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.     

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 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.     

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.     

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.     

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).     

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.     

M4–18: the planetary nebula and its WC10 central star

We present a detailed analysis of the planetary nebula M4–18 (G146.7+07.6) and its WC10-type Wolf–Rayet (WR) central star, based on high‐quality optical spectroscopy (WHT/UES, INT/IDS, WIYN/DensPak) and imaging ( HST /WFPC2). From a non-LTE model atmosphere analysis of the stellar spectrum, we derive T _eff=31 kK,     v _∞=160 km s⁻¹ and abundance number ratios of H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to those of He 2–113 ([WC10]). Assuming an identical stellar mass to that determined by De Marco et al. for He 2–113, we obtain a distance of 6.8 kpc to M4–18 [ E ( B−V )=0.55 mag from nebular and stellar techniques]. This implies that the planetary nebula of M4–18 has a dynamical age of ∼3100 yr, in contrast to ≥270 yr for He 2–113. This is supported by the much higher electron density of the latter. These observations may be reconciled with evolutionary predictions only if [WC]-type stars exhibit a range in stellar masses.
Photoionization modelling of M4–18 is carried out using our stellar WR flux distribution, together with blackbody and Kurucz energy distributions obtained from Zanstra analyses. We conclude that the ionizing energy distribution from the WR model provides the best consistency with the observed nebular properties, although discrepancies remain.     

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.     

A unique Galactic planetary nebula with a [WN] central star

We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s⁻¹, suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula.     

Symbiotic stars with Wolf-Rayet spectra as protoplanetary nebulae?

Symbiotic systems, in particular symbiotic novae, have been suggested to be very early stages of planetary nebulae. Some of them have been described as going through a Wolf-Rayet phase. We argue that there may be a direct relation between symbiotic objects and planetary nebulae, and that the Wolf-Rayet phase is connected to an active spell of the hot companion. Symbiotic stars could lead to planetary nebulae with two central stars with different radiation temperatures and luminosities, where each has the power to ionize a planetary nebula on its own.     

Accretion in brown dwarfs down to nearly planetary masses

We show that in accreting ultra low‐mass stars and brown dwarfs, the CaII λ 8662 emission line flux correlates remarkably well with the mass accretion rate ( ), just as it does in higher mass classical T Tauri stars (CTTs). A straightforward measurement of the CaII flux thus provides an easier determination technique than detailed modeling of the Hα emission line profile (except at the very lowest accretion rates, where CaII does not appear to be in emission for ultra low‐mass objects, and Hα modeling is required). Using optical high‐resolution spectra, we infer from CaII emission for young ultra low‐mass objects down to nearly the deuterium‐burning (planetary‐mass) limit. Our results, in combination with previous determinations of in CTTs, illustrate that the accretion rate declines steeply with mass, roughly as ∝ M_*² (albeit with considerable scatter). A similar relationship has been suggested by previous studies; we extend it down to nearly the planetary regime. The physical reason for this phenomenon is not yet clear; we discuss various possible mechanisms. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Star-like activity from a very young 'isolated planet'

The discovery of isolated bodies of planetary mass has challenged the paradigm that planets form only as companions to stars. To determine whether 'isolated planets', brown dwarfs and stars can have a common origin, we have made deep submillimetre observations of part of the ρ Oph B star formation region. Spectroscopy of the 9-Jupiter-mass core Oph B-11 has revealed carbon monoxide line wings such as those of a protostar. Moreover, the estimated mass of outflowing gas lies on the force versus core-mass relation for protostars and protobrown dwarfs. This is evidence for a common process that can form any object between planetary and stellar masses in a molecular cloud. In a submillimetre continuum map, six compact cores in ρ Oph B were found to have masses presently below the deuterium-burning limit, extending the core mass function down to  0.01 M_⊙  with the approximate form  d N /d M ∝ M ^−3/2  . If these lowest-mass cores are not transient and can collapse under gravity, then isolated planets should be very common in ρ Oph in the future, as is the case in the Orion star formation region. In fact, the isolated planetary objects that may form from these cores would outnumber the massive planets that have been found as companions to stars.     

Kinematic Analysis of Near and Far Stars in the Hipparcos Catalog

The proper motions of stars in the main sequence and of luminosity class III giants are analyzed kinematically. A new method has been used for reliably separating all the parameters of the Ogorodnikov-Milne model based on representing the proper motions of the stars in coordinate systems whose poles lie on each of the three principal axes of the galactic trihedron. Solutions for stars in different spectral classes are obtained. The main sequence is found to subdivide into two zones (near and far stars) with a fairly sharp boundary at B-V=0.5. It is shown that the Parenago effect may be related to the different distances from the sun of the main sequence stars.     

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.     

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.     

Binary (Post) AGB evolution

In this contribution the observational characteristicsof F to G type post-AGB stars, showing both O-rich andC-rich circumstellar chemistry, are reviewed. It turnsout that binarity and the presence of a stablecircumbinary dusty disc are fundamental properties of these objects. The possible common origin of the mixed chemistry observed in these systems and in IR-bright [WC] stars is discussed.     

Similarities and differences between the [WC 2] nucleus of NGC 5189 and the WO star Sand 4

Long slit spectra of the planetary nebula NGC 5189 with its [WC2] type central object and the G2.4+1.4 wind bubble around the evolved Pop. I WO star Sand 4 were obtained in order to investigate the observable effects of different initial masses in stars with otherwise similar spectra. Our preliminary results of a comparative spectroscopic study of these two objects shows that the chemical composition of the two nebuale is completely different, even though their morphology is most probably quite similar. In addition, the PN appears much more chemically homogeneous. These features are clearly associated with the evolutionary paths of the stars.     

Searching for hidden Wolf–Rayet stars in the Galactic plane – 15 new Wolf–Rayet stars

We report the discovery of 15 previously unknown Wolf–Rayet (WR) stars found as part of an infrared (IR) broad-band study of candidate WR stars in the Galaxy. We have derived an empirically based selection algorithm which has selected ∼5000 WR candidate stars located within the Galactic plane drawn from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (mid-IR) and Two-Micron All-Sky Survey (near-IR) catalogues. Spectroscopic follow-up of 184 of these reveals 11 nitrogen-rich (WN) and four carbon-rich (WC) WR stars. Early WC subtypes are absent from our sample and none shows evidence for circumstellar dust emission. Of the candidates which are not WR stars, ∼120 displayed hydrogen emission-line features in their spectra. Spectral features suggest that the majority of these are in fact B supergiants/hypergiants, ∼40 of these are identified Be/B[e] candidates.
Here, we present the optical spectra for six of the newly detected WR stars, and the near-IR spectra for the remaining nine of our sample. With a WR yield rate of ∼7 per cent and a massive star detection rate of ∼65 per cent, initial results suggest that this method is one of the most successful means for locating evolved, massive stars in the Galaxy.