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.
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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.
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Distances to Cepheid open clusters via optical and K-band imaging
We investigate the reddening and main-sequence-fitted distances to 11young, Galactic open clusters that contain Cepheids. Each clustercontains or is associated with at least one Cepheid variable star.Reddening to the clusters is estimated using the U-B:B-V colours of theOB stars and the distance modulus to the cluster is estimated via B-V:Vand V-K:V colour-magnitude diagrams. Our main-sequence fitting assumesthat the solar-metallicity zero-age main sequence of Allen appliesuniversally to all the open clusters, although this point iscontroversial at present. In this way we proceed to calibrate theCepheid period-luminosity (PL) relation and find MV=-2.87× logP- 1.243 +/- 0.09, MK=-3.44 × logP- 2.21 +/-0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band givingan overall distance modulus to the LMC of μ0= 18.51 +/-0.10. This is in good agreement with the previous Cepheid PL-K result ofLaney & Stobie at μ0= 18.51 +/- 0.09 and with theHipparcos parallax-calibrated Cepheid PL-K estimate of Feast &Catchpole at μ0= 18.66 +/- 0.10 when no account is takenof the LMC metallicity.We also find that the two-colour U-B:B-V diagrams of two importantclusters are not well fitted by the standard main-sequence line. In onecase, NGC 7790, we find that the F stars show a UV excess and in thesecond case, NGC 6664, they are too red in U-B. Previous spectroscopicestimates of the metallicity of the Cepheids in these clusters appear tosuggest that the effects are not due to metallicity variations. Otherpossible explanations for these anomalies are positional variations inthe dust reddening law and contamination by foreground or backgroundstars.
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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.
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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.
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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].
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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.
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The overlapping open clusters NGC 1750 and NGC 1758. III. Cluster-field segregation and clusters physical parameters
Cluster-field segregation and determination of physical parameters ofthe open clusters NGC 1750 and NGC 1758 were performed from CCD andphotographic photometric and astrometric surveys previously published bythe authors. Membership probabilities were computed to a brightnesslimit of R=15 mag through a non-standard method in a fullynon-parametric approach, using proper motions, positions and photometry.This study allowed several parameters for NGC 1750 and NGC 1758 to bedetermined, including position, size, density profile, absorption,distance, age, luminosity function and mass. The common colour excess ofboth clusters is E(B-V)=0.34 mag. NGC 1750 is slightly younger (log t =8.3), closer (V-M_v=9.0 mag) and less concentrated than NGC 1758 (log t= 8.60 and V-M_v=9.4 mag). We also conclude that NGC 1746 is anerroneous assignation (not corresponding to any real open cluster), andthat NGC 1750 and NGC 1758 are overlapping clusters, but they do notconstitute a gravitationally bounded system.
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Luminosity function fine structure and the ages of young open clusters
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Absolute proper motions of 181 young open clusters.
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On Coagulation and the Stellar Mass Spectrum
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...452..652A&db_key=AST
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Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
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Young open clusters as probes of the star formation process. 1: an atlas of open cluster photometry
We have obtained charge coupled device (CCD) photometry, in the UBVsystem, for 23 open clusters in order to explore the star formationhistory of the Cassiopeia region of the Perseus spiral arm of ourGalaxy. Magnitudes and colors of 35,788 stars were measured, making thisthe most comprehensive, homogeneous single study of open clusterproperties in one part of the Galaxy. This paper presents an atlas ofopen cluster photometry that serves as the database for an investigationof cluster properties such as their ages, distances, reddenings, sizes,and richnesses. This information provides insight into the spatial andtemporal formation sequence of the clusters and allows an investigationof the stellar content of the clusters to be undertaken.
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Young open clusters as probes of the star-formation process. 2: Mass and luminosity functions of young open clusters
This paper is the second in a series devoted to the use of young,optically revealed open clusters as probes of the star-formationprocess. The first paper in the series presented the dataset used forthe study, and discussed some of the results from the survey of 23 openclusters in the Cassiopeia region of the Perseus spiral arm of ourGalaxy. This paper presents an analysis of the stellar content of asubset of the clusters, including determinations of the luminosity andmass functions of 8 of the clusters to approximately 1 solar mass. Theslope of the average mass function Gamma is found to be -1.40 +/- 0.13over the mass range 1.4 less than M/solar mass less than 7.9.Significant variations from this average value are found for two of theclusters, NGC 581 and NGC 663. Possible structure in the mass functionsis discussed, as is the degree to which the initial mass function (IMF)is universal, or merely an average of many IMFs that differsubstantially on the local level.
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Stellar Photometry Software
We describe the Stellar Photometry Software (SPS), which combines in asingle program procedures for locating stars, computing a mean stellarpoint-spread function (PSF), and performing aperture and/or multiplePSF-fitting photometry, along with related bookkeeping functions. Thesoftware can be run either interactively or in batch mode on computersusing the UNIX operating system. The performance of SPS is compared tothat of the photometry programs DoPHOT and IRAF/DAOPHOT, using both realand simulated CCD observations. A direct comparison of the instrumentalmagnitudes shows that all three programs produce comparable results.
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Numerical taxonomy for open clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..305T
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A CO survey of regions around 34 open clusters. II - Physical properties of cataloged molecular clouds
The physical properties of the 148 molecular clouds found in a CO surveyof regions around 34 young open clusters have been examined. Expressionsare given for the cloud size spectrum and the mass spectrum. Themass-radius relation implies that clouds of all size larger than a fewpc have about the same mean volume density. Power laws with slopes of0.6 and 3 describe, respectively, the relations of CO linewidth andcloud mass to cloud size. The clouds are distinctly nonspherical andappear to be randomly oriented with respect to the Galactic plane. Theobservations can be explained by a model for molecular clouds in whichclouds are ensembles of dense clumps of gas. Based on such a model, itis shown that molecular clouds are perturbed on a time scale shortcompared to the time required for them to reestablish virialequilibrium.
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A CO survey of regions around 34 open clusters
Results are presented from a systematic search for CO emission fromregions around 34 young open clusters in the outer Galaxy. The clustershave well-determined distances ranging from about 1 to 5 kpc and agesnot greater than about 100 Myr. It was found that some moderately youngclusters have no associated CO emission. All the surveyed clustersyounger than about 5 Myr have associated with them at least onemolecular cloud more massive than 10,000 solar mass, while the molecularclouds associated with clusters older than about 10 Myr are not moremassive than a few thousands solar masses. It was also found thatmolecular clouds are receding from young clusters at a rate of about 10km/sec, and that they seem to be destroyed by their interaction with thestars. Sites of ongoing star formation were identified in a number ofclouds associated with young clusters.
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An optical spiral arm beyond the Perseus arm
In the second galactic quadrant, optical spiral arm tracers have beencollected in a systematic literature search. A uniform reduction of thedata led to the detection of a distinct structure (probably a spiralarm) beyond the Perseus arm that is separated by a statisticallysignificant gap from the latter.
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Hypersensibilization of ORWO plates by prolonged heating.
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Component Analysis of Open Clusters
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Young stellar-gas complexes in the Galaxy
It is found that about 90 percent of OB-associations and o-b2 clusterssituated within 3 kpc of the sun can be united into complexes withdiameters of 150-700 pc. Almost all of these clusters contain giantmolecular clouds with a mass greater than about 100,000 solar masses. Anumber of complexes are associated with giant H I clouds; a few of thesmall complexes are situated in the HI caverns. The concentration ofOB-associations and young clusters in star complexes attests to theircommon origin in the supergiant gaseous clouds.
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The classification of open clusters by the centroid method of cluster analysis
The distribution of open clusters in the Galaxy are considered, withspace coordinates including mass, absolute magnitude, integrated colorindex, diameter, metallicity, and age. It is shown that the majority ofclusters belong to several classes which have parameter values in asufficiently narrow range. The classes form a linear sequence by age andmonotonic sequence on a color-magnitude diagram. They are not isolated,but move into each other continuously. This suggests that the process ofcluster formation contains no significant gaps. The bifurcation of theage sequence of classes depending on the mass and diameter values isfound. This bifucation makes an evolutionary interpretation possible.
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On Statistics of Star Complexes
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A cluster analysis of open clusters
The Galactic distribution of 361 open clusters is studied using acluster analysis method. It is shown that more than half of the clustersenter groups with characteristic dimensions of several hundred parsecs.To distinguish physical clusters from random condensations, criteriabased on age similarity, the color of the main-sequence blue end, andthe integrated color and radial velocity of the clusters are used. Theproximity of these values suggests a physical unity and common origin ofclusters in a group.
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Catalog of open clusters and associated interstellar matter.
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Photographic Observations of Ar-Andromedae
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Kinematics of young open clusters and the rotation curve of our Galaxy
Published observational data on a sample of 105 kinematically andspatially distinct open clusters of early spectral type (up to B3) arecompiled in tables, graphs, and diagrams and characterizedstatistically. Findings reported include (1) solar motion expanding atLSR velocity 3 km/s or less (with no noncircular motion in the directionof rotation), (2) Oort constant A = 17.0 + or - 1.5 km/s kpc andsecond-order rotation term alpha = -2.0 + or - 0.6 km/s sq kpc at R-R0between -3 and 5 kpc, (3) maximum rotation-curve deviation + or - 10km/s at R-R0 about - or + 2 kpc, and (4) nondecreasing rotationalvelocities beyond about R-R0 = 3 kpc. The rotational velocities of H IIregions and molecular clouds in the Perseus arm are found to besignificantly lower than those of the open clusters.
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NEWS@SKY (Science&Space News)
