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A close look at the heart of RCW 108
Context: The IRAS 16362-4845 star-forming site in the RCW 108 complexcontains an embedded compact cluster that includes some massive O-typestars. Star formation in the complex, and in particular in IRAS16362-4845, has been proposed to be externally triggered by the actionof NGC 6193. Aims: We present a photometric study of the IRAS16362-4845 cluster sensitive enough to probe the massive brown dwarfregime. In particular, we try to verify an apparent scarcity ofsolar-type and low-mass stars reported in a previous paper(Comerón et al. 2005, A&A, 433, 955). Methods: Using NACO atthe VLT we have carried out adaptive optics-assisted imaging in the{JHK}_SL' bands, as well as through narrow-band filters centered on theBrγ and the H2 S(1) v=1 → 0 lines. We estimate individualline-of-sight extinctions and, for stars detected in the three{JHK}S filters, we estimate the contribution to theKS flux caused by light reprocessed in the circumstellarenvironment. We also resolve close binary and multiple systems. We usethe K luminosity function as a diagnostic tool for the characteristicsof the underlying mass function. Results: IRAS 16362-4845 does containyoung low-mass stars. Nevertheless, they are far less than thoseexpected from the extrapolation of the bright end of the K luminosityfunction towards fainter magnitudes. We estimate a total stellar mass of370 M_ȯ. Nearly all the cluster members display L' excesses,whereas KS excesses are in general either absent or moderate(< 1 mag). We also detect an extremely red object with (K_S-L')>9, likely to be a Class I source. Conclusions: The fact that solar-typeand low-mass stars are present in numbers much smaller than thoseexpected from the number of more massive members hints at an initialmass function deficient in low mass stars as compared to that of otheryoung clusters such as the Trapezium. The origin of this difference isunclear, and we speculate that it might be due to external triggeringhaving started star formation in the cluster, perhaps producing atop-heavy initial mass function. We also note that there are nodetectable systematic differences between the spatial distributions ofbright and faint cluster members. Such absence of mass segregation inthe spatial distribution of stars may also support external triggeringhaving played an important role in the history of the RCW 108 region.Based on observations obtained at the European Southern Observatoryusing the Very Large Telescope (VLT) (programme 077.C-0660(A)), on CerroParanal, Chile. Table 2 is only available in electronic form athttp://www.aanda.org
| Champagne flow and triggered star formation in NGC 1893
Using Hα grism spectroscopy and optical and Two-Micron All-SkySurvey photometry of the fields containing NGC 1893 and two emissionnebulae, Sim 129 and Sim 130, we show the presence of a number ofHα and near-infrared excess sources towards the region between HD242935 and the two emission nebulae. Among them a large majority ofthese pre-main-sequence candidate sources are concentrated towardsregions closer to Sim 129 and Sim 130. Age distribution of these sources(viz. main-sequence stars in the cluster, including HD 242935, youngsources located in the region between HD 242935 and two nebulae andthose located inside Sim 129) obtained from their positions in opticalV,V - I colour-magnitude diagram strongly suggest `small-scalesequential star formation' along the axes of the clouds caused by theoutward advance of ionization/shock front from the HII region. The IRASsource, IRAS 05198+3325, identified with CPM 16 is a Herbig Be (B3) typestar showing Hα and CaII triplet lines in emission. From theorientation of the two emission nebulae it is suggested that HD 242935is most likely responsible for the cometary morphology of the twonebulae and the trigger of star formation in the region. From the globaldistribution of the interstellar material, it is also suggested that thesame source is responsible for the champagne flow causing the entiremolecular cloud to have a cometary appearance.
| Pre-main-sequence stars in the young open cluster NGC 1893. II. Evidence for triggered massive star formation
Context: The open cluster NGC 1893 illuminating the HII region IC 410 contains a moderately large population of O-type starsand is one of the youngest clusters observable in the optical range. Itis suspected of harbouring a large population of pre-main-sequence (PMS)stars. Aims: We have probed the stellar population of NGC 1893 in anattempt to determine its size and extent. In particular, we look forsigns of sequential star formation. Methods: We classify a large sampleof cluster members with new intermediate-resolution spectroscopy. Weused Hα slitless spectroscopy of the field to search foremission-line objects, identifying 18 emission-line PMS stars. We thencombined existing optical photometry with the 2MASS JHKSphotometry to detect stars with infrared excesses, finding close to 20more PMS candidates. Results: While almost all stars earlier than B2indicate standard reddening, all later cluster members show strongdeviations from a standard reddening law, which we interpret in terms ofinfrared excess emission. Emission-line stars and IR-excess objects showthe same spatial distribution, concentrating around two localised areas,the immediate vicinity of the pennant nebulae Sim 129and Sim 130 and the area close to the cluster corewhere the rim of the molecular cloud associated with IC410 is illuminated by the nearby O-type stars. In and aroundthe emission nebula Sim 130, we find three Herbig Be stars with spectraltypes in the B1-4 range and several other fainter emission-line stars.We obtain a complete census of B-type stars by combining Strömgren,Johnson and 2MASS photometry and find a deficit of intermediate massstars compared to massive stars. We observe a relatively extended haloof massive stars surrounding the cluster without an accompanyingpopulation of intermediate-mass stars. Conclusions: Stars in NGC 1893show strong indications of being extremely young. The pennant nebula Sim130 is an area of active massive star formation, displaying very goodevidence of triggering by the presence of nearby massive stars. Theoverall picture of star formation in NGC 1893 suggests a very complexprocess.Partially based on observations obtained at the Nordic Optical Telescopeand the Isaac Newton Telescope (La Palma, Spain) and Observatoire deHaute Provence (CNRS, France).
| Wide-Field CCD Photometry around Nine Open Clusters
In this paper we study the evolution of the core and corona of nine openclusters using the projected radial density profiles derived fromhomogeneous CCD photometric data obtained with the 105 cm Kiso Schmidttelescope. The age and galactocentric distance of the target clustersvary from 16 to 2000 Myr and 9 to 10.8 kpc, respectively. Barring Be 62,which is a young open cluster, other clusters show a uniform reddeningacross the cluster region. The reddening in Be 62 varies fromE(B-V)min=0.70 mag to E(B-V)max=1.00 mag. Thecoronae of six of the clusters in the present sample are found to beelongated; however, on the basis of the present sample it is notpossible to establish any correlation between the age and shape of thecore. The elongated core in the case of the young cluster Be 62 mayreflect the initial conditions in the parental molecular cloud. Theother results of the present study are as follows: (1) Core radiusrc and corona size rcn/cluster radiusrcl are linearly correlated. (2) The rc,rcn, and rcl are linearly correlated with thenumber of stars in that region. (3) In the age range 10-1000 Myr, thecore and corona shrink with age. (4) We find that in the galactocentricdistance range 9-10 kpc, the core and corona/cluster extent of theclusters increase with the galactocentric distance.
| 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.
| 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.
| 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.
| On the distance, reddening and progenitor of V838 Mon
Extensive optical and infrared photometry as well as low and highresolution spectroscopy are used as inputs in deriving robust estimatesof the reddening, distance and nature of the progenitor of V838 Mon, the2002 outbursting event that produced a most spectacular light-echo. Thereddening affecting V838 Mon is found to obey the R_V=3.1 law andamounts to (i) EB-V=0.86 from the interstellar NaI and KIlines; (ii) EB-V=0.88 from the energy distribution of the B3V component; and (iii) EB-V=0.87 from the progression ofextinction along the line of sight. The adoptedEB-V=0.87±0.01 is also the amount required by fittingthe progenitor with theoretical isochrones of appropriate metallicity.The distance is estimated from (a) the galactic kinematics of the threecomponents of the interstellar lines; (b) the amount of extinction vs.the HI column density and vs. the dust emission through the whole Galaxyin that direction; from (c) spectrophotometric parallax to the B3 Vcompanion; from (d) comparison of the observed color-magnitude diagramof field stars with 3D stellar population models of the Galaxy; from (e)comparison of theoretical isochrones with the components of the binarysystem in quiescence and found to be around 10 kpc. Pre-outburst opticaland IR energy distributions show that the component erupting in 2002 wasbrighter and hotter than the B3 V companion. The best fit is obtainedfor a 50 000 K source, 0.5 mag brighter than the B3 V companion. Thelatter passed unaffected through the outburst, which implies an orbitalseparation wide enough to avoid mass exchange during the evolution ofthe binary system, and to allow a safe comparison with theoreticalisochrones for single stars. Such a comparison suggests that theprogenitor of the outbursting component had an initial mass ~65M_ȯ, that it was approaching the carbon ignition stage in its coreat the time it erupted in 2002 and that the age of the V838 Mon binarysystem is close to 4 million yr. The 2002 event is probably just a shellthermonuclear event in the outer envelope of the star.
| Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass(mmax) in an ensemble of N stars, as a function of theinitial mass function (IMF) upper mass cutoff (mup) and N. Westatistically demonstrate that the upper IMF of the local massive starcensus observed thus far in the Milky Way and Magellanic Clouds clearlyexhibits a universal upper mass cutoff around 120-200 Msolarfor a Salpeter IMF, although the result is more ambiguous for a steeperIMF.
| 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.
| Morphology of Galactic Open Clusters
We analyzed the shapes of Galactic open clusters by the star-countingtechnique with the Two Micron All Sky Survey star catalog database.Morphological parameters such as the ellipticity and size have beenderived via stellar density distribution, weighed by clusteringprobability. We find that most star clusters are elongated, even for theyoungest star clusters of a few million years old, which are locatednear the Galactic disk. The shapes of young star clusters must reflectthe conditions in the parental molecular clouds and during the clusterformation process. As an open cluster ages, stellar dynamics cause theinner part of the cluster to circularize, but the overall radius getslarger and the stellar density becomes sparser. We discuss how theinternal relaxation process competes with Galactic external perturbationduring cluster evolution.
| 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.
| Stellar tracers of the Cygnus Arm. I. Spectroscopic study of bright photometric candidates
We present medium-resolution spectroscopy of a sample of stars in thesecond Galactic quadrant selected from the literature because theircolours suggest that they are moderately-reddened early-type stars atvery large distances. From the derived spectral types and observedcolours, we calculate distances to all these objects. For a sizablefraction of our sample, we find distances well in excess of what isexpected for Perseus Arm objects, even allowing for rather generouserrors. In the interval l=150degr -180degr , there is a large number ofobjects with distances in excess of 4 kpc, which are likely tracing theOuter or Cygnus Arm. In particular, we find that the association Cam OB3is placed on this Arm. Based on our results, the extent and definitionof the associations Cas OB4 and Aur OB2 need to be reevaluated.Based on observations made at Observatoire de Haute Provence (CNRS),France.
| Studies of Young Stellar Objects
We present a detailed study of four young open star clusters and two OBassociations using deep CCD U B V R I and 2MASS J H Ks measurements.Physical parameters of the clusters have been estimated using optical aswell as near-IR data. Ages of objects under study range 10-100 Myr. Massfunction slope of the sample of clusters are in agreement with theSalpeter (1955) value. From a comparison of mass function slope ofclusters and OB associations in our Galaxy with those in externalgalaxies, we urgue that the star formation processes are such that theyyield almost similar stellar mass distribution in vastly different starforming environments. Mass segregation suggests that the clusters understudy are dynamically relaxed and hence mass segregation may be due todynamical evolution or imprint of star formation or both. The extinctionproperties of a sample of 18 young star clusters have been investigatedutilizing recent data available in literature. The study suggests thatthere is no uniformity in extinction properties amongst these clusters.A non-uniform extinction has been noticed for the first time in NGC 1502and Tr 37 along with the existence of circumstellar shell around someearly type stars in NGC 884, NGC 2264, Tr 14 and Tr 16.
| 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.
| The Star Population of Young Open Clusters: A Photometric and Spectroscopic Study
Not Available
| 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.
| Pre-main-sequence stars in the young open cluster NGC 1893. I. A spectroscopic search for candidates in the area photometrically surveyed
We present low-resolution spectroscopy of the majority of the stars inthe field of NGC 1893 which were classified asnon-members based on the photometry of Marco et al. (2001). We separatefield stars from pre-main-sequence members based on a combination ofspectral type and photometric indices which allows us to determine thereddening to the objects. For candidates to pre-main-sequence objects,we present intermediate-resolution spectra. We confirm 3 F-typeemission-line objects as massive T Tauri stars in NGC1893 and find two good candidates for being absorption-lineHerbig A stars. A B5 star is a very good candidate for anabsorption-line Herbig B star. We also investigate the nature ofemission-line B-type stars close to the cluster centre. Availableevidence suggests that both classical Be stars and Herbig Be stars arepresent in NGC 1893. Based on observations obtainedat the Asiago Observatory (Italy), the Isaac Newton Telescope (La Palma,Spain) and Observatoire de Haute Provence (CNRS, France).
| 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
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| Non-uniform extinction in young open star clusters
The extinction law and the variation of colour excess with position,luminosity and spectral class in young open star clusters NGC 663, NGC869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14,Tr 15, Tr 16, Coll 228, Tr 37 and Be 86 have been studied. Thedifference in the minimum and maximum values of E(B-V) of clustermembers has been considered as a measure of the presence of non-uniformgas and dust inside the clusters. Its value ranges from 0.22 to 1.03magin the clusters under study, which indicates that non-uniform extinctionis present in all the clusters. It has been noticed for the first timein NGC 1502 and Tr 37. It is also found that the differential colourexcess in open clusters, which may be caused by the presence of gas anddust, decreases systematically with the age of clusters, indicating thatmatter is either used in star formation or blown away by hot stars orboth. There is no uniformity in the variation of E(B-V) with position orspectral class or luminosity. Except in Tr 14, all clusters show arandom spatial distribution of E(B-V), indicating a random distributionof gas and dust inside the clusters. The E(B-V) value correlates withboth luminosity and spectral class only in the case of Coll 228, Tr 16and Be 86. The members of these clusters atλ>=λR show larger values of colour excessratios than the normal ones. The value of E(U-V)/E(B-V) for most of thecluster members is close to the normal interstellar value of 1.73.However, the colour excess ratios with E(B-V) atλ>=λJ are smaller than the normal value forNGC 663, NGC 869, NGC 884 and NGC 1502, while they are larger for NGC6611, Coll 228, Tr 16 and Tr 14. Thus there is no uniformity in therelationship of extinction properties amongst the clusters under study.
| Estimation of Errors in the Distances to Intrinsically Reddened Stars
Not Available
| An optical and near IR study of the old open cluster NGC 2141
We report CCD optical (B and V passbands) and near IR (J and K bands)observations in the region of the old open cluster NGC 2141. Bycombining the two sets of photometry (500 stars in common) we derive newestimates of the cluster's fundamental parameters. We confirm that thecluster is 2.5 Gyrs old, but, with respect to previous investigations,we obtain a slightly larger reddening (E(B-V) = 0.40), and a slightlyshorter distance (3.8 kpc) from the Sun. Finally, we present theLuminosity Function (LF) in the V band, which is another age indicator.We provide a good fit for the age range inferred from isochrones byassuming the Kroupa et al. (\cite{r18}) IMF up to MV = 5.0.We interpret the disagreement at fainter magnitudes as evidence of masssegregation. Based on observations taken at ESO La Silla and TIRGO.Table~2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/372/879
| Photometric and Spectroscopic Study of the Young Open Cluster NGC 1893
We present uvbyβ CCD photometry of the field of the open clusterNGC 1893. Our photometry is deep enough to cover the completemain-sequence B spectral type. We identify ~50 very likely members ofthe cluster down to spectral type B9-A0, some of which have much higherreddenings than the average. We derive a color excess E(b-y)=0.33+/-0.03and a dereddened distance modulusV0-MV=13.9+/-0.2. From the β index, weidentify several candidates as emission-line stars, for which we haveobtained spectroscopy. Three of them display spectra corresponding tospectral type F but showing Hα in emission. Photometricmeasurements in this and previous studies indicate strong variability.These characteristics show them to be pre-main-sequence stars ofspectral type F. We also identify two likely Herbig Be stars. Theseresults hint at the existence of a sizable pre-main-sequence populationin NGC 1893.
| 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
| Observing bright Cepheids whitout optical aids.
The eye can distinguish brightness differences as small as +/- 0.1magnitude near the limit of vision. Bright variable stars can thereforebe studied effectively without optical aid, provided such a restrictionis satisfied. That conclusion has been confirmed by backyardobservation of the bright Cepheids ? Cephei, ? Geminorum, and ? Aquilaemade during 1998-99 using newly-generated reference charts tied tophotoelectric V-magnitudes rather than the usual "visual" magnitudesincluded on AAVSO charts. The derived light curves for the three brightCepheids exhibit surprisingly little scatter, and are of potentialscientific value for the study of period changes in the variables.
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| Transitional YSOs: candidates from flat-spectrum IRAS sources
We are searching for Young Stellar Objects (YSOs) near the boundarybetween protostars and pre-main-sequence objects, what we termTransitional YSOs. We have identified a sample of 125 objects ascandidate transitional YSOs on the basis of IRAS colors and the opticalappearance on POSS plates. We have obtained optical and near-IR imagingof 82 objects accessible from the Northern Hemisphere and optical imagesof 62 sources accessible from the South. We also created deconvolved 60mu m IRAS images of all sources. We have classified the objects on thebasis of their morphology in the optical and near-IR images. We findthat the majority of our objects are associated with star-formingregions, confirming our expectation that the bulk of these objects areYSOs. Of the 125 objects, 28 have a variety of characteristics verysimilar to other transitional YSOs, while another 22 show some of thesecharacteristics. Furthermore we have found seven objects to be goodcandidates for members of the Herbig Ae/Be stellar group, of which threeare newly identified as such. We have placed a set of images for each ofthe objects in the archives of the Centre de Données astronomiquede Strasbourg (CDS). Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile.
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