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Radio Proper Motions of Wolf-Rayet Stars
We present the analysis of observations taken from the Very Large Arrayarchive of six Wolf-Rayet stars with radio emission, with the purpose ofdetermining their proper motions. Typically, these observations coverperiods of 10 to 20 years. To verify the method, we included WR 140 inthe sample, and found that the proper motions determined by us are a fewtimes more accurate than, and consistent within noise, with those ofHipparcos. The other five WR stars were not studied by Hipparcos and wereport their proper motions for the first time. The proper motions forWR 145a = Cyg X-3 are consistent with the source being stationary withrespect to its local standard of rest and suggest that the black hole inthis binary system formed by direct collapse of a massive star, withoutexpulsion of a supernova remnant.
| Wolf-Rayet Stars at the Highest Angular Resolution
Interferometric observations of high-mass evolved stars provide new andvery valuable information of their nature. With the unique capabilitiesof the VLTI, direct images of their closest environment where mass lossand dust formation occur, can be obtained. The breakthrough of the VLTIin terms of angular resolution as well as spectral resolution allowscompeting theoretical models, based on indirect constraints, to betested. The high angular resolution made available by the VLTI showsthat there is still a lot to discover about these massive stars.
| Possible implications of mass accretion in Eta Carinae
We apply the previously suggested accretion model for the behavior ofthe super-massive binary system η Car close to periastron passages.In that model it is assumed that for ˜10 weeks near periastronpassages one star is accreting mass from the slow dense wind blown bythe other star. We find that the secondary, the less massive star,accretes ˜2×10Mȯ. This masspossesses enough angular momentum to form a disk, or a belt, around thesecondary. The viscous time is too long for the establishment ofequilibrium, and the belt must be dissipated as its mass is being blownin the reestablished secondary wind. This process requires about half ayear, which we identify with the recovery phase of η Car. We showthat radiation pressure, termed radiative braking, cannot preventaccretion. In addition to using the commonly assumed binary model forη Car, we also examine alternative models where the stellar massesare larger, and/or the less massive secondary blows the slow dense wind,while the primary blows the tenuous fast wind and accretes mass for˜10 week near periastron passages. We end by some predictions forthe next event (January March 2009).
| Puzzling X-rays from the new colliding wind binary Wolf-Rayet 65 (WC9d)
We report the discovery of variability in the X-ray emission from theWolf-Rayet type star WR65. Using archival Chandra data spanning over5yr, we detect changes of the X-ray flux by a factor of 3 accompanied bychanges in the X-ray spectra. We believe that this X-ray emissionoriginates from wind-wind collision in a massive binary system. Theobserved changes can be explained by the variations in the emissionmeasure of the hot plasma, and in the different absorption column alongthe binary orbit. The X-ray spectra of WR65 display prominent emissionfeatures at wavelengths corresponding to the lines of strongly ionizedFe, Ca, Ar, S, Si and Mg. WR65 is a carbon-rich WC9d star that is apersistent dust maker. This is the first investigation of any X-rayspectrum for a star of this spectral type. There are indications thatthe dust and the complex geometry of the colliding wind region arepivotal in explaining the X-ray properties of WR65.
| Dust formation by colliding-wind binaries
The recent high-resolution infrared images of the enigmatic cocoon starsin the Quintuplet cluster showing them to have ``pinwheel'' tails, andthe deduction that they are colliding-wind Wolf-Rayet binaries, showshow far the study of colliding-wind dust formation has come since thefirst discovery of heated carbon dust around the classical WC9 starVe2-45 (WR 104) in the early days of infrared astronomy. The formationof dust is the least expected, and hardest to understand, processattributed to colliding stellar winds, and direct evidence of theconnection is only now becoming available. I will review recent work andconsider systems showing a range of dust-formation phenomena, includingthe prototypical epsiodic dust-maker WR 140 and the variable dust-makerWR 70, also long studied by Virpi Niemela and her colleagues.
| A 3D dynamical model of the colliding winds in binary systems
We present a three-dimensional (3D) dynamical model of theorbital-induced curvature of the wind-wind collision region in binarystar systems. Momentum balance equations are used to determine theposition and shape of the contact discontinuity between the stars, whilefurther downstream the gas is assumed to behave ballistically. AnArchimedean spiral structure is formed by the motion of the stars, withclear resemblance to high-resolution images of the so-called `pinwheelnebulae'. A key advantage of this approach over grid or smoothedparticle hydrodynamic models is its significantly reduced computationalcost, while it also allows the study of the structure obtained in aneccentric orbit. The model is relevant to symbiotic systems andγ-ray binaries, as well as systems with O-type and Wolf-Rayetstars.As an example application, we simulate the X-ray emission fromhypothetical O+O and WR+O star binaries, and describe a method of raytracing through the 3D spiral structure to account for absorption by thecircumstellar material in the system. Such calculations may be easilyadapted to study observations at wavelengths ranging from the radio toγ-ray.
| Dust Formation and He II λ4686 Emission in the Dense Shell of the Peculiar Type Ib Supernova 2006jc
We present evidence for the formation of dust grains in an unusual TypeIb supernova (SN) based on late-time spectra of SN 2006jc. Theprogenitor suffered an outburst qualitatively similar to those ofluminous blue variables (LBVs) just 2 yr prior to the SN, and we proposethat the dust formation is a consequence of the SN blast wave overtakingthat LBV-like shell. The key evidence for dust formation is (1) theappearance of a red/near-infrared continuum emission source that can befit by T~1600 K graphite grains, and (2) fading of the redshifted sidesof intermediate-width He I emission lines, yielding progressively moreasymmetric blueshifted lines as dust obscures receding material. Thisprovides the strongest case yet for dust formation in any SN Ib/c. Bothdevelopments occurred between 51 and 75 days after peak brightness,while the few other SNe observed to form dust did so after a few hundreddays. Geometric considerations indicate that dust formed in the denseswept-up shell between the forward and reverse shocks, and not in thefreely expanding SN ejecta. The rapid cooling leading to dust formationmay have been aided by extremely high shell densities of 1010cm-3, indicated by He I line ratios. The brief epoch of dustformation is accompanied by He II λ4686 emission and enhancedX-ray emission, suggesting a common link. These clues imply that theunusual dust formation in this object was not attributable to propertiesof the SN itself, but instead-like most peculiarities of SN 2006jc-was aconsequence of interaction with the dense environment created by anLBV-like eruption 2 yr before the SN.
| 3D simulations of RS Ophiuchi: from accretion to nova blast
Context: The binary star system RS Ophiuchi is a recurrent nova, withoutbursts occurring about every 22 years. It consists of a red giantstar (RG) and a wind accreting white dwarf close to the Chandrasekharlimit. This system is considered a prime candidate for evolving into anSNIa. For its most recent outbursts in 1985 and 2006, exquisitemultiwavelength observational data are available. Aims: Deeper physicalinsight is needed regarding the inter-outburst accretion phase and thedynamical effects of the subsequent nova explosion in order to improvethe interpretation of the observed data and to shed light on whether thesystem is an SNIa progenitor. Methods: We present a 3D hydrodynamicsimulation of the quiescent accretion with the subsequent explosivephase. Results: The computed circumstellar mass distribution in thequiescent phase is highly structured with a mass enhancement in theorbital plane of about a factor of 2 as compared to the polewarddirections. The simulated nova remnant evolves aspherically, propagatingfaster toward the poles. The shock velocities derived from thesimulations agree with those derived from observations. For v_RG = 20 kms-1 and for nearly isothermal flows, we find that 10% of themass lost by the RG is transfered to the WD. For an RG mass loss of10-7~Mȯ yr-1, the orbit of thesystem decays by 3% per million years. With the derived mass transferrate, multi-cycle nova models provide a qualitatively correct recurrencetime, amplitude, and fastness of the nova. Conclusions: Our 3Dhydrodynamic simulations provide, along with the observations and novamodels, the third ingredient for a deeper understanding of the recurrentnovae of the RS Oph type. In combination with recent multi-cycle novamodels, our results suggest that the WD in RS Oph will increase in mass.Several speculative outcomes then seem plausible. The WD may reach theChandrasekhar limit and explode as an SN Ia. Alternatively, the massloss of the RG could result in a smaller Roch volume, a common envelopephase, and a narrow WD + WD system. Angular momentum loss due togravitational wave emission could trigger the merger of the two WDs and- perhaps - an SN Ia via the double degenerate scenario.In the frame of the computing project “Cosmic Engines inGalaxies”.The movie is only available inelectronic form at http://www.aanda.org
| The Prototype Colliding-Wind Pinwheel WR 104
Results from the most extensive study of the time-evolving duststructure around the prototype ``pinwheel'' nebula WR 104 are presented.Encompassing 11 epochs in three near-infrared filter bandpasses, ahomogeneous imaging data set spanning more than 6 yr (or 10 orbits) ispresented. Data were obtained from the highly successful Keck ApertureMasking Experiment, which can recover high-fidelity images at extremelyhigh angular resolutions, revealing the geometry of the plume withunprecedented precision. Inferred properties for the (unresolved)underlying binary and wind system are orbital period 241.5+/-0.5 daysand angular outflow velocity 0.28+/-0.02 mas day-1. Anoptically thin cavity of angular size 13.3+/-1.4 mas was found to liebetween the central binary and the onset of the spiral dust plume.Rotational motion of the wind system induced by the binary orbit isfound to have important ramifications: entanglement of the winds resultsin strong shock activity far downstream from the nose of the bow shock.The far greater fraction of the winds participating in the collision mayplay a key role in gas compression and the nucleation of dust at largeradii from the central binary and shock stagnation point. Investigationof the effects of radiative braking points toward significantmodifications of the simple hydrostatic colliding wind geometry,extending the relevance of this phenomenon to wider binary systems thanpreviously considered. Limits placed on the maximum allowed orbitaleccentricity of e<~0.06 argue strongly for a prehistory of tidalcircularization in this system. Finally, we discuss the implications ofEarth's polar (i<~16deg) vantage point onto a systemlikely to host supernova explosions at future epochs.
| Mid-Infrared Interferometry of Dust around Massive Evolved Stars
We report long-baseline interferometric measurements of circumstellardust around massive evolved stars with the MIDI instrument on the VeryLarge Telescope Interferometer and provide spectrally dispersedvisibilities in the 8-13 μm wavelength band. We also presentdiffraction-limited observations at 10.7 μm on the Keck Telescopewith baselines up to 8.7 m, which explore larger scale structure. Wehave resolved the dust shells around the late-type WC stars WR 106 andWR 95 and the enigmatic NaSt 1 (formerly WR 122), suspected to haverecently evolved from a luminous blue variable (LBV) stage. For AG Car,the prototypical LBV in our sample, we marginally resolve structureclose to the star, distinct from the well-studied detached nebula. Thedust shells around the two WC stars show fairly constant size in the8-13 μm MIDI band, with Gaussian half-widths of ~25 to 40 mas, andthe Keck observations reveal an additional extended structure around WR106. The visibility profiles for NaSt 1 obtained from two MIDI baselinesindicate a compact source embedded in an extended structure. The compactdust we detect around NaSt 1 and AG Car favors recent or ongoing dustformation. Using the measured visibilities, we build sphericallysymmetric radiative transfer models of the WC dust shells, which enabledetailed comparison with existing SED-based models. Our results indicatethat the inner radii of the shells are within a few tens of AU from thestars. In addition, our models favor grain size distributions with large(~1 μm) dust grains. This proximity of the inner dust to the hotcentral star emphasizes the difficulty faced by current theories informing dust in the hostile environment around WR stars. Although wedetect no direct evidence for binarity for these objects, dustproduction in a colliding-wind interface in a binary system is afeasible mechanism in WR systems under these conditions.
| H+3 in Diffuse Interstellar Clouds: A Tracer for the Cosmic-Ray Ionization Rate
Using high-resolution infrared spectroscopy we have surveyed twentysight lines for H+3 absorption.H+3 is detected in eight diffuse cloud sight lineswith column densities varying from 0.6×1014 to3.9×1014 cm-2. This brings to 14 the totalnumber of diffuse cloud sight lines where H+3 hasbeen detected. These detections are mostly along sight linesconcentrated in the Galactic plane but are well dispersed in Galacticlongitude. The results imply that abundant H+3 iscommon in the diffuse interstellar medium. Because of the simplechemistry associated with H+3 production anddestruction, these column density measurements can be used in concertwith various other data to infer the primary cosmic-ray ionization rate,ζp. Values range from 0.5×10-16 to3×10-16 s-1 with an average of2×10-16 s-1. Where H+3is not detected, the upper limits on the ionization rate are consistentwith this range. The average value of ζp is about 1order of magnitude larger than both the canonical rate and ratespreviously reported by other groups using measurements of OH and HD. Thediscrepancy is most likely due to inaccurate measurements of rateconstants and the omission of effects which were unknown when thosestudies were performed. We believe that the observed column density ofH+3 is the most direct tracer for the cosmic-rayionization rate due to its simple chemistry. Recent models of diffusecloud chemistry require cosmic-ray ionization rates on the order of10-16 s-1 to reproduce observed abundances ofvarious atomic and molecular species, in rough accord with ourobservational findings.
| Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysicalprocesses likely to play a role in the production of non-thermalemission in massive stars, with emphasis on massive binaries. Eventhough the discussion will start in the radio domain where thenon-thermal emission was first detected, the census of physicalprocesses involved in the non-thermal emission from massive stars showsthat many spectral domains are concerned, from the radio to the veryhigh energies. First, the theoretical aspects of the non-thermalemission from early-type stars will be addressed. The main topics thatwill be discussed are respectively the physics of individual stellarwinds and their interaction in binary systems, the acceleration ofrelativistic electrons, the magnetic field of massive stars, and finallythe non-thermal emission processes relevant to the case of massivestars. Second, this general qualitative discussion will be followed by amore quantitative one, devoted to the most probable scenario wherenon-thermal radio emitters are massive binaries. I will show how severalstellar, wind and orbital parameters can be combined in order to makesome semi-quantitative predictions on the high-energy counterpart to thenon-thermal emission detected in the radio domain. These theoreticalconsiderations will be followed by a census of results obtained so far,and related to this topic. These results concern the radio, the visible,the X-ray and the γ-ray domains. Prospects for the very highenergy γ-ray emission from massive stars will also be addressed.Two particularly interesting examples—one O-type and oneWolf-Rayet binary—will be considered in details. Finally,strategies for future developments in this field will be discussed.
| Physical Properties of Wolf-Rayet Stars
The striking broad emission line spectroscopic appearance of Wolf-Rayet(WR) stars has long defied analysis, owing to the extreme physicalconditions within their line- and continuum-forming regions. Recently,model atmosphere studies have advanced sufficiently to enable thedetermination of stellar temperatures, luminosities, abundances,ionizing fluxes, and wind properties. The observed distributions ofnitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and innearby star-forming galaxies are discussed; these imply lower limits toprogenitor masses of ?25, 40, and 75 Mȯ forhydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars,respectively. WR stars in massive star binaries permit studies ofwind-wind interactions and dust formation in WC systems. They also showthat WR stars have typical masses of 10 25 Mȯ, extendingup to 80 Mȯ for H-rich WN stars. Theoretical andobservational evidence that WR winds depend on metallicity is presented,with implications for evolutionary models, ionizing fluxes, and the roleof WR stars within the context of core-collapse supernovae andlong-duration gamma-ray bursts.
| Wind-wind collision in the η Carinae binary system - III. The HeII λ4686 line profile
We modelled the HeII λ4686 line profiles observed in the ηCarinae binary system close to the 2003.5 spectroscopic event, assumingthat they were formed in the shocked gas that flows at both sides of thecontact surface formed by wind-wind collision. We used a constant flowvelocity and added turbulence in the form of a Gaussian velocitydistribution. We allowed emission from both the primary and secondaryshocks but introduced infinite opacity at the contact surface, implyingthat only the side of the contact cone visible to the observercontributed to the line profile. Using the orbital parameters of thebinary system derived from the 7-mm light curve during the lastspectroscopic event (Paper II) we were able to reproduce the lineprofiles obtained with the Hubble Space Telescope at different epochs,as well as the line mean velocities obtained with ground-basedtelescopes. A very important feature of our model is that the lineprofile depends on the inclination of the orbital plane; we found thatto explain the latitude-dependent mean velocity of the line, scatteredinto the line of sight by the Homunculus, the orbit cannot lie in theHomunculus equatorial plane, as usually assumed. This result, togetherwith the relative position of the stars during the spectroscopic events,allowed us to explain most of the observational features, like thevariation of the `Purple Haze' with the orbital phase, and to conciliatethe X-ray absorption with the postulated shell effect used to explainthe optical and ultraviolet light curves.
| Effect of the Coriolis Force on the Hydrodynamics of Colliding-Wind Binaries
Using fully three-dimensional hydrodynamic simulations, we investigatethe effect of the Coriolis force on the hydrodynamic and observableproperties of colliding-wind binary systems. To make the calculationstractable, we assume adiabatic, constant velocity winds. The neglect ofradiative driving, gravitational deceleration, and cooling limits theapplication of our models to real systems. However, these assumptionsallow us to isolate the effect of the Coriolis force, and by simplifyingthe calculations, allow us to use a higher resolution (up to6403) and to conduct a larger survey of parameter space. Westudy the dynamics of colliding winds with equal mass-loss rates andvelocities emanating from equal-mass stars on circular orbits, with arange of values for the ratio of the wind to orbital velocity. We alsostudy the dynamics of winds from stars on elliptical orbits and withunequal-strength winds. Orbital motion of the stars sweeps the shockedwind gas into an Archimedean spiral with asymmetric shock strengths andtherefore unequal postshock temperatures and densities in the leadingand trailing edges of the spiral. We observe the Kelvin-Helmholtzinstability at the contact surface between the shocked winds in systemswith orbital motion even when the winds are identical. The change inshock strengths caused by orbital motion increases the volume of X-rayemitting postshock gas with T>0.59 keV by 63%, for a typical system,as the ratio of wind velocity to orbital velocity decreases toVw/Vo=2.5. This causes increased free-freeemission from systems with shorter orbital periods and an altered timedependence of the wind attenuation. We comment on the importance of theeffects of orbital motion on the observable properties of colliding-windbinaries.
| The Complex Structure of the Mg II λλ 2795.523, 2802.698 Å Regions of 64 Be Stars
Here we consider the presence of absorption components shifted to theviolet or red side of the main spectral line (satellite or discreteabsorption components, i.e., SACs or DACs) in the regions of the Mg IIresonance lines in Be stars as well as their kinematicalcharacteristics. Namely, our objective is to check whether there existsa common physical structure for the atmospheric regions creating SACs orDACs of the Mg II resonance lines. In order to do this, a statisticalstudy of the Mg II λλ 2795.523, 2802.698 Å lines inthe spectra of 64 Be stars of all spectral subtypes and luminosityclasses was performed. We found that the atmospherical absorptionregions where the Mg II resonance lines originated may be formed ofseveral independent density layers of matter that rotate with differentvelocities. It was also attempted to separate SACs and DACs according tolow or high radial velocity. The emission lines were detected only inthe earliest and latest spectral subtypes.
| The Keck Aperture-masking Experiment: Near-Infrared Sizes of Dusty Wolf-Rayet Stars
We report the results of a high angular resolution near-infrared surveyof dusty Wolf-Rayet stars using the Keck I Telescope, including newmultiwavelength images of the pinwheel nebulae WR 98a, WR 104, and WR112. Angular sizes were measured for an additional eight dustyWolf-Rayet stars using aperture-masking interferometry, allowing us toprobe characteristic sizes down to ~20 mas (~40 AU for typical sources).With angular sizes and specific fluxes, we can directly measure thewavelength-dependent surface brightness and size relations for oursample. We discovered tight correlations of these properties within oursample that could not be explained by simple spherically symmetric dustshells or even the more realistic ``pinwheel nebula''(three-dimensional) radiative transfer model, when using Zubko's opticalconstants. While the tightly correlated surface brightness relations weuncovered offer compelling indirect evidence of a shared and distinctivedust shell geometry among our sample, long-baseline interferometersshould target the marginally resolved objects in our sample in order toconclusively establish the presence or absence of the putativeunderlying colliding-wind binaries thought to produce the dust shellsaround WC Wolf-Rayet stars.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium
We have analyzed the 9.7 and ``18'' μm interstellar silicateabsorption features along the line of sight toward four heavilyextincted galactic WC-type Wolf-Rayet (WR) stars. We construct twointerstellar extinction curves from 1.25 to 25 μm using near-IRextinction measurements from the literature, along with the silicateprofiles of WR 98a (representing the local ISM) and GCS 3 (representingthe Galactic center). We have investigated the mineralogy of theinterstellar silicates by comparing extinction profiles for amorphoussilicates with olivine and pyroxene stoichiometry to the 9.7 and ``18''μm absorption features in the WR 98a spectrum. In this analysis, wehave considered solid and porous spheres and a continuous distributionof ellipsoids. While it is not possible to simultaneously provide aperfect match to both profiles, we find that the best match requires amixture of these two types of compounds. We also consider iron oxides,aluminosilicates, and silicate carbide (SiC) as grain components. Ironoxides cannot be accommodated in the observed spectrum, while the amountof Si in SiC is limited to <4%. Finally, we discuss the cosmicelemental abundance constraints on the silicate mineralogy, grain shape,and porosity.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| An Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])
We present new Spitzer IRS observations of HD 164270 (WC9, WR103). Aquantitative analysis of the UV, optical, near-, and mid-IR spectra ofHD 164270 is presented, allowing for line blanketing and wind clumping,revealing T*~48 kK, logL/Lsolar~4.9, andM˙~10-5 Msolar yr-1 for a volumefilling factor of f~0.1. Our models predict that He is partiallyrecombined in the outer stellar wind, such that recent radio-derivedmass-loss rates of WC9 stars have been underestimated. We obtainC/He~0.2 and O/He~0.01 by number from optical diagnostics. Mid-IRfine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm areobserved, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. Fromthese we obtain Ne/He~Ne+/He=2.2×10-3 bynumber, 7 times higher than the solar value (as recently derived byAsplund et al.), and S/He~S2+/He=5.1×10-5 bynumber. From a comparison with similar results for other WC subtypes weconclude that WC9 stars are as chemically advanced as earlier subtypes.We consider why late WC stars are exclusively observed inhigh-metallicity environments. In addition, we compare theUV/optical/mid-IR spectroscopic morphology of HD 164270 with theplanetary nebula central star BD +30 3639 ([WC9]). Their UV and opticalsignatures are remarkably similar, such that our quantitativecomparisons confirm similarities in stellar temperature, wind densities,and chemistry first proposed by Smith & Aller, in spite ofcompletely different evolutionary histories, with HD 164270 presently afactor of 10 more massive than BD +30 3639. At mid-IR wavelengths, thedust from the dense young nebula of BD +30 3639 completely dominates itsappearance, in contrast with HD 164270.
| Five WC9 stars discovered in the AAO/UKST Hα survey
We report the discovery of five massive Wolf-Rayet (WR) stars resultingfrom a programme of follow-up spectroscopy of candidate emission-linestars in the Anglo-Australian Observatory United Kingdom SchmidtTelescope (AAO/UKST) Southern Galactic Plane Hα survey. The6195-6775 Åspectra of the stars are presented and discussed. A WC9class is assigned to all five stars through comparison of their spectrawith those of known late-type WC stars, bringing the known total numberof Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibitnear-infrared (NIR) excesses characteristic of hot dust emission (asseen in the great majority of known WC9 stars), we find that two of thestars show no discernible evidence of such excesses. This increases thenumber of known WC9 stars without NIR excesses to seven. Reddenings anddistances for all five stars are estimated.
| Gamma-Ray Burst Dust Echoes Revisited: Expectations at Early Times
Gamma-ray burst (GRB) dust echoes were first proposed as an alternativeexplanation for the supernova-like (SN-like) components to theafterglows of GRB 980326 and GRB 970228. However, the spectroscopicidentification of Type Ic SN 2003dh associated with GRB 030329, as wellas the identification of SN-like components of the afterglows of otherGRBs, appears to have confirmed the GRB/SN paradigm. However, the likelyprogenitors of Type Ic SNe are Wolf-Rayet WC stars, and late-type WCstars have been observed to be surrounded by dust at a distance of1014-1015 cm from the star. Consequently, werevisit the possibility of GRB dust echoes, not on a timescale of weeksafter the burst but on a timescale of minutes to hours. We find that ifthe optical flash is sufficiently bright and the jet sufficiently wide,GRB afterglows may be accompanied by chromatic variations on thistimescale. From these signatures, model parameters such as the innerradius of the dust distribution and the initial opening angle of the jetmay be deduced. With rapid and regular localizations of GRBs by HETE-2,INTEGRAL, and now Swift, and new and improved robotic telescope systems,these early-time GRB dust echoes may soon be detected. We describe ingreater detail one such robotic telescope system, called PROMPT, whichthe University of North Carolina at Chapel Hill is building at the CerroTololo Inter-American Observatory.
| FUSE Observations of the SMC 16 day Wolf-Rayet Binary Sanduleak 1 (WO4+O4): Atmospheric Eclipses and Colliding Stellar Winds
In this paper we present the results of a FUSE monitoring campaign ofthe SMC WO4+O4 V Wolf-Rayet binary Sanduleak 1. Our 18 spectra obtainedduring a little more than one orbital cycle in 2000 October combinedwith four archival spectra show variability in the S VI, C III, C IV,and O VI P Cygni profiles, which we attribute to emission from the shockcone resulting from the collision between the two strong winds and toatmospheric eclipses of the O star continuum light by the W-R wind. Allthe lines vary in concert indicating that the cooling is such that evenlines such as the OVI λλ1032, 1038 doublet form in thelinear part of the cone. We have also applied both a simple geometricalmodel and profile fits, including emission from the normal wind, extraemission from the shock cone, and the atmospheric eclipse. Adopting anorbital inclination of ~40°, we deduce a total cone opening angle of~80° and a streaming velocity for the gas along the shock cone of~3000 km s-1. The luminosity ratio required to fit ourspectra is LO/LW-R=3.5, and the stellar radii are3.5 and 12 Rsolar, respectively, for the W-R and O stars. Wealso present radiative driving models for this binary system having twomassive stars with strong winds and discuss radiative inhibition andbraking effects. In particular, we address the coupling of the O starradiation with the W-R star wind. Finally, we present a PICAhydrodynamic colliding-wind model for Sand 1. We find an opening anglefor the shock cone similar to that deduced from the line-profilefitting, but significantly longer cooling lengths along the shock cone.However, the model reveals some cold gas that is stripped off the O4surface and mixed with the hotter WO4 material, thereby accelerating itscooling. This could very well explain why shorter cooling lengths areinferred from the profile fits.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS-32985.
| Evidence of asymmetry in Mira variable U Ori
Near simultaneous, two high angular resolution observations by lunaroccultation technique at the same wavelength (2.2mu m) but at differentposition angles (PA) result in two unique Uniform Disk (UD) angulardiameters of Mira variable U Ori. UD angular diameter obtained fromobservations at Mt. Abu observatory is 11.9(+/-)0.3 milliarcsecond (mas)at PA 136o while from observations at TIRGO observatory UDvalue obtained is 15.14(+/-)0.05 mas at PA 75o. The sourcebrightness profile derived from a model independent analysis shows anasymmetric spatial structure in both cases. Asymmetric structure of thesource at higher spatial scale was also reported by several authors fromOH and H2O maser distribution at radio wavelengths; thesource is more extended at PA of 30 - 60o. Furthermore,moderate level maximum optical intrinsic polarization of ~1-2% at PAapprox 20o - 40o is also detected. All theevidences bring out the spatial asymmetry in U Ori.
| Lunar occultations in the near infrared: achievements and new challenges
A brief review of the lunar occultation program in the near infrared forhigh angular resolution study of bright IR sources carried out at PRL inthe last decade is presented. The development of the two channel Fast IRphotometer is described. Major results pertaining to circumstellar duststructures surrounding occulted objects like IRC+10216 and WR104 areoutlined. The challenges for the future in observing lunar occultationsin the L band and in the use of IR arrays for occultation work arediscussed.
| Sous le vent des étoiles massives
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| Wind-wind collision in the η Carinae binary system: a shell-like event near periastron
The exact nature of η Carinae is still an open issue. Strictperiodicity in the light curves at several wavelengths seem to point toa binary system, but the observed radial velocities, measured from spacewith high spatial resolution, are in conflict with the ground-basedobservations used to calculate the binary orbit. Also, the observed 2-10keV X-ray flux is much larger that what is expected from a single star,and favours the wind-wind collision hypothesis, characteristic ofhigh-mass binary systems. However, to explain the duration of the dip inthe light curve by wind collisions, it is necessary to postulate a verylarge increase in the η Carinae mass loss rate. Finally, the opticaland ultraviolet light curves are better explained by periodicshell-ejection events. In this paper we conciliate the two hypotheses.We still assume a binary system to explain the strong X-ray emission,but we also take into account that, near periastron and because of thehighly eccentric orbit, the wind emerging from η Carinae accumulatesbehind the shock and can mimic a shell-like ejection event. For thisprocess to be effective, at periastron the secondary star should belocated between η Carinae and the observer, solving also thediscrepancy between the orbital parameters derived from ground- andspace-based observations. We show that, as the secondary moves in itsorbit, the shell cools down and the number of available stellar ionizingphotons is not enough to maintain the shell temperature at itsequilibrium value of about 7500 K. The central part of the shell remainscold and under these conditions grain formation and growth can takeplace in time-scales of hours. We also calculated the neutral gas columndensity intercepting the line of sight at each point of the orbit nearperiastron, and were able to reproduce the form and duration of theX-ray light curve without any change in the η Carinae mass lossrate. This same column density can explain the observed Hα lightcurve observed during the 2003 event.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Three-dimensional dust radiative-transfer models: the Pinwheel Nebula of WR 104
We present radiative-transfer modelling of the dusty spiral PinwheelNebula observed around the Wolf-Rayet/OB-star binary WR 104. The modelsare based on the three-dimensional radiative-transfer code TORUS,modified to include an adaptive mesh that allows us to adequatelyresolve both the inner spiral turns (subau scales) and the outer regionsof the nebula (distances of 104 au from the central source).The spiral model provides a good fit to both the spectral energydistribution and Keck aperture masking interferometry, reproducing boththe maximum entropy recovered images and the visibility curves. Wededuce a dust creation rate of 8 +/- 1 × 10-7Msolar yr-1, corresponding to approximately 2 percent by mass of the carbon produced by the Wolf-Rayet star. Simultaneousmodelling of the imaging and spectral data enables us to constrain boththe opening angle of the wind-wind collision interface and the dustgrain size. We conclude that the dust grains in the inner part of thePinwheel Nebula are small (~100 Å), in agreement with theoreticalpredictions, although we cannot rule out the presence of larger grains(~1 μm) further from the central binary. The opening angle of thewind-wind collision interface appears to be about 40°, in broadagreement with the wind parameters estimated for the central binary. Wediscuss the success and deficiencies of the model, and the likelybenefits of applying similar techniques to the more complex nebulaeobserved around other WR/O star binaries.
| A Very Large Array 3.6 Centimeter Continuum Survey of Galactic Wolf-Rayet Stars
We report the results of a survey of radio continuum emission ofGalactic Wolf-Rayet (WR) stars north of δ=-46°. Theobservations were obtained at 8.46 GHz (3.6 cm) using the Very LargeArray, with an angular resolution of ~6"×9" and typical rms noiseof ~0.04 mJy beam-1. Our survey of 34 WR stars resulted in 15definite and five probable detections, 13 of these for the first time atradio wavelengths. All detections are unresolved (θ<~5"). Timevariations in flux are confirmed in the cases of WR 98a, 104, 105, and125. WR 79a and WR 89 are also variable in flux, and we suspect they arealso nonthermal emitters. Thus, of our sample 20%-30% of the detectedstars are nonthermal emitters. Average mass-loss rate determinationsobtained excluding definite and suspected nonthermal cases give similarvalues for WN (all subtypes) and WC5-7 stars[M(WN)=(4+/-3)×10-5 Msolar yr-1and M(WC5-7)=(4+/-2)×10-5 Msolaryr-1], while a lower value was obtained for WC8-9 stars[M(WC8-9)=(2+/-1)×10-5 Msolaryr-1]. Uncertainties in stellar distances largely contributeto the observed scatter in mass-loss rates. Upper limits to themass-loss rates were obtained in cases of undetected sources and forsources that probably show additional nonthermal emission.
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