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Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| H2O maser emission from bright rimmed clouds in the southern hemisphere
Context: Water maser emission is a powerful tracer of the presence ofembedded sources in dense clouds since it requires elevated temperatures(>100 K) and densities (>107 cm-3) that canbe found in circumstellar disks and/or jets/outflows associated withYoung Stellar Objects. Bright rimmed clouds compressed by ionizationfronts from nearby massive stars are considered good examples ofexternally triggered star formation, possibly resulting in the formationof massive stars. Aims: We aim to determine the water maser emissionfrequency and characteristics of 45 bright rimmed clouds in the southernhemisphere identified by Sugitani & Ogura (1994, ApJS, 92, 163). Methods: We have used the Tidbinbilla 70-m radiotelescope to perform ahigh sensitivity survey at 22.2 GHz of the maser emission from the616-523 rotational transition of H2O molecules. Results: We found 7 water maser sources out of 44 (16% detection rate),5 being new detections. With the exception of the maser associated withBRC 68, all the other maser are characterized by low integrated fluxesand luminosities. Conclusions: Most maser sources fall below thecorrelation between the H2O and far-infrared luminosity found in otherstudies towards a variety of star forming regions. These results aresimilar to those found in the companion survey of BRCs in the northernhemisphere by Valdettaro et al. (2005, A&A, 443, 535). The lowdetection frequency and the properties of water maser emission from BRCsindicate that low-mass star formation is the most natural outcome of theexternal compression induced by the ionization front from nearby massivestars.Based on observations obtained with the 70-m Tidbinbilla radiotelescope.
| Image-Processing Techniques for the Creation of Presentation-Quality Astronomical Images
The quality of modern astronomical data and the agility of currentimage-processing software enable the visualization of data in a way thatexceeds the traditional definition of an astronomical image. Twodevelopments in particular have led to a fundamental change in howastronomical images can be assembled. First, the availability ofhigh-quality multiwavelength and narrowband data allow for images thatdo not correspond to the wavelength sensitivity of the human eye,thereby introducing ambiguity in the usage and interpretation of color.Second, many image-processing software packages now use a layeringmetaphor that allows for any number of astronomical data sets to becombined into a color image. With this technique, images with as many aseight data sets have been produced. Each data set is intensity-scaledand colorized independently, creating an immense parameter space thatcan be used to assemble the image. Since such images are intended fordata visualization, scaling and color schemes must be chosen that bestillustrate the science. A practical guide is presented on how to use thelayering metaphor to generate publication-ready astronomical images fromas many data sets as desired. A methodology is also given on how to useintensity scaling, color, and composition to create contrasts in animage that highlight the scientific detail. Examples of image creationare discussed.
| Orbital period investigations of two short-period early-type overcontact binaries BH Cen and V701 Sco in two extremely young galactic clusters IC 2944 and NGC 6383
Both V701 Sco and BH Cen are two early-type short-period overcontactsystems (P = 0.d762 and P = 0.d792, respectively).V701 Sco is a member of the young galactic cluster NGC 6383, while BHCen is a component of a younger galactic cluster IC 2944 where starformation is in process. They provide good opportunity to understand theformation and evolution of binary stars. In the present paper, orbitalperiod changes of the two binaries are investigated. It is discoveredthat the orbital period of BH Cen shows a long-term increase with a rateof dP/dt = +1.70(±0.39) × 10‑7 days/yearwhile it undergoes a cyclic oscillation with a period of 44.6 years andan amplitude of A3 = 0.d0216. For V701 Sco, itsO-C curve reveals a periodic change with a period of 41.2 years andamplitude of A3 = 0.d0158. The mass ratio of BHCen is 0.84, but V701 Sco contains twin B1-1.5V type stars with a massratio of unit. The continuous period increase of BH Cen is caused by themass transfer from the less massive component to the more massive one ata rate of dM2/dt = 3.5 × 10‑6days/year. The cyclic period changes of both systems can be plausiblyexplained as the results of light-travel time effects suggesting thatthey are triple systems. The astrophysical parameters of the unseentertiary components in the two systems have been determined. We thinkthat the invisible tertiary components in both binaries played animportant role in the formations and evolutions of the overcontactconfigurations by bringing angular momentum out from the centralsystems. For BH Cen, this process created the initial short period andwill support its evolution into an overcontact configuration via a CaseA mass transfer within the life time of the extremely young cluster IC2944. For V701 Sco, two identical zero-age main-sequence components inan overcontact configuration suggest that it may have been formed byfission, possibly by the fission of the third body. The fact that nolong-term continuous period variations were found for V701 Sco maysuggest that an overcontact binary with the mass ratio of unity can bein an equilibrium revealing that the original configuration of thebinary was overcontact as is its present state. It has been reportedthat faint stars in the two extremely young clusters are relativelyscare. From the present study, it is shown that faint stars in youngclusters are usually formed as companions of OB stars (includingbinaries). It is very difficult to detect them because of their lowluminosity when compared with the more luminous OB stars.
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| 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.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| 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.
| 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.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Galactic Metallicity Gradients Derived from a Sample of OB Stars
The distribution of stellar abundances along the Galactic disk is animportant constraint for models of chemical evolution and Galaxyformation. In this study we derive radial gradients of C, N, O, Mg, Al,and Si, as well as S, from abundance determinations in young OB stars.Our database is composed of a sample of 69 members of 25 open clusters,OB associations, and H II regions with Galactocentric distances between4.7 and 13.2 kpc. An important feature of this abundance database is thefact that the abundances were derived self-consistently in non-LTE usinga homogeneous set of stellar parameters. Such an uniform analysis isexpected to reduce the magnitude of random errors, as well as theinfluence of systematics in the gradients defined by the abundance andGalactocentric distance. The metallicity gradients obtained in thisstudy are, in general, flatter than the results from previous recentabundance studies of early-type stars. The slopes are found to bebetween -0.031 (for oxygen) and -0.052 dex kpc-1 (formagnesium). The gradients obtained for the studied elements are quitesimilar, and if averaged they can be represented by a single slope of-0.042+/-0.007dexkpc-1. This value is generally consistentwith an overall flattening of the radial gradients with time.
| Carina's defiant Finger: HST observations of a photoevaporating globule in NGC 3372*
We present Hubble Space Telescope Wide Field Planetary Camera 2 imagesof a prominent externally ionized molecular globule in the Carina Nebula(NGC 3372), supplemented with ground-based infrared images andvisual-wavelength spectra. This molecular globule has a shape resemblinga human hand, with an extended finger that points toward its likelysource of ionizing radiation. Following an analysis of the spatiallyresolved ionization structure and spectrum of the photoevaporative flowfrom the Finger, we conclude that the dominant ionizing source is eitherthe WNL star WR25 (HD 93162), the adjacent O4 If-type star Tr16-244, orperhaps both. We estimate a mass-loss rate of ~2 × 10-5Msolar yr-1 from the main evaporating surface ofthe globule, suggesting a remaining lifetime of105.3-106 yr. We find a total mass for the entireglobule of more than 6 Msolar, in agreement with previousestimates. The hydrogen column density through the globule derived fromextinction measurements is a few times 1022 cm-2,so the photodissociation region behind the ionization front should belimited to a thin layer compared to the size of the globule, inagreement with the morphology seen in H2 images. Although afew reddened stars are seen within the boundary of the globule innear-infrared continuum images, these may be background stars. We do notdetect a reddened star at the apex of the finger, for example, down to alimiting magnitude of mK~= 17. However, considering thephysical properties of the globule and the advancing ionization front,it appears that future star formation is likely in the Finger globule,induced by radiation-driven implosion.
| A Hot Envelope around the Southern Coalsack: X-Ray and Far-Ultraviolet Observations
We present Far Ultraviolet Spectroscopic Explorer and ROSAT X-rayobservations toward the Southern Coalsack. An almost complete X-ray halocan be seen around the cloud in the 0.75 and 1.5 keV images, and most ofthe observed stars show O VI absorption. Both the cloud and the starshave highly accurate distance determinations, allowing us to reliablyplace the stars and the cloud relative to each other. Using thesedistance determinations, we find no O VI-bearing gas in the foregroundof the Coalsack, while for stars in the background of the cloud, O VIabsorption is the norm. The column density of O VI correlates with the0.75 and 1.5 keV intensities. These results suggest that theX-ray-emitting hot plasma is associated with the dense cloud. We proposethat the heating of the Coalsack envelope is due to the hot gas in theinterior of the Upper Cen-Lup superbubble. The Coalsack interactionregion provides a nearby example of the hot-cold gas interfaces thoughtto be responsible for the O VI absorptions seen on many sight linesthroughout the Galaxy.
| Chemical Abundances for a Sample of Southern OB Stars
Chemical abundances are presented for carbon, nitrogen, oxygen,magnesium, aluminum, silicon, and sulfur in a sample of 25 OB stars thatare members of open clusters, OB associations, and H II regions havingGalactocentric distances that lie inside the solar Galactic orbit. Theelemental abundances are derived from fits of observed high-resolutionspectra to non-LTE synthetic line profiles. Our effective temperaturescale is based on calibrations of the Strömgren photometric indicescoupled to Hγ line profiles. For stars without Strömgrenindices, the reddening-free Q-parameter, defined from UBV photometry isused, with the Q-parameter calibrated against Teff for starsthat have Strömgren photometry. We also investigated othereffective temperature scales from the literature and conclude thatadoption of these other Teff scales, which typically resultin lower temperatures, produces dependences of the derived abundanceswith Teff. Our non-LTE abundances reveal dispersions that arevery similar to those expected from the analysis uncertainties. Thesolar abundances, some of which have been recently revised, fall withinthe abundance distributions defined by this sample of inner disk OBstars for all seven studied elements.Based on observations collected with the 1.52 m telescope at theEuropean Southern Observatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| New Elements for 80 Eclipsing Binaries
This research presents new elements for 80 eclipsing binaries found withthe help of the ASAS-3, Hipparcos and TASS databases.
| Fragmentation of Globules in H II Regions: Hubble Space Telescope Images of Thackeray's Globules
We present Hubble Space Telescope WFPC2 images through an Hαfilter of Thackeray's globules in the southern H II region IC 2944. Theimages document the state of the globule complex during its currenthighly dynamical phase of breakup. A population of very small andshort-lived splinters suggests that continuous fragmentation must playan important role in increasing the surface-to-mass ratio of the neutralgas, thus accelerating the processes of photoevaporation andphotoablation. We are not able to single out which of varioustheoretical mechanisms for breakup is operating in the globule complex.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| New infrared star clusters in the southern Milky Way with 2MASS
We carried out a 2MASS J, H and Ks survey of infrared starclusters in the Milky Way sector 230deg< l <350deg. This zone was the least studied in the literature,previously including only 12 infrared clusters or stellar groups with|b|< 10deg, according to the recent catalogue by Bica etal. (2003). We concentrated efforts on embedded clusters, which arethose expected in the areas of known radio and optical nebulae. Thepresent study provides 179 new infrared clusters and stellar groups,which are interesting targets for detailed future infrared studies. Thesample of catalogued infrared clusters and stellar groups in the Galaxyis now increased by 63%.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Gems from the Stars
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| Tomographic Separation of Composite Spectra. X. The Massive Close Binary HD 101131
We present the first orbital elements for the massive close binary HD101131, one of the brightest objects in the young open cluster IC 2944.This system is a double-lined spectroscopic binary in an ellipticalorbit with a period of 9.64659+/-0.00012 days. It is a young system ofunevolved stars (approximately 2 million yr old) that are well withintheir critical Roche surfaces. We use a Doppler tomography algorithm toreconstruct the individual component optical spectra, and we applywell-known criteria to arrive at classifications of O6.5 V((f)) and O8.5V for the primary and secondary, respectively. We compare thereconstructed spectra of the components to single-star spectrumstandards to determine a flux ratio off2/f1=0.55+/-0.08 in the V band. Both componentsare rotating faster than synchronously. We estimate the temperatures andluminosities of the components from the observed spectralclassifications, composite V magnitude, and cluster distance modulus.The lower limits on the masses derived from the orbital elements and thelack of eclipses are 25 and 14Msolar for the primary andsecondary, respectively. These limits are consistent with the somewhatlarger masses estimated from the positions of the stars in theHertzsprung-Russell diagram and evolutionary tracks for single stars.
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are 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/388/168
| The blue to red supergiant ratio in young clusters at various metallicities
We present new determinations of the blue to red supergiant ratio (B/R)in young open clusters at various metallicities. For this purpose, weexamine the HR diagrams of 45 clusters in the Galaxy and of 4 clustersin the Magellanic Clouds. The identification of supergiants is based onspectroscopic measurements (with photometric counts to check theresults). The new counts confirm the increase of the B/R ratio when themetallicity increases with the following normalized relation:(B/R)/((B/R)sun) =~ 0.05* e3(Z)/(Zsun)}, where Zsun=0.02 and(B/R)sun is the value of B/R at Zsun which dependson the definition of B and R and on the age interval considered (e.g.for spectroscopic counts including clusters with log age between 6.8 and7.5, (B/R)sun =~ 3 when B includes O, B and A supergiants).
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| The Galactic metallicity gradient
We have previously published intermediate to high resolutionspectroscopic observations of approximately 80 early B-typemain-sequence stars situated in 19 Galactic open clusters/associationswith Galactocentric distances distributed over 6<= Rg<=18 kpc. This current study collates and re-analyses theseequivalent-width datasets using LTE and non-LTE model atmospheretechniques, in order to determine the stellar atmospheric parameters andabundance estimates for C, N, O, Mg, Al and Si. The latter should berepresentative of the present-day Galactic interstellar medium. Ourextensive observational dataset permits the identification ofsub-samples of stars with similar atmospheric parameters and ofhomogeneous subsets of lines. As such, this investigation represents themost extensive and systematic study of its kind to date. We concludethat the distribution of light elements (C, O, Mg & Si) in theGalactic disk can be represented by a linear, radial gradient of-0.07+/-0.01 dex kpc-1. Our results for nitrogen and oxygenviz. (-0.09+/-0.01 dex kpc-1 and -0.067+/-0.008 dexkpc-1) are in excellent agreement with that found from thestudy of H II regions. We have also examined our datasets for evidenceof an abrupt discontinuity in the metallicity of the Galactic disk neara Galactocentric distance of 10 kpc (see Twarog et al. \cite{twa97}).However, there is no evidence to suggest that our data would be betterfitted with a two-zone model. Moreover, we observe a N/O gradient of-0.04+/-0.02 dex kpc-1 which is consistent with that foundfor other spiral galaxies (Vila-Costas & Edmunds \cite{vil93}).
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Dust properties in NGC 6611
An investigation on dust properties in the NW portion of NGC6611 has been conducted polarimetrically to test the existenceof abnormal extinction in that section of the open cluster in anindependent way. As previously suggested for the eta Carinae nebula byTapia et al. (\cite{TRMR88b}) and subsequently confirmed by Marraco etal. (\cite{MVV93}), the canonical relation between {E_{ V-K}/E_{ B-V}}and lambda_max (the wavelength of maximun interstellar polarization) isnot valid for stars belonging to dusty H ii regions, as is the case forthe observed portion of M 16. This may arise mainlyfrom the presence of silicate grains of a slightly larger size than thestandard ISM and also from a considerable increase in mean graphitegrain size, according to previous results from Chini & Wargau(\cite{CW90}). About 50% of the observed stars in NGC6611 present indications of intrinsic polarization in theirmeasurements; a similar percentage was found in IC2944 (Vega et al. \cite{VOM94}), another young open cluster inclose relation with an H ii region. Table 1 only available in electronicform at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
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