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Non-thermal radio emission from O-type stars. II. HD 167971
HD 167971 is a triple system consisting of a 3.3-day eclipsing binary(O5-8 V + O5-8 V) and an O8 supergiant. It is also a well knownnon-thermal radio emitter. We observed the radio emission of HD 167971with the Very Large Array (VLA) and the Australia Telescope CompactArray (ATCA). By combining these data with VLA archive observations weconstructed a radio lightcurve covering a 20-yr time-range. We searchedfor, but failed to find, the 3.3-day spectroscopic period of the binaryin the radio data.This could be due to the absence of intrinsic synchrotron radiationin the colliding-wind region between the two components of the eclipsingbinary,or due to the largeamount of free-free absorption that blocks the synchrotronradiation. We are able to explain many of the observed characteristicsof the radio dataif the non-thermal emission is produced in a colliding-wind regionbetween the supergiant and the combined winds of the binary.Furthermore, if the system is gravitationally bound, the orbital motionoccurs over a period of ~20 years or longer, as suggested by thelong-term variability in the radio data. We argue that the variabilityis due to the free-free absorption that changes with orbital phase ormay also in part be due to changes in separation, should the orbit beeccentric.Table A.1 is only available in electronic form at http://www.aanda.org
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| 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.
| Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
| 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.
| An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604
We discuss the results of two XMM-Newton observations of the opencluster NGC 6604 obtained in April and September 2002. We concentratemainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). Thesoft part of the EPIC spectrum of this system is thermal with typicaltemperatures of about 2 × 106 to 9 ×106 K. The nature (thermal vs. non-thermal) of the hard partof the spectrum is not unambiguously revealed by our data. If theemission is thermal, the high temperature of the plasma (~2.3 ×107 to 4.6 × 107 K) would be typical of whatshould be expected from a wind-wind interaction zone within a longperiod binary system. This emission could arise from an interactionbetween the combined winds of the O5-8V + O5-8V close binary system andthat of the more distant O8I companion. Assuming instead that the hardpart of the spectrum is non-thermal, the photon index would be rathersteep (~3). Moreover, a marginal variability between our two XMM-Newtonpointings could be attributed to an eclipse of the O5-8V + O5-8V system.The overall X-ray luminosity points to a significant X-ray luminosityexcess of about a factor 4 possibly due to colliding winds. ConsideringHD 167971 along with several recent X-ray and radio observations, wepropose that the simultaneous observation of non-thermal radiation inthe X-ray (below 10.0 keV) and radio domains appears rather unlikely.Our investigation of our XMM-Newton data of NGC 6604 reveals a rathersparse distribution of X-ray emitters. Including the two brightnon-thermal radio emitters HD 168112 and HD 167971, we present a list of31 X-ray sources along with the results of the cross-correlation withoptical and infrared catalogues. A more complete spectral analysis ispresented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-mainsequence star candidates.
| The origin of massive O-type field stars: II. Field O stars as runaways
In two papers we try to confirm that all Galactic high-mass stars areformed in a cluster environment, by excluding that O-type stars found inthe Galactic field actually formed there. In de Wit et al. (2004) wepresented deep K-band imaging of 5 arcmin fields centred on 43 massiveO-type field stars that revealed that the large majority of theseobjects are single objects. In this contribution we explore thepossibility that the field O stars are dynamically ejected from youngclusters, by investigating their peculiar space velocity distribution,their distance from the Galactic plane, and their spatial vicinity toknown young stellar clusters. We (re-)identify 22 field O-type stars ascandidate runaway OB-stars. The statistics show that 4 ± 2% ofall O-type stars with V<8m can be considered as formedoutside a cluster environment. Most are spectroscopically singleobjects, some are visual binaries. The derived percentage for O-typestars that form isolated in the field based on our statistical analysesis in agreement with what is expected from calculations adopting auniversal cluster richness distribution with power index of β= 1.7,assuming that the cluster richness distribution is continuous down tothe smallest clusters containing one single star.
| 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.
| 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.
| The origin of massive O-type field stars. I. A search for clusters
We present a study aimed at clarifying the birthplace for 43 massiveO-type field stars. In this first paper we present the observationalpart: a search for stellar clusters near the target stars. We derivestellar density maps at two different resolving scales, viz. 0.25pc and 1.0 pc from NTT and TNG imaging and the 2MASS catalogue.These scales are typical for cluster sizes. The main result is that thelarge majority of the O-type field population are isolated stars: only12% (5 out of 43) of the O-type field stars is found to harbour asmall-scale stellar cluster. We review the literature and aim atcharacterizing the stellar field of each O-type field star with theemphasis on star formation and the presence of known young stellarclusters. An analysis of the result of this paper and a discussion ofthe O-type field population as products of a dynamical ejection event ispresented in an accompanying paper.Based on observations collected at the European Southern Observatory,Chile, and at the Italian Telescopio Nazionale Galileo (TNG) operated onthe island of La Palma by the Centro Galileo Galilei of the CNAA(Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.Table 2 and Figs. 4 to 17 are available in electronic form athttp://www.edpsciences.org
| Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD 168112 (O5.5III(f+))
We report the results of a multiwavelength study of the non-thermalradio emitter HD 168112 (O5.5III(f+)). The detailed analysisof two quasi-simultaneous XMM-Newton and VLA observations reveals strongvariability of this star both in the X-ray and radio ranges. The X-rayobservations separated by five months reveal a decrease of the X-rayflux of 30%. The radio emission on the other hand increases by afactor 5-7 between the two observations obtained roughly simultaneouslywith the XMM-Newton pointings. The X-ray data reveal a hard emissionthat is most likely produced by a thermal plasma at kT 2-3 keVwhile the VLA data confirm the non-thermal status of this star in theradio waveband. Comparison with archive X-ray and radio data confirmsthe variability of this source in both wavelength ranges over a yet illdefined time scale. The properties of HD 168112 in the X-ray and radiodomain point towards a binary system with a significant eccentricity andan orbital period of a few years. However, our optical spectra reveal nosignificant changes of the star's radial velocity suggesting that if HD168112 is indeed a binary, it must be seen under a fairly lowinclination.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA). Also based on observations collected with the VLA, aninstrument of the National Radio Astronomy Observatory, which is afacility of the National Science Foundation operated by AssociatedUniversities, Inc. Optical data were collected at the European SouthernObservatory (La Silla, Chile), and at the ObservatorioAstronómico Nacional of San Pedro Mártir (Mexico).
| Chemical Abundances for a Sample of Southern OB Stars. II. The Outer Disk
Non-LTE abundances of carbon, nitrogen, oxygen, aluminum, magnesium,silicon, and sulfur are derived for a sample of nine OB stars that aremembers of seven clusters or H II regions located outside the solarcircle, with Galactocentric distances between 9 and 13 kpc. Theabundance distribution for these stars is found to have a peak roughly0.3 dex lower than the average of the abundance distribution for OBstars from the inner disk. This result is in general agreement with thedisk metallicity discontinuity claimed from observations of openclusters and Cepheids. The abundances obtained for this outer disksample are all subsolar, with an average for C, N, O, Mg, and S of-0.39+/-0.05. For Si and Al, we find even lower average abundances of-0.56 and -0.58, respectively.Based on observations collected with the 1.52 m telescope at theEuropean Southern Observatory (La Silla, Chile) under agreement with theObservatório Nacional, Brazil.
| 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.
| 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.
| 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.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| VLA Radio Continuum and IRAS Observations of the Ring Nebulae around WR 101 and WR 113
We report radio continuum observations at 1465 MHz obtained with theVery Large Array (VLA) in the DnC configuration toward the ring nebulaeassociated with the stars WR 101 and WR 113, with resolutions of ~38"and 30", respectively. IRAS images of the nebulae with resolutions ofabout 2' (90 Msolar, 40 cm-3) are also analyzed. Aremarkable resemblance among the optical, infrared, and radio images ofthese ring nebulae is observed. The VLA data indicate that Anon. WR 101is thermal in nature. An ionized mass of ~230+/-40 Msolar andelectron densities in the range ~40-55 cm-3 were estimatedfor Anon. WR 101. The derived ionized masses and electron densities inthe inner and outer shells of the nebula related to WR 113 are ~20+/-10Msolar, 180-500 cm-3 and ~90 Msolar 40cm-3, respectively. Based on infrared data at 60 and 100μm, the derived masses and temperatures for the dust component in thering nebula around WR 101 are 0.3-1 Msolar and ~40 K. Theassociated masses suggest that the ring nebula related to WR 101 and theouter arc associated with WR 113 consist of swept-up interstellarmatter, while the relatively low ionized mass associated with the innershell of the nebula around WR 113 may contain a nonnegligiblecontribution of expelled ejecta material. The derived electron densitiesfor the nebula around WR 101 and the inner shell around WR 113 arecomparable to electron densities for other W-R ring nebulae. Low fillingfactors are inferred for both nebulae. The nebulae probably originatedduring the current W-R phase of the stars.
| 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).
| On the oxygen abundances in H II regions of the Galaxy
Oxygen abundances in H II regions of the Galaxy were derived with theclassical Te-method within the framework of the two-zone H IIregion model using published spectro-photometric data (69 spectra of 11H II regions in the range of galactocentric distances from 6.6 to 14.8kpc). The derived radial distribution of the oxygen abundance wascompared with that from Shaver et al. (1983), which is widely used inconstructing the model of the chemical evolution of the Galaxy. It wasfound that the oxygen abundances given by Shaver et al. areoverestimated by 0.2-0.3 dex.
| 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 Serpens OB2 Association and Its Thermal ``Chimney''
UBVRI photometry and MK spectral types have been obtained for a nearlycomplete sample of the massive star population of the Serpens OB2association. This relatively neglected association is found to containover 100 OB stars at a distance of 1.9+/-0.3 kpc and with a common ageof 5+/-1 Myr. These stars illuminate the large H II region S54, and aredirectly connected to the unusual thermal ``chimney'' detected byMüller et al. Evidence is given of continuing star formation withinthe association, and the relationship between the association stars andthe chimney is discussed.
| 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
| Spectroscopy and BVI_C photometry of the young open cluster NGC 6604
BVI_C photometry and classification spectroscopy is presented for theyoung open cluster NGC 6604. Additional Echelle spectroscopy of thebrightest members is used to check the reddening against theinterstellar NaI and KI absorption lines, to measure the cluster radialvelocity and to derive the individual rotational velocities. We obtain1.7 kpc as the cluster distance, an age of ~ 5 106 years anda reddening EB-V= 1.02 (+/- 0.01 from three independentmethods). Pre-ZAMS objects are apparently not detected over thebigtriangleup m = 8.5 mag explored range. The cluster radial velocity isin agreement with the \cite[Hron (1987)]{hro87} model for the Galaxydisk rotation. Based on observations secured with the telescopes of theSouth African Astronomical Observatory (SAAO) and those of the Padovaand Asiago Astronomical Observatories.
| Oxygen and helium abundances in Galactic Hii regions - I. Observations
Absolute integrated line fluxes of Hii regions have been measured usinga Fabry-Perot spectrophotometer. We describe the observations andcalibration procedures. Fluxes are given for 36 Hii regions withGalactocentric distances ranging from 6.6 to 17.7kpc. Several emissionlines have been measured, mainly [Oii] λλ3726 and 3629,Hβ, [Oiii] λ5007, Hei λ5876 and Hα. The veryfaint [Oiii] λ4363 line has been measured in six regions,allowing a direct determination of the electron temperature. Newphotometric distances have been derived based on data from theliterature. A discussion of these results in terms of extinction,electron density and temperature, and oxygen and helium abundances isgiven in Paper II.
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| Absolute proper motions of 181 young open clusters.
Not Available
| Radio Recombination Lines from a Thermal Spur Associated with the Region S54: A Model to Explain the Observational Results
Not Available
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