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An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| An XMM-Newton view of the young open cluster NGC 6231 - III. Optically faint X-ray sources
We discuss the properties of the X-ray sources with faint opticalcounterparts in the very young open cluster NGC 6231. From theirpositions in the Hertzsprung-Russell diagram, we find that the bulk ofthese objects probably consists of low-mass pre-main-sequence (PMS)stars with masses in the range 0.3-3.0 Msolar. The agedistribution of these objects indicates that low-mass star formation inNGC 6231 started more than 10Myr ago and culminated in a starburst-likeevent about 1-4Myr ago when the bulk of the low-mass PMS stars as wellas the massive cluster members formed. We find no evidence for a spatialage gradient that could point towards a sequential star formationprocess. Only a few X-ray sources have counterparts with a reddeningexceeding the average value of the cluster or with infrared coloursindicating the presence of a moderate near-IR excess. The X-ray spectraof the brightest PMS sources are best fitted by rather hard thermalplasma models and a significant fraction of these sources display flaresin their light curve. The X-ray brightest flaring sources have decaytimes between 2 and 16ks. The X-ray selected PMS stars in NGC 6231 havelogLX/Lbol values that increase strongly withdecreasing bolometric luminosity and can reach a saturation level(logLX/Lbol ~ -2.4) for non-flaring sources andeven more extreme values during flares.Based on observations with XMM-Newton, an ESA science mission withinstruments and contributions directly funded by ESA member states andthe USA (NASA).E-mail: hsana@eso.org ‡FNRS Research Associate (Belgium).
| Runaway Massive Binaries and Cluster Ejection Scenarios
The production of runaway massive binaries offers key insights into theevolution of close binary stars and open clusters. The stars HD 14633and HD 15137 are rare examples of such runaway systems, and in this workwe investigate the mechanism by which they were ejected from theirparent open cluster, NGC 654. We discuss observational characteristicsthat can be used to distinguish supernova ejected systems from thoseejected by dynamical interactions, and we present the results of a newradio pulsar search of these systems, as well as estimates of theirpredicted X-ray flux assuming that each binary contains a compactobject. Since neither pulsars nor X-ray emission are observed in thesesystems, we cannot conclude that these binaries contain compactcompanions. We also consider whether they may have been ejected bydynamical interactions in the dense environment where they formed, andour simulations of four-body interactions suggest that a dynamicalorigin is possible but unlikely. We recommend further X-ray observationsthat will conclusively identify whether HD 14633 or HD 15137 containneutron stars.
| The Young Stellar Population in M17 Revealed by Chandra
We report here results from a Chandra ACIS observation of the stellarpopulations in and around the M17 H II region. The field reveals 886sources with observed X-ray luminosities (uncorrected for absorption)between ˜29.3 ergs s-1< log LX<32.8ergs s-1, 771 of which have stellar counterparts in infraredimages. In addition to comprehensive tables of X-ray source properties,several results are presented:1. The X-ray luminosity function is calibrated to that of the OrionNebula Cluster population to infer a total population of roughly8000-10,000 stars in M17, one-third lying in the central NGC 6618cluster.2. About 40% of the ACIS sources are heavily obscured withAV>10 mag. Some are concentrated around well-studiedstar-forming regions -- IRS 5/UC1, the Kleinmann-Wright Object, andM17-North -- but most are distributed across the field. As previouslyshown, star formation appears to be widely distributed in the molecularclouds. X-ray emission is detected from 64 of the hundreds of Class Iprotostar candidates that can be identified by near- and mid-infraredcolors. These constitute the most likely protostar candidates known inM17.3. The spatial distribution of X-ray stars is complex: in addition tothe central NGC 6618 cluster and well-known embedded groups, we find anew embedded cluster (designated M17-X), a 2 pc long arc of young starsalong the southwest edge of the M17 H II region, and 0.1 pc substructurewithin various populations. These structures may indicate that thepopulations are dynamically young.4. All (14/14) of the known O stars but only about half (19/34) of theknown B0-B3 stars in the M17 field are detected. These stars exhibitthe long-reported correlation between X-ray and bolometric luminositiesof LX˜10-7Lbol. While many O andearly-B stars show the soft X-ray emission expected from microshocks intheir winds or moderately hard emission that could be caused bymagnetically channeled wind shocks, six of these stars exhibit very hardthermal plasma components (kT>4 keV) that may be due to collidingwind binaries. More than 100 candidate new OB stars are found, including28 X-ray detected intermediate- and high-mass protostar candidates withinfrared excesses.5. Only a small fraction (perhaps 10%) of X-ray selected high- andintermediate-mass stars exhibit K-band-emitting protoplanetary disks,providing further evidence that inner disks evolve very rapidly aroundmore massive stars.
| Constraining the Fundamental Parameters of the O-Type Binary CPD -41 7733
Using a set of high-resolution spectra, we studied the physical andorbital properties of the O-type binary CPD -41 7733, located in thecore of NGC 6231. We report the unambiguous detection of a secondaryspectral signature and we derive the first SB2 orbital solution of thesystem. The period is 5.6815+/-0.0015 days, and the orbit has nosignificant eccentricity. CPD -41 7733 probably consists of stars ofspectral types O8.5 and B3. As for other objects in the cluster, weobserve discrepant luminosity classifications while using spectroscopicor brightness criteria. Still, the present analysis suggests that bothcomponents display physical parameters close to those of typical O8.5and B3 dwarfs. We also analyze the X-ray light curves and spectraobtained during six 30 ks XMM-Newton pointings spread over the 5.7 dayperiod. We find no significant variability between the differentpointings, nor within the individual observations. The CPD -41 7733X-ray spectrum is well reproduced by a three-temperature thermal mekalmodel with temperatures of 0.3, 0.8, and 2.4 keV. No X-rayoverluminosity, resulting, e.g., from a possible wind interaction, isobserved. The emission of CPD -41 7733 is thus very representative oftypical O-type star X-ray emission.
| Spectral atlas of massive stars around He I 10 830 Å
We present a digital atlas of peculiar, high-luminosity massive stars inthe near-infrared region (10 470-11 000 Å) at medium resolution(R≃7000). The spectra are centered around He I 10 830 Å,which is formed in the wind of those stars, and is a crucial line toobtain their physical parameters. The instrumental configuration alsosampled a rich variety of emission lines of Fe II, Mg II, C I, N I, andPa γ. Secure identifications for most spectral lines are given,based on synthetic atmosphere models calculated by our group. We alsopropose that two unidentified absorption features have interstellarand/or circumstellar origin. For the strongest one (10 780 Å) anempirical calibration between E(B-V) and equivalent width is provided.The atlas displays the spectra of massive stars organized in fourcategories, namely Be stars, OBA Iape (or luminous blue variables, LBVcandidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars.For comparison, the photospheric spectra of non emission-line stars arepresented. Selected LBVs were observed in different epochs from 2001 to2004, and their spectral variability reveals that some stars, such asη Car, AG Car and HR Car, suffered dramatic spectroscopic changesduring this time interval.Based on observations made at Observatório do Pico dos Dias/LNA(Brazil). Figures 5 to 18 are only available in electronic form athttp://www.aanda.org Electronic version of the spectra (fichiers FITS)is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/993
| Towards an understanding of the Of?p star HD191612: phase-resolved multiwavelength observations
We present the analysis of phase-resolved X-ray and optical observationsof the peculiar hot star HD191612 (Of?p). This star is known to displayline-profile variations that are recurrent with a period of 538d and itsspectrum was found to present the signature of a magnetic field. In theX-rays, it is slightly overluminous compared to the canonicalLX/LBOL) relation and appears brighter when theoptical lines are strongest. Our XMM-Newton observations further revealthat the X-ray spectrum of HD191612 exhibits rather broad lines and isdominated by a `cool' (0.2-0.6keV) thermal component, twocharacteristics at odds with the proposed magnetic rotator model. Wealso report for the first time the low-level variability of the metallic(absorption/emission) lines and HeII absorptions that appear to beassociated with radial-velocity shifts. Finally, we compare our resultswith observations of the early-type stars and discuss several possiblescenarios.Based on observations collected at the Haute-Provence Observatory(France) and with XMM-Newton, an ESA Science Mission with instrumentsand contributions directly funded by ESA Member States and the USA(NASA).E-mail: naze@astro.ulg.ac.be ‡Post-doctoral Researcher FNRS (Belgium). §Research Associate FNRS (Belgium). ¶Operated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555.
| The Monitor project: searching for occultations in young open clusters
The Monitor project is a photometric monitoring survey of nine young(1-200Myr) clusters in the solar neighbourhood to search for eclipses byvery low mass stars and brown dwarfs and for planetary transits in thelight curves of cluster members. It began in the autumn of 2004 and usesseveral 2- to 4-m telescopes worldwide. We aim to calibrate the relationbetween age, mass, radius and where possible luminosity, from the Kdwarf to the planet regime, in an age range where constraints onevolutionary models are currently very scarce. Any detection of anexoplanet in one of our youngest targets (<~10Myr) would also provideimportant constraints on planet formation and migration time-scales andtheir relation to protoplanetary disc lifetimes. Finally, we will usethe light curves of cluster members to study rotation and flaring inlow-mass pre-main-sequence stars.The present paper details the motivation, science goals and observingstrategy of the survey. We present a method to estimate the sensitivityand number of detections expected in each cluster, using a simplesemi-analytic approach which takes into account the characteristics ofthe cluster and photometric observations, using (tunable) best-guessassumptions for the incidence and parameter distribution of putativecompanions, and we incorporate the limits imposed by radial velocityfollow-up from medium and large telescopes. We use these calculations toshow that the survey as a whole can be expected to detect over 100 younglow and very low mass eclipsing binaries, and ~3 transiting planets withradial velocity signatures detectable with currently availablefacilities.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| Optical multicolor polarization observations in the region of the open cluster NGC 5749
We present (UBVRI ) multicolor linear polarimetric data for 31 of thebrightest stars in the area of the open cluster NGC 5749 considered tostudy the properties of the interstellar medium (ISM) towards thecluster. Our data yield a mean polarization percentage of PV˜ 1.7%, close to the polarization value produced by the ISM withnormal efficiency (P(%)=3.5 {E(B-V)}0.8) for amean color excess of EB-V=0.42. The mean angle ofpolarization vectors, θ=74°, agrees quite well with theexpected angle produced by dust particles aligned in the direction ofthe galactic disk (and the magnetic field) in the region. Our analysisindicates that the visual absorption affecting the stars in NGC 5749 ispartially produced by a dust layer located up to 300 pc from the Sun andalso by a second layer of dust closer to the cluster (located at leastat 700 pc). The observed photometry and our polarization data areconsistent with the existence of dust within the cluster. We also showin this work how polarimetry could be an excellent technique foridentifying nonmember stars.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina. Table 1 is 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/462/621
| A comprehensive NBODY study of mass segregation in star clusters: energy equipartition and escape
We address the dynamical evolution of an isolated self-gravitatingsystem with two stellar mass groups. We vary the individual ratio of theheavy to light bodies, μ from 1.25 to 50 and alter also the fractionof the total heavy mass from 5 to 40 per cent of the whole cluster mass.Clean-cut properties of the cluster dynamics are examined, like corecollapse, the evolution of the central potential, as well as escapers.We present in this work collisional N-body simulations, using thehigh-order integrator NBODY6++ with up to particles improving thestatistical significancy of the lower- simulations by ensemble averages.Equipartition slows down the gravothermal contraction of the coreslightly. Beyond a critical value of μ ~ 2, no equipartition can beachieved between the different masses; the heavy component decouples andcollapses. For the first time, the critical boundary betweenSpitzer-stable and Spitzer-unstable systems is demonstrated in directN-body models. We also present the measurements of the Coulomb logarithmand discuss the relative importance of the evaporation and ejection ofescapers.
| A search for counterparts to massive X-ray binaries using photometric catalogues
Context: The X-ray and γ-ray observatory INTEGRAL has discoveredlarge numbers of new hard X-ray sources, many of which are believed tobe high mass X-ray binaries. However, for a significant fraction, theircounterparts remain unidentified. Aims: We explore the use ofphotometric catalogues to find optical counterparts to high mass X-raybinaries and search for objects likely to be early-type stars within theerror circles of several INTEGRAL sources. Methods: Candidates wereselected from 2MASS photometry by means of a reddening free Q parameter.Sufficiently bright candidates were spectroscopically observed. Results: Many of the candidates selected turned out to be moderatelyreddened late A or early F stars. Optically visible OB stars are veryscarce even in these Galactic Plane fields. Our method is able toidentify the counterpart to IGR J16207-5129, confirmed by a Chandralocalisation. We classify this object as a B0 supergiant. In the fieldof AX J1820.5-1434, we find a mid or early B-type star, but we cannotconfirm it as the counterpart. For IGR J16320-4751 we rule out theoptically visible candidate as a possible counterpart. For AXJ1700.2-4220, we do not find any suitable candidate within the ASCAerror circle. We classify HD 153295, a marginal candidate to be thecounterpart, as B0.5 IVe, and find a distance compatible with membershipin Sco OB1. In the case of IGR J17091-3624, the object associated with avariable radio source in the field is a late F star. We derive aspectral type B0 IIIe for HD 100199, previously identified as thecounterpart to IGR J11305-6256. Conclusions: .The procedure used isable to correctly identify OB stars and, in about one third of thecases, may lead to the localisation of the correct counterpart. However,the majority of INTEGRAL error circles do not contain any suitableoptically visible counterpart. Deep infrared searches are going to benecessary in order to locate the counterparts to these sources.
| The young star cluster NGC 2362: low-mass population and initial mass function from a Chandra X-ray observation
Context: . We study the stellar population of the very young cluster NGC2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only5 Myr old, has already cleared most of its inter- and circumstellardust, and with its small and uniform reddening offers a uniqueopportunity of studying its pre-main-sequence stellar population withminimal disturbance from a dense interstellar medium. Aims: .Ourmain purposes are to select cluster members down to low masses and tostudy their properties as a population (spatial properties, initial massfunction, and coronal properties). Methods: .We compare existingdeep optical photometry and Hα data with new X-ray data. We usecombined optical and X-ray criteria to select cluster members.Results: .We detect 387 X-ray sources down to log LX = 29.0(erg/s), and identify most of them (308) with star-like objects. Themajority (88%) of optically identified X-ray sources are found to bevery good candidate low-mass pre-main-sequence stars, with minimalfield-object contamination. This increases the known cluster census by asubstantial amount at low masses, with respect to previous optical/IRstudies. The fraction of stars with active accretion is found to be inthe range 5-9%. We find a significantly wider spatial distribution forlow-mass stars than for massive stars (mass segregation). We find only asmall spread around the low-mass cluster sequence in the HR diagram,indicating that star formation lasted only about 1-2 Myr. We havederived the cluster initial mass function, which appears to flatten (onthe low-mass side) at higher masses with respect to other very youngclusters. The quiescent X-ray emission of low-mass cluster stars isfound to be rather strictly correlated with the stellar bolometricluminosity: the small spread in this correlation puts an upper bound onthe amplitude of X-ray variability on time scales longer than one day(e.g., activity cycles) in such young coronal sources. We findsignificant X-ray spectral differences between low-mass stars brighterand fainter than log LX ˜ 30.3 (erg/s), respectively,with X-ray brighter stars showing hotter components (kT ˜ 2 keV),absent in fainter stars.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| The OB binary HD152219: a detached, double-lined, eclipsing system
We present the results of an optical spectroscopic campaign on themassive binary HD152219 located near the core of the NGC6231 cluster.Though the primary to secondary optical brightness ratio is probablyabout 10, we clearly detect the secondary spectral signature and wederive the first reliable SB2 orbital solution for the system. Theorbital period is close to 4.2403d and the orbit is slightly eccentric(e = 0.08 +/- 0.01). The system is most probably formed by an O9.5 giantand a B1-2 V-III star. We derive minimal masses of 18.6 +/- 0.3 and 7.3+/- 0.1Msolar for the primary and secondary, respectively,and we constrain the stellar radius at values about 11 and5Rsolar. INTEGRAL-Optical Monitoring Camera (OMC) data revealthat HD152219 is the third O-type eclipsing binary known in NGC6231. Inthe Hertzsprung-Russell (HR) diagram, the primary component lies on theblue edge of the β Cep-type instability strip and its spectrallines display clear profile variations that are reminiscent of thoseexpected from non-radial pulsations. Finally, we report the analysis ofXMM-Newton observations of the system. The X-ray spectrum is relativelysoft and is well reproduced by a two-temperature mekal model withkT1 = 0.26keV and kT2 = 0.67keV. The X-ray flux ismost probably variable on a time-scale of days. The average X-rayluminosity during our campaign is log(LX) ~ 31.8(ergs-1), but shows fluctuations of about 10 per cent aroundthis value.Based on observations collected at the European Southern Observatory (LaSilla, Chile) and with the XMM-Newton satellite, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA); also based on data from the Optical MonitoringCamera (OMC) Archive at LAEFF, processed by ISDC.E-mail: hsana@eso.org ‡FNRS Research Associate (Belgium).
| Early-type stars in the young open cluster IC 1805. II. The probably single stars HD 15570 and HD 15629, and the massive binary/triple system HD 15558
Aims.We address the issue of the multiplicity of the three brightestearly-type stars of the young open cluster IC 1805, namely HD 15570, HD15629 and HD 15558. Methods: .For the three stars, we measured theradial velocity by fitting Gaussian curves to line profiles in theoptical domain. In the case of the massive binary HD 15558, we also useda spectral disentangling method to separate the spectra of the primaryand of the secondary in order to derive the radial velocities of the twocomponents. These measurements were used to compute orbital solutionsfor HD 15558. Results: .For HD 15570 and HD 15629, the radialvelocities do not present any significant trend attributable to a binarymotion on time scales of a few days, nor from one year to the next. Inthe case of HD 15558 we obtained an improved SB1 orbital solution with aperiod of about 442 days, and we report for the first time on thedetection of the spectral signature of its secondary star. We derivespectral types O5.5III(f) and O7V for the primary and the secondary ofHD 15558. We tentatively compute a first SB2 orbital solution althoughthe radial velocities from the secondary star should be considered withcaution. The mass ratio is rather high, i.e. about 3, and leads to veryextreme minimum masses, in particular for the primary object. Minimummasses of the order of 150 ± 50 and 50 ± 15 M_ȯ arefound respectively for the primary and the secondary. Conclusions:.We propose that HD 15558 could be a triple system. This scenario couldhelp to reconcile the very large minimum mass derived for the primaryobject with its spectral type. In addition, considering new andpreviously published results, we find that the binary frequency amongO-stars in IC 1805 has a lower limit of 20%, and that previouslypublished values (80%) are probably overestimated.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| Population analysis of open clusters: radii and mass segregation
Aims.Based on our well-determined sample of open clusters in the all-skycatalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, andinvestigate the effect of mass segregation of stars in open clusters. Methods: .Using statistical methods, we study the distribution oflinear sizes as a function of spatial position and cluster age. We alsoexamine statistically the distribution of stars of different masseswithin clusters as a function of the cluster age. Results: .Nosignificant dependence of the cluster size on location in the Galaxy isdetected for younger clusters (<200 Myr), whereas older clustersinside the solar orbit turned out to be, on average, smaller thanoutside. Also, small old clusters are preferentially found close to theGalactic plane, whereas larger ones more frequently live farther awayfrom the plane and at larger Galactocentric distances. For clusters with(V - M_V) < 10.5, a clear dependence of the apparent radius on agehas been detected: the cluster radii decrease by a factor of about 2from an age of 10 Myr to an age of 1 Gyr. A detailed analysis shows thatthis observed effect can be explained by mass segregation and does notnecessarily reflect a real decrease of cluster radii. We found evidencefor the latter for the majority of clusters older than 30 Myr. Among theyoungest clusters (between 5 and 30 Myr), there are some clusters with asignificant grade of mass segregation, whereas some others show nosegregation at all. At a cluster age between 50 and 100 Myr, thedistribution of stars of different masses becomes more regular overcluster area. In older clusters the evolution of the massive stars isthe most prominent effect we observe.
| 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.
| An XMM-Newton view of the young open cluster NGC 6231. I. The catalogue
This paper is the first of a series dedicated to the X-ray properties ofthe young open cluster NGC 6231. Our data set relies on an XMM-Newtoncampaign of a nominal duration of 180 ks and reveals that NGC 6231 isvery rich in the X-ray domain too. Indeed, 610 X-ray sources aredetected in the present field of view, centered on the cluster core. Thelimiting sensitivity of our survey is approximately 6 ×10-15 erg cm-2 s-1 but clearly dependson the location in the field of view and on the source spectrum. Usingdifferent existing catalogues, over 85% of the X-ray sources could beassociated with at least one optical and/or infrared counterpart withina limited cross-correlation radius of 3´´ at maximum. Thesurface density distribution of the X-ray sources presents a slight N-Selongation. Once corrected for the spatial sensitivity variation of theEPIC instruments, the radial profile of the source surface density iswell described by a King profile with a central density of about 8sources per arcmin2 and a core radius close to 3.1 arcmin.The distribution of the X-ray sources seems closely related to theoptical source distribution. The expected number of foreground andbackground sources should represent about 9% of the detected sources,thus strongly suggesting that most of the observed X-ray emitters arephysically belonging to NGC 6231. Finally, beside a few bright but softobjects - corresponding to the early-type stars of the cluster - most ofthe sources are relatively faint (~5 × 10-15 ergcm-2 s-1) with an energy distribution peakedaround 1.0-2.0 keV.
| Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity
Aims.We present new CORAVEL radial-velocity observations andphotoelectric photometry in the UBV and DDO systems for a sample ofpotential members of the red-giant branches of NGC 6192, NGC 6208 andNGC 6268, three open clusters projected close to the Galactic centerdirection. We also examine the properties of a sample of 42 inner diskopen clusters projected towards almost the same direction as the threeclusters. Methods: .Cluster members and red field giants werediscriminated by using the CORAVEL radial-velocity data and by applyingtwo photometric criteria. Interstellar reddening and metal content ofthe clusters were derived from combined BV and DDO data. Results:.Cluster membership for five red giants in NGC 6192, three in NGC 6208and three in NGC 6268 has been confirmed by the analysis of thephotometric and kinematic data. Photometric membership probabilitiesshow very good agreement with those obtained from CORAVEL radialvelocities. Three new spectroscopic binaries were discovered among thered giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B-V)colour excesses were derived. Conclusions: .The overallmetallicities were found to be nearly solar for NGC 6208 and above solarfor NGC 6192 and NGC 6268. Most of the clusters located closer than 2kpc from the Sun in the considered direction are slightly more reddenedthan the absorption resulting from the Baade's window absorption law.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Automated analysis of eclipsing binary light curves - II. Statistical analysis of OGLE LMC eclipsing binaries
In the first paper of this series, we presented EBAS - Eclipsing BinaryAutomated Solver, a new fully automated algorithm to analyse the lightcurves of eclipsing binaries, based on the EBOP code. Here, we apply thenew algorithm to the whole sample of 2580 binaries found in the OpticalGravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC)photometric survey and derive the orbital elements for 1931 systems. Toobtain the statistical properties of the short-period binaries of theLMC, we construct a well-defined subsample of 938 eclipsing binarieswith main-sequence B-type primaries. Correcting for observationalselection effects, we derive the distributions of the fractional radiiof the two components and their sum, the brightness ratios and theperiods of the short-period binaries. Somewhat surprisingly, the resultsare consistent with a flat distribution in log P between 2 and 10 d. Wealso estimate the total number of binaries in the LMC with the samecharacteristics, and not only the eclipsing binaries, to be about 5000.This figure leads us to suggest that (0.7 +/- 0.4) per cent of themain-sequence B-type stars in the LMC are found in binaries with periodsshorter than 10 d. This frequency is substantially smaller than thefraction of binaries found by small Galactic radial-velocity surveys ofB stars. On the other hand, the binary frequency found by Hubble SpaceTelescope (HST) photometric searches within the late main-sequence starsof 47 Tuc is only slightly higher and still consistent with thefrequency we deduced for the B stars in the LMC.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| Scorpius the Winter-Bug.
Not Available
| The physical parameters of the binary V1034 Sco .
Strömgren photometric monitoring of the open cluster NGC 6231 wascarried out during several years. Several stars are under study, one ofthem is the eclipsing binary V1034 Sco for which more than 3000 datapoints were collected. The derived masses of the componenents are M_1 =16.8±0.5 M_ȯ and M_2 = 9.4±0.3 M_ȯ, and acluster distance modulus V_0 - M_V = 10.73±0.02 was found.
| The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
| The massive binary CPD - 41° 7742. II. Optical light curve and X-ray observations
In the first paper of this series, we presented a detailedhigh-resolution spectroscopic study of CPD - 41°7742, deriving for the first time an orbital solution for bothcomponents of the system. In this second paper, we focus on the analysisof the optical light curve and on recent XMM-Newton X-ray observations.In the optical, the system presents two eclipses, yielding aninclination i77°. Combining the constraints from the photometrywith the results of our previous work, we derive the absolute parametersof the system. We confirm that the two components of CPD -41° 7742 are main sequence stars (O9 V + B1-1.5 V) withmasses (M_118 Mȯ and M_210 Mȯ)and respective radii (R_17.5 Rȯ and R_25.4Rȯ) close to the typical values expected for such stars.We also report an unprecedented set of X-ray observations that almostuniformly cover the 2.44-day orbital cycle. The X-ray emission fromCPD - 41° 7742 is well described by atwo-temperature thermal plasma model with energies close to 0.6 and 1.0keV, thus slightly harder than typical early-type emission. The X-raylight curve shows clear signs of variability. The emission level ishigher when the primary is in front of the secondary. During the highemission state, the system shows a drop of its X-ray emission thatalmost exactly matches the optical eclipse. We interpret the mainfeatures of the X-ray light curve as the signature of a wind-photosphereinteraction, in which the overwhelming primary O9 star wind crashes intothe secondary surface. Alternatively the light curve could result from awind-wind interaction zone located near the secondary star surface. As asupport to our interpretation, we provide a phenomenological geometricmodel that qualitatively reproduces the observed modulations of theX-ray emission.
| VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours
We have obtained high resolution (R = 10 000) K-band spectra ofcandidate young massive stars deeply embedded in (ultra-) compact H IIregions (UCHIIs). These objects were selected from a near-infraredsurvey of 44 fields centered on IRAS sources with UCHII colours. Often,the near-infrared counterpart of the IRAS source is a young embeddedcluster hosting massive stars. In these clusters, three types of objectsare identified. The first type (38 objects) consists of "naked" OB starswhose K-band spectra are dominated by photospheric emission. We classifythe K-band spectra of the OB-type cluster members using near-infraredclassification criteria. A few of them have a very early (O3-O4 V)spectral type, consistent with a young age of the embedded clusters. Thespectral classification provides an important constraint on the distanceto the embedded cluster. The ionising power of the population thusderived is compared to the information obtained from the infrared andradio flux of these sources. In most cases these two differentdeterminations of the ionising flux are consistent, from which weconclude that we have identified the ionising star(s) in about 50% ofthe embedded clusters. The second type (7 objects) are point sourcesassociated with UCHII radio emission, that exhibit nebular emissionlines in the near-infrared. Six of the objects in this group produce HeI emission indicative of an embedded O-type star. These objects are moreembedded than the OB stars and probably do not dominate the infraredflux as measured by IRAS. They may emit the bulk of their reprocessed UVradiation at mm wavelengths. The third type (20 objects) ischaracterised by broad (100-200 km s-1) Brγ emissionand no photospheric absorption profiles. Bik et al. (2005, A&A,submitted) show that these objects are massive YSO candidates surroundedby dense circumstellar disks.
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