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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
| Measurements and Analysis of Helium-like Triplet Ratios in the X-Ray Spectra of O-Type Stars
We discuss new methods of measuring and interpreting theforbidden-to-intercombination line ratios of helium-like triplets in theX-ray spectra of O-type stars, including accounting for the spatialdistribution of the X-ray-emitting plasma and using the detailedphotospheric UV spectrum. Measurements are made for four O stars usingarchival Chandra HETGS data. We assume an X-ray-emitting plasmaspatially distributed in the wind above some minimum radiusR0. We find minimum radii of formation typically in the rangeof 1.25
| XMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter CygOB2#8A [O6If + O5.5III(f)]
We report on the results of four XMM-Newton observations separated byabout ten days from each other of CygOB2#8A [O6If + O5.5III(f)]. Thismassive colliding wind binary is a very bright X-ray emitter - one ofthe first X-ray emitting O-stars discovered by the Einstein satellite -as well as a confirmed non-thermal radio emitter whose binarity wasdiscovered quite recently. The X-ray spectrum between 0.5 and 10.0keV isessentially thermal, and is best fitted with a three-component modelwith temperatures of about 3, 9 and 20MK. The X-ray luminosity correctedfor the interstellar absorption is rather large, i.e. about1034ergs-1. Compared to the `canonical'LX/Lbol ratio of O-type stars, CygOB2#8A was afactor of 19-28 overluminous in X-rays during our observations. The EPICspectra did not reveal any evidence for the presence of a non-thermalcontribution in X-rays. This is not unexpected considering that thesimultaneous detections of non-thermal radiation in the radio and softX-ray (below 10.0keV) domains is unlikely. Our data reveal a significantdecrease in the X-ray flux from apastron to periastron with an amplitudeof about 20 per cent. Combining our XMM-Newton results with those fromprevious ROSAT-PSPC and ASCA-SIS observations, we obtain a light curvesuggesting a phase-locked X-ray variability. The maximum emission leveloccurs around phase 0.75, and the minimum is probably seen shortly afterthe periastron passage. Using hydrodynamic simulations of the wind-windcollision, we find a high X-ray emission level close to phase 0.75, anda minimum at periastron as well. The high X-ray luminosity, the strongphase-locked variability and the spectral shape of the X-ray emission ofCygOB2#8A revealed by our investigation point undoubtedly to X-rayemission dominated by colliding winds.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA).E-mail: debecker@astro.ulg.ac.be ‡Research Associate FNRS (Belgium).
| The Effect of Porosity on X-Ray Emission-Line Profiles from Hot-Star Winds
We investigate the degree to which the nearly symmetric form of X-rayemission lines seen in Chandra spectra of early-type supergiant starscould be explained by the possibly porous nature of their spatiallystructured stellar winds. Such porosity could effectively reduce thebound-free absorption of X-rays emitted by embedded wind shocks, andthus allow a more similar transmission of redshifted and blueshiftedemission from the back and front hemispheres, respectively. To obtainthe localized self-shielding that is central to this porosity effect, itis necessary that the individual clumps be optically thick. In a mediumconsisting of clumps of size l and volume filling factor f, we arguethat the general modification in effective opacity should scaleapproximately as κeff~κ/(1+τc),where, for a given atomic opacity κ and mean density ρ, theclump optical thickness scales as τc=κρl/f. Fora simple wind structure parameterization in which the ``porositylength'' h≡l/f increases with local radius r as h=h'r,we find that a substantial reduction in wind absorption requires a quitelarge porosity scale factor, h'>~1, implying largeporosity lengths h>~r. The associated wind structure must thus haveeither a relatively large scale l<~r, or a small volume fillingfactor f~l/r<<1, or some combination of these. We argue that therelatively small-scale, moderate compressions generated by intrinsicinstabilities in line driving are unlikely to give such large porositylengths. This raises questions about whether porosity effects could playa significant role in explaining nearly symmetric X-ray line profiles,leaving the prospect of instead having to invoke a substantial(approximately a factor of 5) downward revision in the assumed mass-lossrates.
| The XMM-Newton view of Plaskett's star and its surroundings
XMM-Newton data of Plaskett's star (HD 47129) are used in order toanalyse its X-ray spectrum and variability and hence to derive furtherconstraints on the wind interaction in this early-type binary (O6 I +O7.5 I) system.Conventional models fail to provide a consistent fit of the EuropeanPhoton Imaging Camera (EPIC) and Reflexion Grating Spectrometer (RGS)spectra. The lines seen in the RGS spectrum have a temperature ofmaximum emissivity between 0.18 and 1.4 keV. The EPIC and RGS spectraare best fitted by a non-equilibrium model consisting of abremsstrahlung continuum at 2.2 +/- 0.1 keV and a number of independentemission lines. Our tests also suggest that an overabundance in nitrogenby a factor of ~6 might be indicated to best represent the RGS spectrum.On the other hand, a short-term variability study of the light curves ofthe system indicates that the X-ray flux of Plaskett's star did notdisplay any significant variability during our observation. This resultholds for all time-scales investigated here (from a few minutes to aboutone hour). Combining our XMM-Newton data with ROSAT archivalobservations, we find, however, a significant variability on the orbitaltime-scale. If this behaviour is indeed phase locked, it suggests aminimum in the X-ray flux when the primary star is in front. This mightbe attributed to an occultation of the colliding wind region by the bodyof the primary.Finally, 71 other X-ray sources have been detected in the field aroundPlaskett's star and most of them have a near-infrared (near-IR)counterpart with colours that are consistent with those of slightlyreddened main-sequence objects. Actually, a sizeable fraction of theX-ray sources in the EPIC images could be either foreground orbackground sources with no direct connection to HD 47129.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.E-mail: linder@astro.ulg.ac.be (NL), rauw@astro.ulg.ac.be (GR) ‡Research Associate FNRS, Belgium.
| Bright OB stars in the Galaxy. III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Hα, IR and radio analysis
Context: .Recent results strongly challenge the canonical picture ofmassive star winds: various evidence indicates that currently acceptedmass-loss rates, {dot M}, may need to be revised downwards, by factorsextending to one magnitude or even more. This is because the mostcommonly used mass-loss diagnostics are affected by "clumping"(small-scale density inhomogeneities), influencing our interpretation ofobserved spectra and fluxes. Aims: .Such downward revisions wouldhave dramatic consequences for the evolution of, and feedback from,massive stars, and thus robust determinations of the clumping propertiesand mass-loss rates are urgently needed. We present a first attemptconcerning this objective, by means of constraining the radialstratification of the so-called clumping factor. Methods: .To thisend, we have analyzed a sample of 19 Galactic O-type supergiants/giants,by combining our own and archival data for Hα, IR, mm and radiofluxes, and using approximate methods, calibrated to more sophisticatedmodels. Clumping has been included into our analysis in the"conventional" way, by assuming the inter-clump matter to be void.Because (almost) all our diagnostics depends on the square of density,we cannot derive absolute clumping factors, but only factors normalizedto a certain minimum. Results: .This minimum was usually found tobe located in the outermost, radio-emitting region, i.e., the radiomass-loss rates are the lowest ones, compared to {dot M} derived fromHα and the IR. The radio rates agree well with those predicted bytheory, but are only upper limits, due to unknown clumping in the outerwind. Hα turned out to be a useful tool to derive the clumpingproperties inside r < 3{ldots}5 Rstar. Our most importantresult concerns a (physical) difference between denser and thinnerwinds: for denser winds, the innermost region is more strongly clumpedthan the outermost one (with a normalized clumping factor of 4.1± 1.4), whereas thinner winds have similar clumping properties inthe inner and outer regions. Conclusions: .Our findings arecompared with theoretical predictions, and the implications arediscussed in detail, by assuming different scenarios regarding the stillunknown clumping properties of the outer wind.
| Can single O stars produce non-thermal radio emission?
We present a model for the non-thermal radio emission from presumablysingle O stars, in terms of synchrotron emission from relativisticelectrons accelerated in wind-embedded shocks. These shocks areassociated with an unstable, chaotic wind. The main improvement withrespect to earlier models is the inclusion of the radial dependence ofthe shock velocity jump and compression ratio, based on one-dimensionaltime-dependent hydrodynamical simulations. The decrease of the velocityjump and the compression ratio as a function of radius produces arapidly decreasing synchrotron emissivity. This effectively prohibitsthe models from reproducing the spectral shape of the observednon-thermal radio emission. We investigate a number of “escaperoutes” by which the hydrodynamical predictions might bereconciled with the radio observations. We find that the observedspectral shape can be reproduced by a slower decline of the compressionratio and the velocity jump, by the re-acceleration of electrons in manyshocks or by adopting a lower mass-loss rate. However, none of theseescape routes are physically plausible. In particular, re-accelerationby feeding an electron distribution through a number of shocks, is incontradiction with current hydrodynamical simulations. Thesehydrodynamical simulations have their limitations, most notably the useof one-dimensionality. At present, it is not feasible to performtwo-dimensional simulations of the wind out to the distances requiredfor synchrotron-emission models. Based on the current hydrodynamicmodels, we suspect that the observed non-thermal radio emission from Ostars cannot be explained by wind-embedded shocks associated with theinstability of the line-driving mechanism. The most likely alternativemechanism is synchrotron emission from colliding winds. That would implythat all O stars with non-thermal radio emission should be members ofbinary or multiple systems.
| Winds from OB Stars: A Two-Component Scenario?
X-ray spectroscopy of several OB stars with massive winds has revealedthat many X-ray line profiles exhibit unexpectedly small blueshifts andare almost symmetric. Moreover, the hottest X-ray lines appear tooriginate closest to the star. These properties appear to beinconsistent with the standard model of X-rays originating in shockedmaterial in line-driven spherically symmetric winds. Here we raise thequestion, can the X-ray line data be understood in terms of atwo-component wind? We consider a scenario in which one component of thewind is a standard line-driven wind that emerges from a broad range oflatitudes centered on the equator. The second component of the windemerges from magnetically active regions in extensive polar caps. Theexistence of such polar caps is suggested by a recent model of dynamoaction in massive stars. We describe how the two-component model isconsistent with a variety of observational properties of OB star winds.
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
| Spectral analysis of early-type stars using a genetic algorithm based fitting method
We present the first automated fitting method for the quantitativespectroscopy of O- and early B-type stars with stellar winds. The methodcombines the non-LTE stellar atmosphere code fastwind from Puls et al.(2005, A&A, 435, 669) with the genetic algorithm based optimizationroutine pikaia from Charbonneau (1995, ApJS, 101, 309), allowing for ahomogeneous analysis of upcoming large samples of early-type stars (e.g.Evans et al. 2005, A&A, 437, 467). In this first implementation weuse continuum normalized optical hydrogen and helium lines to determinephotospheric and wind parameters. We have assigned weights to theselines accounting for line blends with species not taken into account,lacking physics, and/or possible or potential problems in the modelatmosphere code. We find the method to be robust, fast, and accurate.Using our method we analysed seven O-type stars in the young cluster CygOB2 and five other Galactic stars with high rotational velocities and/orlow mass loss rates (including 10 Lac, ζ Oph, and τ Sco) thathave been studied in detail with a previous version of fastwind. Thefits are found to have a quality that is comparable or even better thanproduced by the classical “by eye” method. We defineerrorbars on the model parameters based on the maximum variations ofthese parameters in the models that cluster around the global optimum.Using this concept, for the investigated dataset we are able to recovermass-loss rates down to ~6 × 10-8~Mȯyr-1 to within an error of a factor of two, ignoringpossible systematic errors due to uncertainties in the continuumnormalization. Comparison of our derived spectroscopic masses with thosederived from stellar evolutionary models are in very good agreement,i.e. based on the limited sample that we have studied we do not findindications for a mass discrepancy. For three stars we findsignificantly higher surface gravities than previously reported. Weidentify this to be due to differences in the weighting of Balmer linewings between our automated method and “by eye” fittingand/or an improved multidimensional optimization of the parameters. Theempirical modified wind momentum relation constructed on the basis ofthe stars analysed here agrees to within the error bars with thetheoretical relation predicted by Vink et al. (2000, A&A, 362, 295),including those cases for which the winds are weak (i.e. less than a fewtimes 10-7 Mȯ yr-1).
| Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution
In this paper we have analyzed 25 Galactic O and early B-stars by meansof H and K band spectroscopy, with the primary goal to investigate towhat extent a lone near-IR spectroscopy is able to recover stellar andwind parameters derived in the optical. Most of the spectra have beentaken with subaru-ircs, at an intermediate resolution of 12 000, andwith a very high S/N, mostly on the order of 200 or better. In order tosynthesize the strategic H/He lines, we have used our recent,line-blanketed version of fastwind (Puls et al. 2005, A&A, 435,669). In total, seven lines have been investigated, where for two starswe could make additional use of the Hei2.05 singlet which has beenobserved with irtf-cshell. Apart from Brγ and Heii2.18, the otherlines are predominately formed in the stellar photosphere, and thusremain fairly uncontaminated from more complex physical processes,particularly clumping. First we investigated the predicted behaviour ofthe strategic lines. In contradiction to what one expects from theoptical in the O-star regime, almost all photospheric H/Hei/Heii H/Kband lines become stronger if the gravity decreases. Concerning H andHeii, this finding is related to the behaviour of Stark broadening as afunction of electron density, which in the line cores is different formembers of lower (optical) and higher (IR) series. Regarding Hei, thepredicted behaviour is due to some subtle NLTE effects resulting in astronger overpopulation of the lower level when the gravity decreases.We have compared our calculations with results from the alternative NLTEmodel atmosphere code cmfgen (Hillier & Miller 1998, ApJ, 496, 407).In most cases, we found reasonable or nearly perfect agreement. Only theHei2.05 singlet for mid O-types suffers from some discrepancy, analogouswith findings for the optical Hei singlets. For most of our objects, weobtained good fits, except for the line cores of Brγ in earlyO-stars with significant mass-loss. Whereas the observations showBrγ mostly as rather symmetric emission lines, the models predicta P Cygni type profile with strong absorption. This discrepancy (whichalso appears in lines synthesized by cmfgen) might be an indirect effectof clumping. After having derived the stellar and wind parameters fromthe IR, we have compared them to results from previous optical analyses.Overall, the IR results coincide in most cases with the optical oneswithin the typical errors usually quoted for the correspondingparameters, i.e., an uncertainty in T_eff of 5%, in log g of 0.1 dex andin {dot M} of 0.2 dex, with lower errors at higher wind densities.Outliers above the 1-σ level where found in four cases withrespect to log g and in two cases for {dot M}.
| An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604
We discuss the results of two XMM-Newton observations of the opencluster NGC 6604 obtained in April and September 2002. We concentratemainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). Thesoft part of the EPIC spectrum of this system is thermal with typicaltemperatures of about 2 × 106 to 9 ×106 K. The nature (thermal vs. non-thermal) of the hard partof the spectrum is not unambiguously revealed by our data. If theemission is thermal, the high temperature of the plasma (~2.3 ×107 to 4.6 × 107 K) would be typical of whatshould be expected from a wind-wind interaction zone within a longperiod binary system. This emission could arise from an interactionbetween the combined winds of the O5-8V + O5-8V close binary system andthat of the more distant O8I companion. Assuming instead that the hardpart of the spectrum is non-thermal, the photon index would be rathersteep (~3). Moreover, a marginal variability between our two XMM-Newtonpointings could be attributed to an eclipse of the O5-8V + O5-8V system.The overall X-ray luminosity points to a significant X-ray luminosityexcess of about a factor 4 possibly due to colliding winds. ConsideringHD 167971 along with several recent X-ray and radio observations, wepropose that the simultaneous observation of non-thermal radiation inthe X-ray (below 10.0 keV) and radio domains appears rather unlikely.Our investigation of our XMM-Newton data of NGC 6604 reveals a rathersparse distribution of X-ray emitters. Including the two brightnon-thermal radio emitters HD 168112 and HD 167971, we present a list of31 X-ray sources along with the results of the cross-correlation withoptical and infrared catalogues. A more complete spectral analysis ispresented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-mainsequence star candidates.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| A layered model for non-thermal radio emission from single O stars
We present a model for the non-thermal radio emission from bright Ostars, in terms of synchrotron emission from wind-embedded shocks. Themodel is an extension of an earlier one, with an improved treatment ofthe cooling of relativistic electrons. This improvement limits thesynchrotron-emitting volume to a series of fairly narrow layers behindthe shocks. We show that the width of these layers increases withincreasing wavelength, which has important consequences for the shape ofthe spectrum. We also show that the strongest shocks produce the bulk ofthe emission, so that the emergent radio flux can be adequatelydescribed as coming from a small number of shocks, or even from a singleshock. A single shock model is completely determined by four parameters:the position of the shock, the compression ratio and velocity jump ofthe shock, and the surface magnetic field. Applying a single shock modelto the O5 If star Cyg OB2 No. 9 allows a gooddetermination of the compression ratio and shock position and, to alesser extent, the magnetic field and velocity jump. Our main conclusionis that strong shocks need to survive out to distances of a few hundredstellar radii. Even with multiple shocks, the shocks needed to explainthe observed emission are stronger than predictions from time-dependenthydrodynamical simulations.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| High-Resolution X-Ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association
The Cyg OB2 association contains some of the most luminous OB stars inour Galaxy, the brightest of which are also among the most luminous inX-rays. We obtained a Chandra High Energy Transmission GratingSpectrometer observation centered on Cyg OB2 No. 8A, the most luminousX-ray source in the association. Although our analysis focuses on theX-ray properties of Cyg OB2 No. 8A, we also present limited analyses ofthree other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standarddiagnostic techniques as used in previous studies of early-type stars,we find that the X-ray properties of Cyg OB2 No. 8A are very similar tothose of other OB stars that have been observed using high-resolutionX-ray spectroscopy. From analyses of the He-like ion fir emission lines(Mg XI, Si XIII, S XV, and Ar XVII), we derive radial distances of theHe-like line emission sources and find that the higher energy ions havetheir lines form closer to the stellar surface than those of lower ionstates. These fir-inferred radii are also found to be consistent withtheir corresponding X-ray continuum optical depth unity radii. Both ofthese findings are in agreement with previous O star studies and againsuggest that anomalously strong shocks or high-temperature zones may bepresent near the base of the wind. The observed X-ray emission-linewidths (HWHM~1000 km s-1) are also compatible with theobservations of other O star supergiants. Since Cyg OB2 No. 8A issimilar in spectral type to ζ Pup (the only O star that clearlyshows asymmetric X-ray emission-line profiles with large blueshifts), weexpected to see similar emission-line characteristics. Contrary to otherO star results, the emission lines of Cyg OB2 No. 8A show a large rangein line centroid shifts (~-800 to +250 km s-1). However, weargue that most of the largest shifts may be unreliable and theresulting range in shifts is much less than those observed in ζPup. Although there is one exception, the H-like Mg XII line, whichshows a blueshift of -550 km s-1, there are problemsassociated with trying to understand the nature of this isolated largeblueshifted line. To address the degree of asymmetry in these lineprofiles, we present Gaussian best-fit line profile model spectra fromζ Pup to illustrate the expected asymmetry signature in theχ2 residuals. Comparisons of the Cyg OB2 No. 8A best-fitline profile residuals with those of ζ Pup suggest that there areno indications of any statistical significant asymmetries in these lineprofiles. Both the line shift characteristics and lack of lineasymmetries are very puzzling results. Given the very high mass-lossrate of Cyg OB2 No. 8A (approximately 5 times larger than previousChandra-observed O supergiants), the emission lines from this starshould display a significant level of line asymmetry and blueshifts ascompared to other OB stars. We also discuss the implications of ourresults in light of the fact that Cyg OB2 No. 8A is a member of a rathertight stellar cluster, and shocks could arise at interfaces with thewinds of these other stars.
| A Spectroscopic study of the non-thermal radio emitter Cyg OB2 #8A: Discovery of a new binary system
We present the results of a spectroscopic campaign revealing that thenon-thermal radio emitter Cyg OB2 #8A is an O6 + O5.5 binary system. Wepropose the very first orbital solution indicating a period of about21.9 days. The system appears to be eccentric (0.24 ± 0.04) andis likely seen under a rather low inclination angle. The mass ratio ofthe components is close to unity. The impact of the binarity of thisstar in the framework of our understanding of non-thermal radio emissionfrom early-type stars is briefly discussed.Based on observations collected at the Observatoire de Haute Provence,France.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| A Westerbork Synthesis Radio Telescope 1400 and 350 MHz Continuum Survey of the Cygnus OB2 Association, in Search of Hot Massive Stars
We present a radio continuum survey at 1400 and 350 MHz of a region of2deg×2deg centered on the Cygnus OB2association (d=1.7 kpc), using the Westerbork Synthesis Radio Telescope(WSRT) with angular resolutions of, respectively, 13" and 55". Theresulting 5 σ flux-density limits of, respectively, ~2 mJy and~10-15 mJy are a significant improvement over previous surveys. Wedetected 210 discrete sources with sizes less than1.9θbeam (beam size), 98 of which at both frequencies.We also detected 28 resolved sources(sizes>1.9θbeam) still having well-defined peakintensities. The observed spectral indexα1400350 distribution and source countstrongly suggest an excess of sources of Galactic origin in thedirection of Cyg OB2.We have searched for positional coincidences of the detected sources inour list with other radio, infrared, and optical objects from varioussurveys by using the likelihood ratio (LR) method. Furthermore, welooked for objects that show characteristics of either optically thickstellar winds (α >~ +0.6), or nonthermal emission(-1<~α<~+0.6) and/or variable spectral flux density. The LRmethod resulted in 108 identifications. Eighty unidentified sources,i.e., ~2/3, show characteristics of sources of Galactic origin, 10 ofwhich may be stars. The remaining unidentified sources are probably ofextragalactic origin.We identified one source with the O7 star Cyg OB2-335 and consider it tobe a candidate colliding-wind binary. We also identify 19 point sourceswith known infrared and optical objects: these have nearly flat orinverted spectral indices, and some of them show flux-densityvariability. Follow-up multifrequency monitoring of these sources willbe important in establishing the reality of the flux variabilities andto assess the nature of these sources.Based on observations made with the Westerbork Synthesis Radio Telescope(WSRT), operated by the Netherlands Foundation for Research in Astronomy(NFRA).
| A Study of Cygnus OB2: Pointing the Way toward Finding Our Galaxy's Super-Star Clusters
New optical MK classification spectra have been obtained for 14 OB starcandidates identified by Comerón et al. and presumed to bepossible members of the Cyg OB2 cluster as recently described byKnödlseder. All 14 candidate OB stars observed are indeedearly-type stars, strongly suggesting the remaining 31 candidates byComerón et al. are also early-type stars. A thoroughinvestigation of the properties of these new candidate members comparedwith the properties of the Cyg OB2 cluster star has been completed,using traditional as well as newly revised effective temperature scalesfor O stars. The cooler O star effective temperature scale of Martins etal. gives a very close distance for the cluster (DM=10.4). However, evenusing traditional effective temperature scales, Cyg OB2 appears to beslightly closer (DM=10.8) than previous studies determined (DM=11.2;Massey & Thompson), when the very young age of the stellar cluster(~2×106 yr) is taken into account in fitting the late-Oand early-B dwarfs to model isochrones. Of the 14 new OB stars observedfor this study, as many as half appear to be significantly older thanthe previously studied optical cluster, making their membership in CygOB2 suspect. So, while some of the newly identified OB stars mayrepresent a more extended halo of the Cyg OB2 cluster, the survey ofComerón et al. also picked up a large fraction of nonmembers.Presently, estimates of the very high mass of this cluster(Mcl~104 Msolar and over 100 O stars)first made by Knödlseder remain higher than this study can support.Despite this, the recognition of Cyg OB2 as a more massive and extensivestar cluster than previously realized using 2MASS images, along with therecently recognized candidate super-star cluster Westerlund 1 only a fewkiloparsecs away (Clark & Negueruela), reminds us that we arewoefully underinformed about the massive cluster population in ourGalaxy. Extrapolations of the locally derived cluster luminosityfunction indicate that tens to perhaps a hundred of these very massiveopen clusters (Mcl~104 Msolar,MV~-11) should exist within our galaxy. Radio surveys willnot detect these massive clusters if they are more than a few millionyears old. Our best hope for remedying this shortfall is through deepinfrared searches and follow-up near-infrared spectroscopicobservations, as were used by Comeron et al. to identify candidatemembers of the Cyg OB2 association.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/905
| Does Cyg OB2 harbour any open cluster?
We report the detection of three new open cluster candidates located inCygnus. Two of them are projected near the center of the rich compactassociation Cyg OB2. These two objects have been found during an opticalinspection of that region with Digitized Sky Survey images. Using 2MASSphotometry we determined their fundamental parameters. We assumed an ageof 1-4 Myr and obtained a distance from the Sun d_sun = 1.8 +/- 0.2 kpcfor both objects. Reddening values for Objects 1 and 2 are E(B-V) = 2.12+/- 0.06 and E(B-V)=1.82+/-0.06, respectively. A normal luminosityfunction is observed down to A0 stars, similar to that of NGC 6910. Thetwo open cluster candidates may form a physical pair in Cyg OB2, similarto Tr 14/Tr 16 in Car OB1. Subsequently, we carried out a systematicsearch with 2MASS in a region 3degx3degencompassing Cyg OB2. A third uncatalogued object resulted from thissearch which is projected just outside Cyg OB2.
| Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus
We have made a multiwavelength study of the overlapping error boxes ofthe unidentified γ-ray sources TeV J2032+4130 and 3EG J2033+4118in the direction of the Cygnus OB2 association (d=1.7 kpc) in order tosearch for a point-source counterpart of the first unidentified TeVsource. Optical identifications and spectroscopic classifications forthe brighter X-ray sources in ROSAT PSPC and Chandra ACIS images areobtained, without finding a compelling counterpart. The classified X-raysources are a mix of early- and late-type stars, with one exception. Thebrightest source in the Chandra observation is a new, hard absorbedsource that is both transient and rapidly variable. It lies 7' from thecentroid of the TeV emission, which places it outside of the claimed 2σ location (r~4.8′). A possible eclipse or ``dip''transition is seen in its light curve. With a peak 1-10 keV luminosityof ~7×1032(d/1.7 kpc)2 ergs s-1,this source could be a quiescent low-mass X-ray binary that lies beyondthe Cyg OB2 association. A coincident, reddened optical object ofR=20.4, J=15.4, H=14.2, and K=13.4 is observed but not yet classified asa result of the lack of obvious emission or absorption features in itsspectrum. Alternatively, this Chandra and optical source might be aconsidered a candidate for a ``proton blazar,'' a long hypothesized typeof radio-weak γ-ray source. More detailed observations will beneeded to determine the nature of this variable X-ray source and toassess the possibility of its connection with TeV J2032+4130.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Fundamental parameters of Galactic luminous OB stars VI. Temperatures, masses and WLR of Cyg OB2 supergiants
We have analyzed six OB supergiants and one giant covering spectraltypes from O3 to B1 in the Galactic OB association Cyg OB2 by means ofan updated version of FASTWIND (Santolaya-Rey et al. \cite{sph97}) thatincludes an approximate treatment of metal line blocking and blanketing.This large coverage in spectral type allows us to derive a newtemperature scale for Galactic O supergiants that is lower than the oneobtained by using pure H-He models, either plane-parallel andhydrostatic or spherical with mass-loss. The lower temperatures are thusa combined effect of line blanketing and the large mass-loss rates. Insome cases, the newly derived effective temperature is reduced by up to8000 K. Changes are larger for earlier stars with large mass-loss rates.As a consequence, luminosities are modified as well, which results in alower number of emerging ionizing photons and reduces the massdiscrepancy. Although there are still significant differences betweenspectroscopic and evolutionary masses, we do not find any obvioussystematic pattern of those differences. We derive mass-loss rates andthe corresponding wind momentum-luminosity relation for the analyzedstars. Although consistent with previous results by Puls et al.(\cite{puls96}) for Galactic stars, our relation is better defined dueto a reduction of errors related to stellar distances and points to apossible separation between extreme Of stars (Of+,Of*) and stars with more moderate morphologies. However thisfinding is only tentative, as the statistics are still scarce. The INTis operated in the island of La Palma by the ING in the SpanishObservatorio de El Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| On the massive star contents of Cygnus OB2
We present a near-infrared spectroscopic survey of a large area centeredon the Cygnus OB2 association aimed at constraining its massive starcontents. Our goal is to establish a nearly complete list of O-typemembers of the association, both to examine recent claims based onstarcounts that suggest a richer content than previously thought, and toprovide a suitable database for further studies of the entire high-massend of one of the richest associations of the Galaxy. The targetselection is based on the JHK photometry published in the 2MASS all-skysurvey. We identify 46 new early-type candidates, most of them expectedto be O-type stars, plus 16 new stars with emission in Brgamma and oftenin other lines as well, characteristic of evolved massive starsundergoing intense mass loss. We also present spectra of three luminousstars with CO overtone emission, one of them having also intenseH2 emission and being associated with compact nebulosity. Byconsidering our findings, those of other authors, and plausiblecompleteness corrections, we estimate the number of O-type stars orstars having evolved from a O-type progenitor to be 90-100, slightlybelow, but compatible with, most recent starcounts estimates byKnödlseder (2000, A&A, 360, 539). These results support thenotion that Cygnus OB2 may be considered as a young globular cluster.The lists of new members that we provide, in particular those withemission lines, should be a useful resource for future investigations ofCygnus OB2 itself, as well as of very massive stellar evolution byproviding a nearby, abundant sample of stars sharing a commonenvironment. Based on observations collected at the German-SpanishAstronomical Center, Calar Alto.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| Absorption Wavelengths and Bandwidths for Interstellar Searches of C60 in the 2400-4100 Å Region
Low-temperature gas-phase wavelengths and bandwidths for astronomicalsearches for C60 in the 2400-4100 Å region aredetermined from the absorption spectra of C60 in rare gasmatrices at 4 K, coupled with information on the dependence of thevibronic transition frequencies of this fullerene on the Lorentz-Lorenzpolarizability parameter of matrices and solvents. These bandwidthsincrease from 25+/-5 cm-1 for the 11T1u-1 1Ag band at4024.0+/-0.5 Å to 2300+/-200 cm-1 for the 61T1u-1 1Ag band at 2488+/-8Å. Spectral observations of astrophysical objects considered to belikely sites of C60 have been examined in the context of thewavelengths and bandwidths of bands of the allowed transitions ofC60 which we recommend for searches of this species. Nocertain positive assignment of a C60 band has been made inthe 2400-4100 Å spectral region of these objects. The physicalorigin of the observed C60 absorption bandwidths is analyzedand discussed in an appendix.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/375/670
| Interstellar C2 and CN toward the Cyg OB2 association. A case study of X-ray induced chemistry
An analysis of deep optical echelle spectra towards six stars in the CygOB2 (VI Cygni) association is presented. Interstellar absorption linesup to J''=18 in the (2,0) and (3,0) bands of the C2A1Piu - X1Sigmag+system are detected towards Cyg OB2 No. 12. The large number ofrotational lines accurately constrains the gas-kinetic temperature T andthe density n to T = 35 K and n = (600 +/- 100) cm-3. Theinferred C2 column density is N(C2) =(20+4-2) 1013 cm-2. Thedetection of various lines in the (1,0) and (2,0) band of the CNA2Piu - X2Sigma + red systemsuggest a column density of N(CN) = (8-13)x 1013cm-2. C2 absorption lines are also detectedtowards Cyg OB2 No. 5 and No. 9. Inferred parameters are T = 50 K, n =(600 +/- 200) cm-3,N(C2)=(10+3.5-1.5)x 1013cm-2 towards No. 5, and T = 100 K, n >= 800cm-3, N(C2)=(5.2+/- 1)x 1013cm-2 towards No. 9. Marginal detections of C2towards Cyg OB2 No. 8A indicate N(C2) ~ 3.3x 1013cm-2 and T ~ 100 K. Upper limits are N(C2) <=3.3x 1013 cm-2 toward Cyg OB2 No. 7 and No. 11.The C2 observations eliminate the possibility that themolecular material along the line of sight towards Cyg OB2 No. 12 isspread over a pathlength of several hundred parsecs of very low densityn ~ 10 cm-3. The observations provide some support to arecent chemical model which assumes a nested structure of the moleculargas. Alternatively, the C2 and CN abundances obtained towardsCyg OB2 No. 12 are in agreement with the predictions of an X-ray inducedchemistry driven by an ionisation rate of zeta = (0.6-3)x10-15 s-1. Calculated equilibrium temperatures ofT = 25-50 K agree with temperatures inferred from C2. Themodel also reproduces the observed column densities of CO and CH, andthat of H3+ to within a factor of two. We predicta H2O+ column density of 2x 1012cm-2 towards Cyg OB2 No. 12 and H2O+absorption lines which are detectable by optical absorption linetechniques. We report the detection of interstellar Rubidium towards CygOB2 No. 12 and No. 5. Inferred column densities are N(Rb) = (13+/- 2)x109 cm-2 towards No. 12 and N(Rb) = (13+/- 2)x109 cm-2 towards No. 5.
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