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Runaway Massive Binaries and Cluster Ejection Scenarios
The production of runaway massive binaries offers key insights into theevolution of close binary stars and open clusters. The stars HD 14633and HD 15137 are rare examples of such runaway systems, and in this workwe investigate the mechanism by which they were ejected from theirparent open cluster, NGC 654. We discuss observational characteristicsthat can be used to distinguish supernova ejected systems from thoseejected by dynamical interactions, and we present the results of a newradio pulsar search of these systems, as well as estimates of theirpredicted X-ray flux assuming that each binary contains a compactobject. Since neither pulsars nor X-ray emission are observed in thesesystems, we cannot conclude that these binaries contain compactcompanions. We also consider whether they may have been ejected bydynamical interactions in the dense environment where they formed, andour simulations of four-body interactions suggest that a dynamicalorigin is possible but unlikely. We recommend further X-ray observationsthat will conclusively identify whether HD 14633 or HD 15137 containneutron stars.
| A Spectroscopic Study of Field and Runaway OB Stars
Identifying binaries among runaway O- and B-type stars offers valuableinsight into the evolution of open clusters and close binary stars. Herewe present a spectroscopic investigation of 12 known or suspectedbinaries among field and runaway OB stars. We find new orbital solutionsfor five single-lined spectroscopic binaries (HD 1976, HD 14633, HD15137, HD 37737, and HD 52533), and we classify two stars thought to bebinaries (HD 30614 and HD 188001) as single stars. In addition, wereinvestigate their runaway status using our new radial velocity datawith the UCAC2 proper-motion catalogs. Seven stars in our study appearto have been ejected from their birthplaces, and at least three of theserunaways are spectroscopic binaries and are of great interest for futurestudy.
| 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.
| The Optical Gravitational Lensing Experiment. Variable Stars in the Field of Open Cluster NGC6259
We present the results of a search for variable stars in the field ofNGC6259. Altogether 85 variable stars have been discovered. 36 of themare eclipsing systems. This group contains 13 EA, 2 EB and 21 EW-typestars. Light curves of two variable stars resemble those of Miras. Theremaining 47 variable stars detected in the field of this open clusterare most probably highly obscured red giants (OSARG, irregular). For alldetected variable stars we provide their light curves, preliminaryclassification, discussion on the possible cluster membership,equatorial coordinates, finding charts and periods when possible.
| 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.
| Stellar contents of two young open clusters: NGC 663 and 654
UBVRI CCD photometry in a wide field around two young open clusters, NGC663 and 654, has been carried out. Hα and polarimetricobservations for the cluster NGC 654 have also been obtained. We use thephotometric data to construct colour-colour and colour-magnitudediagrams, from which we can investigate the reddening, age, mass andevolutionary states of the stellar contents of the these clusters. Thereddening across the cluster regions is found to be variable. There isevidence for anomalous reddening law in both clusters; however, moreinfrared and polarimetric data are needed to conclude about thereddening law. Both clusters are situated at about a distance of 2.4kpc. Star formation in both clusters is found to be a continuousprocess. In the case of NGC 663, star formation seems to have takenplace sequentially, in the sense that formation of low-mass starsprecedes the formation of most massive stars. Whereas, in the case ofNGC 654, formation of low-mass stars did not cease after the formationof most massive stars in the cluster.
| The Massive Runaway Stars HD 14633 and HD 15137
We present results from a radial velocity study of two runaway O-typestars, HD 14633 (ON8.5 V) and HD 15137 [O9.5 III(n)]. We find that HD14633 is a single-lined spectroscopic binary with an orbital period of15.4083 days. The second target, HD 15137, is a radial velocity variableand a possible single-lined spectroscopic binary with a period close to1 month. Both binaries have large eccentricity, small semiamplitude, anda small mass function. We show the trajectories of the stars in the skybased on an integration of motion in the Galactic potential, and wesuggest that both stars were ejected from the vicinity of the opencluster NGC 654 in the Perseus spiral arm. The binary orbital parametersand runaway velocities are consistent with the idea that both thesestars were ejected by supernova explosions in binaries and that theyhost neutron star companions. We find that the time of flight sinceejection is longer than the predicted evolutionary timescales for thestars. This discrepancy may indicate that the stars have a lower massthan normally associated with their spectral classifications, that theywere rejuvenated by mass transfer prior to the supernova, or that theirlives have been extended through rapid rotation.Based in part on observations made at the Observatoire de Haute Provence(CNRS), France.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| The Anomalous X-ray Pulsar 4U 0142+61: Variability in the infrared and a spectral break in the optical
We present new optical and infrared observations of the counterpart tothe Anomalous X-ray Pulsar (AXP) 4U 0142+61 taken with the Keck Itelescope. The counterpart is found to be variable in the infrared. Thiscontrasts with our optical observations, which do not show any evidencefor variability. Apart from the variability the AXP shows a remarkablespectral energy distribution. In particular, we find a sudden drop influx going from V to B, presumably due to a spectral feature. We compareour results to those obtained for the two other securely identified AXPcounterparts, to 1E 2259+586 and 1E 1048.1-5937. 4U 0142+61 is verysimilar to the former source in its X-ray timing and spectralproperties, and we find that this similarity extends to the quiescentinfrared to X-ray flux ratio. For 1E 1048.1-5937, which has differentX-ray properties, the situation is less clear: in one observation, theinfrared to X-ray flux ratio was much larger, but another observationgave an upper limit which is consistent with that observed for 4U0142+61. Assuming the quiescent ratios are all similar, we estimate theoptical and infrared brightnesses for the three AXPs that remain to beidentified as well as for the four Soft Gamma-ray Repeaters. We alsodiscuss briefly how the observed optical and infrared emission mightarise, in particular in the context of the magnetar model.Table 3 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/416/1037
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| 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.
| Interstellar extinction in the open clusters towards galactic longitude around 130deg
In this paper we present a detailed study of the intra-cluster reddeningmaterial in the young open clusters located around l ~ 130degusing colour-excess diagrams and two-colour diagrams. The study supportsthe universality of the extinction curves for lambda >= lambdaJ, whereas for shorter wavelengths the curve depends upon thevalue of the Rcluster (total-to-selective absorption in thecluster region). The value of Rcluster in the case of NGC654, NGC 869 and NGC 884 is found to be normal, whereas the value ofRcluster in the cluster regions NGC 1502 and IC 1805indicates an anomalous reddening law in these regions. In the case ofNGC 1502 the value of Rcluster is found to be lower(2.57+/-0.27) whereas in the case of IC 1805 it is higher (3.56+/-0.29)than the normal value of 3.1. Although the intra-cluster materialindicates a higher value of Rcluster in the NGC 663 region,the error in the estimation of Rcluster is too large toconclude anything. It is also found that the extinction process in the Uband in the case of NGC 663 seems to be less efficient, whereas in thecase of NGC 869 the process is more efficient.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| 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.
| 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).
| Variable Stars in the Field of Young Open Cluster NGC 581
We present results of the search for variable stars in the field ofyoung open cluster NGC 581. Based on 19 nights of observations, 6 newvariable stars were discovered. Two of them turned out to be eclipsingbinary systems. Another two detected variable stars are most probably ofgamma Dor type. During our observations one of the known Be starslocated in our field of view showed irregular variations of brightness,typical for this class of stars. The sixth variable star is a pulsatingred giant.
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| 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.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| Variable Stars in the Field of Young Open Cluster NGC 659
We present results of the search for variable stars in young opencluster NGC 659. Based on 35 nights of observations, 7 new variablestars were discovered. Three of them are Be stars. Another three aremost probably pulsating variables of gamma Dor type. One star is adetached binary system.
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| An optical counterpart to the anomalous X-ray pulsar 4U0142+61
The energy source of the anomalous X-ray pulsars (AXPs) is notunderstood, hence their designation as anomalous. Unlike binary X-raypulsars, no companions are seen, so the energy cannot be supplied byaccretion of matter from a companion star. The loss of rotationalenergy, which powers radio pulsars, is insufficient to power AXPs. Twomodels are generally considered: accretion from a large disk left overfrom the birth process, or decay of a very strong magnetic field(1015G) associated with a `magnetar'. The lack ofcounterparts at other wavelengths has hampered progress in ourunderstanding of these objects. Here we report deep optical observationsof the field around 4U0142+61, which is the brightest AXP in X-rays. Thesource has no associated supernova remnant, which, together with itsspin-down timescale of ~105yr (ref. 5), suggests that it maybe relatively old. We find an object with peculiar optical colours atthe position of the X-ray source, and argue that it is the opticalcounterpart. The optical emission is too faint to admit the presence ofa large accretion disk, but may be consistent with magnetosphericemission from a magnetar.
| 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
| 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.
| Spectroscopic observations of young open clusters: IC 1805, NGC 654 and NGC 6823
Spectroscopic observations were made for a sample of 263 stars fromthree young open clusters IC 1805, NGC 654 and NGC 6823 with CCDdetectors in the blue and yellow-red spectral regions. MKK types wereproperly determined for these stars by carefully comparing spectra ofprogram stars with those of a complete grid of MKK standards, which wereobserved with the same instruments as the program stars. For most of theobserved stars precise MKK types were assigned for the first time. A fewof early-type emission-line stars were discovered or confirmed. Thepresence of Be stars in the studied clusters is discussed. Spectra ofthe observed MKK standards also reveal lots of features showingtemperature or luminosity effect in the yellow-red region, which helpedin our stellar spectral classification.
| Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
| A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics
We use optical and near-infrared star counts to explore the structureand dynamics of the Orion Nebula Cluster (ONC). This very young (<1 Myr)cluster is not circularly symmetric in projection but is elongatednorth-south in a manner similar to the molecular gas distribution in theregion, suggesting that the stellar system may still reflect thegeometry of the protocluster cloud. Azimuthally averaged stellar sourcecounts compare well with simple spherically symmetric, single-mass Kingcluster models. The model fits suggest that the inner Trapezium regionshould be regarded as the core of the ONC, not as a distinct entity assometimes advocated. We estimate that the core radius of the cluster is0.16-0.2 1 pc and that the central stellar density approaches 2 x104 stars/pc3. Adopting the stellar velocitydispersion from published proper-motion studies, virial equilibriumwould require a total mass within about 2 pc of the Trapezium of 4500Mo, slightly more than twice the mass of the known stellarpopulation and comparable to the estimated mass in molecular gasprojected onto the same region of the sky. If >~20% of the remainingmolecular gas is converted into stars, thus adding to the binding mass,given that the present stellar population alone has a total energy closeto zero, the ONC is likely to produce a gravitationally bound cluster.The ONC also exhibits mass segregation, with the most massive(Trapezium) stars clearly concentrated toward the center of the clusterand some evidence for the degree of central concentration to decreasewith decreasing mass down to 1-2 Mo, as would be expected forgeneral mass segregation. Given the extreme youth of the stars comparedwith the estimated range of collisional relaxation times, the masssegregation is unlikely to be the result of cluster relaxation. Instead,we suggest that the mass segregation reflects a preference for highermass stars to form in dense, central cluster regions.
| A report on the studies of star clusters with the UPSO 104-cm Sampurnanand telescope during last 25 years
Not Available
| Binary stars in the vicinity of the open cluster NGC 6913.
Not Available
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