences may reflect a compositional difference that plays a role in determining which of the WC9 stars make dust.     

Dust Formation Chemistry in Carbon-Rich Wolf-Rayet Winds

Dust formation processes are investigated in the hot hydrogen-deficient wind of carbon-rich Wolf-Rayet (WC) stars. Results show that dust precursors (small linear carbon chains) form in large quantities only at high gas densities. This suggests that a clumpy wind or an equatorial disc are necessary to trigger dust formation in WC stars. This revised version was published online in September 2006 with corrections to the Cover Date.     

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).     

X-ray Observations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra

We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. These observations are aimed at determining the differences in X-ray properties between massive WR + OB binary systems and putatively single WR stars. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission (including the Fe Kα line complex), characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to detectable levels.     

New Wolf–Rayet central stars of planetary nebulae identified on the AAO/UKST Hα Survey

The central stars of two of the new planetary nebulae found during scans of the AAO/UKST H α Survey of the Milky Way have been found to exhibit Wolf–Rayet (WR) emission features. One (PMR 1) is an early-type star of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC] star which, depending on the classification scheme used, is either intermediate in class between [WC9] and [WC10] or the sole member of the [WC10] class. Both stars exhibit unusual spectral features which may be attributed to enhanced nitrogen in their atmospheres and could be indicative of unusual stellar evolution.     

Secular contraction in extreme helium stars and the future of V4334 Sgr

Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated as the precursors of extreme helium stars (EHes). The question arises to whether Sakurai's Object will evolve down the [WC]-PG1159 evolution track, or the RCB-EHe-HesdO track. From a number of IUE observations, we have measured thesecular contraction rates and pulsation masses of several EHes. These are inconsistent with the predicted contraction rates for helium-shell burning giants produced by a final helium-shell flash (the [WC]-PG1159 track). Although there may be some similarities between Sakurai's Object and some RCBs, if the RCB-EHe conection is valid then these must be superficial rather than structural.     

Magellanic Cloud WC/WO Wolf–Rayet stars – II. Colliding winds in binaries

A search for evidence of colliding winds is undertaken among the four certain Magellanic Cloud WC/WO spectroscopic binaries found in the companion Paper I, as well as among two Galactic WC/WO binaries of very similar subtype. Two methods of analysis, which allow the determination of orbital inclination and parameters relating to the shock cone from spectroscopic studies of colliding winds, are attempted. In the first method, Lührs' spectroscopic model is fitted to the moderately strong C  iii 5696-Å excess line emission arising in the shock cone for the stars Br22 and WR 9. The four other systems show only very weak C  iii 5696-Å emission. Lührs' model follows well the mean displacement of the line in velocity space, but is unable to reproduce details in the line profile and fails to give a reliable estimate of the orbital inclination. In the second method, an alternative attempt is also made to fit the variation of more global quantities, full width at half-maximum and radial velocity of the excess emission, with phase. This method also gives satisfactory results in a qualitative way, but shows numerical degeneracy with orbital inclination. Colliding wind effects on the very strong C  iv 5808-Å Wolf–Rayet emission line, present in all six binaries, are also found to behave qualitatively as expected. After allowing for line enhancement in colliding wind binaries, it now appears that all Magellanic Cloud WC/WO stars occupy a very narrow range in spectral subclass: WC4/WO3.     

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.     

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.     

Magellanic Cloud WC/WO Wolf–Rayet stars – I. Binary frequency and Roche lobe overflow formation

A nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf–Rayet stars is studied spectroscopically and photometrically to determine its binary frequency. Theory predicts the Roche lobe overflow produced Wolf–Rayet binary frequency to be 52±14 per cent in the Large Magellanic Cloud and 100 per cent in the Small Magellanic Cloud, not counting non-Roche lobe overflow Wolf–Rayet binaries. Lower ambient metallicity ( Z ) leads to lower opacity, preventing all but the most massive (hence luminous) single stars from reaching the Wolf–Rayet stage. However, theory predicts that Roche lobe overflow even in binaries of modest mass will lead to Wolf–Rayet stars in binaries with periods below approximately 200 d, for initial periods below approximately 1000 d, independent of Z . By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths and full widths at half-maximum, a WC/WO binary frequency of only 13 per cent, significantly lower than the prediction, is found in the Large Magellanic Cloud. In the unlikely event that all of the cases with a less certain binary status actually turn out to be binary, current theory and observation would agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC+O binaries in the Large Magellanic Cloud all have periods well below 1000 d. The large majority of WC/WO stars in such environments apparently can form without the aid of a binary companion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf–Rayet stars in the Magellanic Clouds, or the formation of Wolf–Rayet stars in general.     

Hydrogen-poor planetary nebulae

Five planetary nebulae are known to show hydrogen-poor material nearthe central star. In the case of A58, this gas was ejected following alate thermal pulse similar to Sakurai's Object. In this paper I will reviewthese five objects. One of them, IRAS 18333 –2357, may not be a truePN. I will show that there is a strong case for a relation to the [WC]stars and their relatives, the weak emission-line stars. The surfaceabundances of the [WC] stars are explained via diffuse overshoot intothe helium layer. The hydrogen-poor PNe do not support this: theirabundances indicate a change of abundance with depth in the heliumlayer. A short-lived phase of very high mass loss, the r-AGB, isindicated. Sakurai's Object may be at the start of such a phase, and mayevolve to very low stellar temperatures.     

The variability of HD 76536

The spectrophotometric and photometric observations of the WC star HD 76536 are reported. The flux and the magnitude variations appear to be periodic. The possibilities of a compact binary are discussed.     

Dust properties of [WCL] CSPNS

The rather rare class of central stars of planetary nebulae thatshow Wolf-Rayet spectra have been a subject of great interest,particularly in the infrared, since their discovery in the late1960s. I will focus on further peculiarities found within thepast 1-2 years with the advent of infrared spectroscopy fromISO. Notably, these stars simultaneously betray the presenceof regions of carbon-rich and oxygen-rich dust chemistry. Icompare and contrast complete ISO spectra from 2 to 200 micronsof a small sample of [WC9] to [WC11] central stars.