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Five More Massive Binaries in the Cygnus OB2 Association
We present the orbital solutions for four OB spectroscopic binaries,MT145, GSC 03161 - 00815, 2MASS J20294666+4105083, and Schulte 73,and the partial orbital solution to the B spectroscopic binary, MT372,as part of an ongoing study to determine the distribution of orbitalparameters for massive binaries in the Cygnus OB2 Association. MT145 isa new, single-lined, moderately eccentric (e = 0.291 ± 0.009)spectroscopic binary with period of 25.140 ± 0.008 days. GSC03161 - 00815 is a slightly eccentric (e = 0.10 ± 0.01),eclipsing, interacting and double-lined spectroscopic binary with aperiod of 4.674 ± 0.004 days. 2MASS J20294666+4105083 is amoderately eccentric (e = 0.273 ± 0.002) double-linedspectroscopic binary with a period of 2.884 ± 0.001 days. Schulte73 is a slightly eccentric (e = 0.169 ± 0.009), double-linedspectroscopic binary with a period of 17.28 ± 0.03 days and thefirst "twin" in our survey with a mass ratio of q = 0.99 ± 0.02.MT372 is a single-lined, eclipsing system with a period of 2.228 daysand low eccentricity (e ~ 0). Of the now 18 known OB binaries in CygOB2, 14 have periods and mass ratios. Emerging evidence also shows thatthe distribution of log(P) is flat and consistent with "Öpik'sLaw."
| A multiwavelength investigation of the massive eclipsing binary Cygnus OB2 #5
Context: The properties of the early-type binary Cyg OB2#5 have been debated for many years and spectroscopic andphotometric investigations yielded conflicting results. Aims: Wehave attempted to constrain the physical properties of the binary bycollecting new optical and X-ray observations. Methods: Theoptical light curves obtained with narrow-band continuum andline-bearing filters are analysed and compared. Optical spectra are usedto map the location of the He ii λ 4686 and Hαline-emission regions in velocity space. New XMM-Newton as well asarchive X-ray spectra are analysed to search for variability andconstrain the properties of the hot plasma in this system. Results: We find that the orbital period of the system slowly changesthough we are unable to discriminate between several possibleexplanations of this trend. The best fit solution of the continuum lightcurve reveals a contact configuration with the secondary star beingsignificantly brighter and hotter on its leading side facing theprimary. The mean temperature of the secondary star turns out to be onlyslightly lower than that of the primary, whilst the bolometricluminosity ratio is found to be 3.1. The solution of the light curveyields a distance of 925 ± 25 pc much lower than the usuallyassumed distance of the Cyg OB2 association. Whilst we confirm theexistence of episodes of higher X-ray fluxes, the data reveal nophase-locked modulation with the 6.6 day period of the eclipsing binarynor any clear relation between the X-ray flux and the 6.7 yr radiocycle. Conclusions: The bright region of the secondary star isprobably heated by energy transfer in a common envelope in this contactbinary system as well as by the collision with the primary's wind. Theexistence of a common photosphere probably also explains the oddmass-luminosity relation of the stars in this system. Most of the X-ray,non-thermal radio, and possibly γ-ray emission of Cyg OB2 #5 islikely to arise from the interaction of the combined wind of theeclipsing binary with at least one additional star of this multiplesystem.Based on observations collected at the Observatoire de Haute Provence(France), the Observatorio Astronómico Nacional of San PedroMártir (Mexico) and XMM-Newton, an ESA science mission withinstruments and contributions funded by ESA member states and the USA(NASA). Light curves of Cyg OB2 #5 are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/495/231
| Spectrophotometry with Hectospec, the MMT's Fiber-Fed Spectrograph
The spectrophotometric calibration of surveys is a significant, butoften neglected, issue when measuring the history of star formation bycombining spectroscopic surveys conducted with different instruments. Wedescribe techniques for photometric calibration of optical spectraobtained with the MMT's fiber-fed spectrograph, Hectospec. Theatmospheric dispersion compensation prisms built into the MMT's f/5wide-field corrector effectively eliminate errors due to differentialrefraction and simplify the calibration procedure. The procedures thatwe describe here are applicable to all spectra obtained to date withHectospec because the instrument response is stable. We estimate theinternal error in the Hectospec measurements by comparing duplicatemeasurements of galaxies. For a sample of 400 galaxies in theSmithsonian Hectospec Lensing Survey (SHELS) with a median , we compareline and continuum fluxes measured by Hectospec through a 1.5'' diameteroptical fiber with those measured by the Sloan Digital Sky Survey (SDSS)through a 3'' diameter optical fiber. Agreement of the [O II] andHα SHELS and SDSS line fluxes, after scaling by the -band flux inthe different apertures, suggests that the spatial variation instar-formation rates over a 1.5 to 3 kpc radial scale is small. Themedian ratio of the Hectospec and SDSS spectra, smoothed over 100Å scales, is remarkably constant to over the range of 3850 to8000 Å. Offsets in the ratio of the median [O II] and Hαfluxes, the equivalent width of Hδ and the continuum index d4000are a few percent, small compared with other sources of scatter. We alsoexplore the impact of atmospheric absorption. Observing redward of 6500Å, it is impossible to remove the effects of atmosphericabsorption perfectly because the variation of absorption with wavelengthis not resolved by a moderate dispersion spectrograph. Thus measurementsof spectral line fluxes including Hα, and derived physicalquantities including star-formation rates, may have sizable systematicerrors where the redshifted spectral features land on strong atmosphericabsorption troughs.
| New very massive stars in Cygnus OB2
Context: The compact association Cygnus OB2 is known to contain a largepopulation of massive stars, but its total mass is currently a matter ofdebate. While recent surveys have uncovered large numbers of OB stars inthe area around Cyg OB2, detailed study of the optically brightest amongthem suggests that most are not part of the association. Aims: Weobserved an additional sample of optically faint OB star candidates,with the aim of checking if more obscured candidates are correspondinglymore likely to be members of Cyg OB2. Methods: Low resolutionspectra of 9 objects allow the rejection of one foreground star and theselection of four O-type stars, which were later observed at higherresolution. In a subsequent run, we observed three more stars in theclassification region and three other stars in the far red. Results: We identify five (perhaps six) new evolved very massive starsand three main sequence O-type stars, all of which are likely to bemembers of Cyg OB2. The new findings allow a much better definition ofthe upper HR diagram, suggesting an age ~2.5 Myr for the association andhinting that the O3-5 supergiants in the association are bluestragglers, either younger or following a different evolutionary pathfrom other cluster members. Though the bulk of the early stars seems tobelong to an (approximately) single-age population, there is ampleevidence for the presence of somewhat older stars at the same distance. Conclusions: Our results suggest that, even though Cyg OB2 isunlikely to contain as many as 100 O-type stars, it is indeedsubstantially more massive than was thought prior to recent infraredsurveys.Figure [see full textsee full textsee full textsee full textsee fulltextsee full textsee full text] and Table [see full textsee full textseefull textsee full textsee full textsee full textsee full text] are onlyavailable in electronic form at http://www.aanda.org
| Non-thermal radio emission from O-type stars. III. Is Cygnus OB2 No. 9 a wind-colliding binary?
The star Cyg OB2 No. 9 is a well-known non-thermal radio emitter. Recenttheoretical work suggests that all such O-stars should be in a binary ora multiple system. However, there is no spectroscopic evidence of abinary component. Re-analysis of radio observations from the VLA of thissystem over 25 years has revealed that the non-thermal emission varieswith a period of 2.35±0.02 yr. This is interpreted as a strongsuggestion of a binary system, with the non-thermal emission arising ina wind-collision region. We derived some preliminary orbital parametersfor this putative binary and revised the mass-loss rate of the primarystar downward from previous estimates.Appendix A is only available in electronic form at http://www.aanda.org
| A binary signature in the non-thermal radio-emitter Cyg OB2 #9
Aims: Non-thermal radio emission associated with massive stars isbelieved to arise from a wind-wind collision in a binary system.However, the evidence of binarity is still lacking in some cases,notably Cyg OB2 #9. Methods: For several years, we have beenmonitoring this heavily-reddened star from various observatories. Thiscampaign allowed us to probe variations both on short and longtimescales and constitutes the first in-depth study of the visiblespectrum of this object. Results: Our observations provide thevery first direct evidence of a companion in Cyg OB2 #9, confirming thetheoretical wind-wind collision scenario. These data suggest a highlyeccentric orbit with a period of a few years, compatible with the 2yr-timescale measured in the radio range. In addition, the signature ofthe wind-wind collision is very likely reflected in the behaviour ofsome emission lines.Based on observations obtained at the OHP and the Asiago observatory.
| On the evolution and fate of super-massive stars
Context: We study the evolution and fate of solar compositionsuper-massive stars in the mass range 60-1000 Mȯ. Ourstudy is relevant for very massive objects observed in young stellarcomplexes as well as for super-massive stars that could potentially formthrough runaway stellar collisions. Aims: We predict the outcomes ofstellar evolution by employing a mass-loss prescription that isconsistent with the observed Hertzsprung-Russell Diagram location of themost massive stars. Methods: We compute a series of stellar models withan appropriately modified version of the Eggleton evolutionary code. Results: We find that super-massive stars with initial masses up to 1000Mȯ end their lives as objects less massive than≃150 Mȯ. These objects are expected to collapseinto black holes (with M ⪉ 70 ~Mȯ) or explode aspair-instability supernovae. Conclusions: We argue that ifultraluminous X-ray sources (ULXs) contain intermediate-mass blackholes, these are unlikely to be the result of runaway stellar collisionsin the cores of young clusters.
| 2MASS Two-Color Interstellar Reddening Line in the Direction of the North America and Pelican Nebulae and the CYG OB2 Association
The slope of the interstellar reddening line in the J--H vs.H--Ks diagram of the 2MASS survey in the direction of theNorth America and Pelican nebulae, the L 935 dust cloud and the Cyg OB2association is determined. The MK types were either classified by C. J.Corbally or collected from the literature. The ratioEJ-H/EH-K_s = 2.0 is obtained by taking theaverage for the four groups of spectral classes: O3--B1, B2--B6,B7--B9.5 and red clump giants. The obtained ratio is among the largestvalues of EJ-H/EH-K_s determined till now.
| A Preliminary Investigation of the Diffuse Interstellar Line at 8621 Å
We have obtained high-resolution spectra at the Dominion AstrophysicalObservatory, the Calar Alto Observatory, and the European SouthernObservatory of hot stars in the near-infrared region to study thediffuse interstellar band (DIB) at 8621 Å. Field stars as well asselected members of the Perseus spiral arm, the ρ Ophiuchi complex,and the Cygnus OB2 association were observed and the equivalent widthsof the band measured, as well as interstellar K I absorption at 7699Å. In total we measure the equivalent width of the 8621 band in 64stars. In a series of figures we show the correlations of the DIB withreddening, polarization, K I, and strengths of other DIBs at 5780, 5797,and 6613 Å. The quality of the correlations are discussed by meansof the Spearman rank correlation test. Good correlations are found withreddening and, among the other DIBs, 8621 correlates best with 5780. Byexamining the three special regions mentioned above we confirm theweakness of the 8621 DIB in the hot stars of the ρ Oph star-formingregion. In the Perseus spiral arm region we find that the DIBs areformed largely in foreground clouds but not in the gas that appears tohave been blown away from the young massive stars of the Perseus arm. InCyg OB2 we find that the equivalent widths of the 8621 line lie abovethe linear correlation of equivalent width with E(B-V) and theirstrength does not vary as strongly with reddening. The heavily reddenedCyg OB2 star 12 is the most extreme example of the relativeinsensitivity of the 8621 equivalent width to reddening in the CygnusOB2 region.
| 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.
| An Extensive Collection of Stellar Wind X-Ray Source Region Emission Line Parameters, Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars
Chandra high energy resolution observations have now been obtained fromnumerous nonpeculiar O and early B stars. The observed X-ray emissionline properties differ from prelaunch predictions, and theinterpretations are still problematic. We present a straightforwardanalysis of a broad collection of OB stellar line profile data to searchfor morphological trends. X-ray line emission parameters and the spatialdistributions of derived quantities are examined with respect toluminosity class. The X-ray source locations and their correspondingtemperatures are extracted by using the He-like f/i line ratios and theH-like-to-He-like line ratios, respectively. Our luminosity class studyreveals line widths increasing with luminosity. Although the majority ofthe OB emission lines are found to be symmetric, with little centralline displacement, there is evidence for small, but finite, bluewardline shifts that also increase with luminosity. The spatial X-raytemperature distributions indicate that the highest temperatures occurnear the star and steadily decrease outward. This trend is mostpronounced in the OB supergiants. For the lower density wind stars, bothhigh and low X-ray source temperatures exist near the star. However, wefind no evidence of any high-temperature X-ray emission in the outerwind regions for any OB star. Since the temperature distributions arecounter to basic shock model predictions, we call this the ``near-starhigh-ion problem'' for OB stars. By invoking the traditional OB stellarmass-loss rates, we find a good correlation between the fir-inferredradii and their associated X-ray continuum optical depth unity radii. Weconclude by presenting some possible explanations of the X-ray sourceproblems that have been revealed by this study.
| Deep radio images of the HEGRA and Whipple TeV sources in the Cygnus OB2 region
Context: The modern generation of Cherenkov telescopes has revealed anew population of gamma-ray sources in the Galaxy. Some of them havebeen identified with previously known X-ray binary systems while otherremain without clear counterparts a lower energies. Our initial goalhere was reporting on extensive radio observations of the first extendedand yet unidentified source, namely TeV J2032+4130. This object wasoriginally detected by the HEGRA telescope in the direction of theCygnus OB2 region and its nature has been a matter of debate during thelatest years. The situation has become more complex with the Whipple andMILAGRO telescopes new TeV detections in the same field which could beconsistent with the historic HEGRA source, although a different origincannot be ruled out. Aims: We aim to pursue our radio explorationof the TeV J2032+4130 position that we initiated in a previous paper buttaking now into account the latest results from new Whipple and MILAGROTeV telescopes. The data presented here are an extended follow up of ourprevious work. Methods: Our investigation is mostly based oninterferometric radio observations with the Giant Metre Wave RadioTelescope (GMRT) close to Pune (India) and the Very Large Array (VLA) inNew Mexico (USA). We also conducted near infrared observations with the3.5 m telescope and the OMEGA2000 camera at the CentroAstronómico Hispano Alemán (CAHA) in Almería(Spain). Results: We present deep radio maps centered on the TeVJ2032+4130 position at different wavelengths. In particular, our 49 and20 cm maps cover a field of view larger than half a degree that fullyincludes the Whipple position and the peak of MILAGRO emission. Our mostimportant result here is a catalogue of 153 radio sources detected at 49cm within the GMRT antennae primary beam with a full width half maximum(FWHM) of 43 arc-minute. Among them, peculiar sources inside the Whippleerror ellipse are discussed in detail, including a likely double-doubleradio galaxy and a one-sided jet source of possible blazar nature. Thislast object adds another alternative counterpart possibility to beconsidered for both the HEGRA, Whipple and MILAGRO emission. Moreover,our multi-configuration VLA images reveal the non-thermal extendedemission previously reported by us with improved angular resolution. Itsnon-thermal spectral index is also confirmed thanks to matching beamobservations at the 20 and 6 cm wavelengths.Table 3 is only available in electronic form at http://www.aanda.org
| Radio observations of candidate magnetic O stars
Context: Some O stars are suspected to have to have (weak) magneticfields because of the observed cyclical variability in their UVwind-lines. However, direct detections of these magnetic fields usingoptical spectropolarimetry have proven to be very difficult. Aims: Non-thermal radio emission in these objects would most likely bedue to synchrotron radiation. As a magnetic field is required for theproduction of synchrotron radiation, this would be strong evidence forthe presence of a magnetic field. Such non-thermal emission has alreadybeen observed from the strongly magnetic Ap/Bp stars. Methods: Wehave performed 6 and 21 cm observations using the WSRT and use these, incombination with archival VLA data at 3.6 cm and results from theliterature, to study the radio emission of 5 selected candidate magneticO stars. Results: Out of our five targets, we have detectedthree: ξ Per, which shows a non-thermal radio spectrum, and αCam and λ Cep, which show no evidence of a non-thermal spectrum.In general we find that the observed free-free (thermal) flux of thestellar wind is lower than expected. This is in agreement with recentfindings that the mass-loss rates from O stars as derived from theHα line are overestimated because of clumping in the inner part ofthe stellar wind.
| A very massive runaway star from Cygnus OB2
Aims:We investigate the star BD+43° 3654 and the possibility that itmay have originated in the massive OB association Cygnus OB2. Methods: We present new spectroscopic observations allowing a reliablespectral classification of the star, and discuss existing MSXobservations of its associated bow shock and astrometric information notpreviously studied. Results: Our observations reveal thatBD+43° 3654 is a very early and luminous star of spectral type O4If,with an estimated mass of (70 ± 15) M_ȯ and an age of ~1.6Myr. The high spatial resolution of the MSX observations allows us todetermine its direction of motion in the plane of the sky by means ofthe symmetry axis of the well-defined bow shock, which matches well theorientation expected from the proper motion. Tracing back its pathacross the sky we find that BD+43° 3654 was located near thecentral, densest region of Cygnus OB2 at a time in the past similar toits estimated age. Conclusions: BD+43° 3654 turns out to beone of the three most massive runaway stars known, and it most likelyformed in the central region of Cygnus OB2. A runaway formationmechanism by means of dynamical ejection is consistent with our results.Based on observations collected at the Centro AstronómicoHispano-Alemán (CAHA) at Calar Alto, operated jointly by theMax-Planck Institut für Astronomie and the Instituto deAstrofísica de Andalucía (CSIC).
| Unveiling the Cygnus OB2 stellar population with Chandra
Aims.The aim of this work is to identify the so far unknown low massstellar population of the ~2 Myr old Cygnus OB2 star forming region, andto investigate the X-ray and near-IR stellar properties of its members. Methods: We analyzed a 97.7 ks Chandra ACIS-I observation pointed at thecore of the Cygnus OB2 region. Sources were detected using the PWDetectcode and were positionally correlated with optical and near-IR catalogsfrom the literature. Source events were extracted with the Acis Extractpackage. X-ray variability was characterized through theKolmogorov-Smirnov test and spectra were fitted using absorbed thermalplasma models. Results: We detected 1003 X-ray sources. Of these, 775have near-IR counterparts and are expected to be almost all associatedwith Cygnus OB2 members. From near-IR color-color and color-magnitudediagrams we estimate a typical absorption toward Cygnus OB2 ofAv ≈ 7.0 mag. Although the region is young, very few stars(~4.4%) show disk-induced excesses in the near-IR. X-ray variability isdetected in ~13% of the sources, but this fraction increases, up to 50%,with increasing source statistics. Flares account for at least 60% ofthe variability. Despite being generally bright, all but 2 of the 26detected O- that early B-type stars are not significantly variable.Typical X-ray spectral parameters are log N_H˜ 22.25(cm-2) and kT˜ 1.35 keV with 1σ dispersion of 0.2dex and 0.4 keV, respectively. Variable and flaring sources have harderspectra with median kT= 3.3 and 3.8 keV, respectively. OB stars aretypically softer (kT ˜ 0.75 keV). X-ray luminosities range between1030 and 1031 erg s-1 for intermediate-and low-mass stars, and 2.5×1030 and between6.3×1033 erg s-1 for OB stars. Conclusions:.The Cygnus OB2 region has a very rich population of low-mass X-rayemitting stars. Circumstellar disks seem to be very scarce. X-rayvariability is related to the magnetic activity of low-mass stars(M/M_ȯ ˜0.5 to 3.0) display X-ray activity levels comparableto those of Orion Nebular Cluster (ONC) sources in the same mass range.Tables 1-3 are only available in electronic form at http://www.aanda.org
| Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group
We analyse the properties of three unusual dwarf galaxies in theCentaurus A group discovered with the HIPASS survey. From their opticalmorphology, they appear to be low surface brightness dwarf spheroidals,yet they are gas rich (MHI/LB > 1) withgas-mass-to-stellar light ratios larger than typical dwarf irregulargalaxies. Therefore these systems appear different from any dwarfs ofthe Local Group. They should be favoured hosts for starburst, whereas wefind a faint star formation region in only one object. We have obtained21-cm data and Hubble Space Telescope photometry in V and I bands, andhave constructed colour magnitude diagrams (CMDs) to investigate theirstellar populations and to set a constraint on their age. From thecomparison of the observed and model CMDs, we infer that all threegalaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr)and remain gas rich because their star formation rates have been verylow (<~10-3Msolaryr-1) throughout.In such systems, star formation appears to have been sporadic and local,though one object (HIPASS J1321-31) has a peculiar red plume in its CMDsuggesting that many of its stars were formed in a `miniburst' 300-500Myr ago. The question of why there are no similar dwarf galaxies in theLocal Group remains open.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociations of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555; the Australia Telescope Compact Arraywhich is part of the Australia Telescope, funded by the Commonwealth ofAustralia for operation as a National Facility managed by CSIRO.E-mail: marco.grossi@roma 1.infn.it ‡Visiting Astronomer, Kitt Peak National Observatory, National OpticalAstronomy Observatories, which is operated by the Association ofUnicersities for for Reasearch in Astronomy. Inc. (AURA) undercooperative agreement with the National Science Foundation.
| 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
| 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).
| 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.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary 4U 1907+09
High-resolution VLT/UVES spectra of the strongly reddened O supergiantcompanion to the X-ray pulsar 4U 1907+09 provide aunique opportunity to study the nature of the diffuse interstellar bands(DIBs) at unprecedented strength. We detect about 180 known DIBs, ofwhich about 25 were listed as tentative and are now confirmed. A dozennew DIB candidates longwards of 6900 Å are identified. We showthat the observed 5797 Å DIB strength is in line with the Galacticcorrelation with reddening, whereas the 5780 Å DIB strength isrelatively weak. This indicates the contribution of denser regions,where the UV penetration is reduced. The presence of dense cloud coresis supported by the detection of C2 rotational transitions. Members ofone DIB family (5797, 6379 Å and 6196, 6613 Å) behavecoherently, although one can make a distinction between the twocorrelated pairs. The broadened profiles of narrow DIBs are shown to beconsistent with the premise that each of the main clouds in the line ofsight discerned in the interstellar K I profile is contributingproportionally to the DIB profile. We complement and extend the relationof DIB strength with reddening {E}(B-V), as well as withneutral hydrogen column density N(H I), respectively, using stronglyreddened sightlines towards another four distant HMXBs. The 5780 ÅDIB, and tentatively also the 5797 and 6613 Å DIBs, are bettercorrelated to the gas tracer H I than to the dust tracer{E}(B-V). The resulting relationship can be applied to anyline of sight to obtain an estimate of the H I column density. In thesearch for the nature of the DIB carrier, this strongly reddened line ofsight is a complementary addition to single cloud line of sight studies.
| Non-thermal radio emission from O-type stars. I. HD168112
We present a radio lightcurve of the O5.5 III(f^+) star HD 168112, basedon archive data from the Very Large Array (VLA) and the AustraliaTelescope Compact Array (ATCA). The fluxes show considerable variabilityand a negative spectral index, thereby confirming that HD 168112 is anon-thermal radio emitter. The non-thermal radio emission is believed tobe due to synchrotron radiation from relativistic electrons that havebeen Fermi accelerated in shocks. For HD 168112, it is not known whetherthese shocks are due to a wind-wind collision in a binary system or tothe intrinsic instability of the stellar wind driving mechanism.Assuming HD 168112 to be a single star, our synchrotron model shows thatthe velocity jump of the shocks should be very high, or there should bea very large number of shocks in the wind. Neither of these iscompatible with time-dependent hydrodynamical calculations of O starwinds. If, on the other hand, we assume that HD 168112 is a binary, thehigh velocity jump is easily explained by ascribing it to the wind-windcollision. By further assuming the star to be an eccentric binary, wecan explain the observed radio variability by the colliding-wind regionmoving in and out of the region where free-free absorption is important.The radio data presented here show that the binary has a period ofbetween one and two years. By combining the radio data with X-ray data,we find that the most likely period is ~1.4 yr.
| 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.
| A pseudo-planar, periodic-box formalism for modelling the outer evolution of structure in spherically expanding stellar winds
We present an efficient technique to study the 1D evolution ofinstability-generated structure in winds of hot stars out to very largedistances (˜ 1000 stellar radii). This technique makes use of ourprevious finding that external forces play little rôle in theouter evolution of structure. Rather than evolving the entire wind, asis traditionally done, the technique focuses on a representative portionof the structure and follows it as it moves out with the flow. Thisrequires the problem to be formulated in a moving reference frame. Thelack of Galilean invariance of the spherical equations of hydrodynamicsis circumvented by recasting them in a pseudo-planar form. By applyingthe technique to a number of problems we show that it is fast andaccurate, and has considerable conceptual advantages. It is particularlyuseful to test the dependence of solutions on the Galilean frame inwhich they were obtained. As an illustration, we show that, in aone-dimensional approximation, the wind can remain structured out todistances of more than 1300 stellar radii from the central star.
| K 4-47: a planetary nebula excited by photons and shocks
K 4-47 is an unusual planetary nebula (PN) composed of a compacthigh-ionization core and a pair of low-ionization knots. Long-slitmedium-resolution spectra of the knots and core are analyzed in thispaper. Assuming photoionization from the central star, we have derivedphysical parameters for all the nebular components, and the ionizationcorrection factors (icf) chemical abundances of the core, which appearsimilar to Type I PNe for He and N/O but significantly deficient inoxygen. The nebula has been further modelled using both photoionization(CLOUDY) and shock (MAPPINGS) codes. From the photoionization modellingof the core, we find that both the strong auroral [OIII] 4363-Åand[NII] 5755-Åemission lines observed and the optical size of thecore cannot be accounted for if a homogeneous density is adopted. Wesuggest that a strong density stratification, matching the high-densitycore detected at radio wavelengths and the much lower density of theoptical core, might solve the problem. From the bow-shock modelling ofthe knots, on the other hand, we find that the chemistry of the knots isalso represented by Type I PN abundances, and that they would move withvelocities of 250-300kms-1.
| 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.
| Non-thermal radio emission from single hot stars
We present a theoretical model for the non-thermal radio emission fromsingle hot stars, in terms of synchrotron radiation from electronsaccelerated in wind-embedded shocks. The model is described by fiveindependent parameters each with a straightforward physicalinterpretation. Applying the model to a high-quality observation ofCyg OB2 No. 9 (O5 If), we obtain meaningfulconstraints on most parameters. The most important result is that theouter boundary of the synchrotron emission region must lie between 500and 2200 stellar radii. This means that shocks must persist up to thatdistance. We also find that relatively weak shocks (with a compressionratio <3) are needed to produce the observed radio spectrum. Theseresults are compatible with current hydrodynamical predictions. Most ofour models also show a relativistic electron fraction that increasesoutwards. This points to an increasing efficiency of the accelerationmechanism, perhaps due to multiple acceleration, or an increase in thestrength of the shocks. Implications of our results for non-thermalX-ray emission are discussed.
| 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).
| The Physical Parameters, Excitation, and Chemistry of the Rim, Jets, and Knots of the Planetary Nebula NGC 7009
We present long-slit optical spectra along the major axis of theplanetary nebula NGC 7009. These data allow us to discuss the physical,excitation, and chemical properties of all the morphological componentsof the nebula, including its remarkable systems of knots and jets. Themain results of this analysis are the following: (1) the electrontemperature throughout the nebula is remarkably constant,Te[OIII]=10,200 K; (2) the bright inner rim and inner pair ofknots have similar densities of Ne~6000 cm-3,whereas a much lower density of Ne~1500 cm-3 isderived for the outer knots as well as for the jets; (3) all the regions(rim, inner knots, jets, and outer knots) are mainly radiativelyexcited; and (4) there are no clear abundance changes across the nebulafor He, O, Ne, or S. There is marginal evidence for an overabundance ofnitrogen in the outer knots (ansae), but the inner ones (caps) and therim have similar N/H values that are at variance with previous results.Our data are compared with the predictions of theoretical models, fromwhich we conclude that the knots at the head of the jets are not matteraccumulated during the jet expansion through the circumstellar medium;nor can their origin be explained by the proposed hydrodynamic or MHDinteracting wind models for the formation of jets/ansae, since thedensities, as well as the main excitation mechanisms of the knots,disagree with model predictions.Based on observations obtained at the 2.5 m Isaac Newton Telescope (INT)of the European Northern Observatory and with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA for NASA under contract NAS5-26555.
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Observation and Astrometry data
| Constellation: | Cygnus |
| Right ascension: | 20h33m10.73s |
| Declination: | +41°15'08.3" |
| Apparent magnitude: | 10.957 |
| Proper motion RA: | -7.4 |
| Proper motion Dec: | -2.5 |
| B-T magnitude: | 12.952 |
| V-T magnitude: | 11.122 |
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