New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is 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/463/789
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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.
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Seven-Color Photometry of the Open Cluster NGC 1647 Area
The area of the open cluster NGC 1647 in Taurus is investigated by CCDphotometry in the Vilnius seven-color system. Magnitudes and colorindices are determined for 433 stars down to V = 15.0 mag in the 45arcmin diameter area. For 252 of them photometric spectral andluminosity classes, interstellar reddenings, extinctions and distancesare obtained. According to the CDS WEBDA database, 89 of them have ahigh cluster membership probability. Their mean distance from the Sunis 555±74 pc, excluding four stars which seem to be field stars.The main sequence starts at spectral class B7 V which corresponds to acluster age of about 150 million years. Cluster members show adifferential interstellar extinction ranging from 0.8 to 1.8 mag. Themean extinction of the cluster stars is 1.12±0.25 mag.Interstellar extinction in the area is dominated by the Taurus darkcloud complex at 160 pc. Color excesses of individual stars correlatewell with the 100 μm dust thermal emission intensity. The clustershape is investigated by counting stars down to K=15.6 mag and is foundto be elongated in the direction roughly perpendicular to the Milky Way,with the flattening 0.4.
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Seven-Color Photometry of the Open Cluster NGC 2395 Area
The area of the open cluster NGC 2395 in Gemini is investigated by CCDphotometry in the Vilnius seven-color system. Magnitudes, color indices,photometric spectral types, color excesses, interstellar extinctions anddistances are determined for 163 stars down to {\msf V} = 15.75 mag inthe 25' diameter area. Twenty stars at a mean distance of 410 pc aresuspected to be cluster members. The main sequence starts at thespectral class F5. Two evolved F5 stars and two red giants are present;the cluster's age should be about 1.5 Gyr. The suspected cluster starsshow a differential interstellar extinction ranging from 0.0 to 0.6 mag.Interstellar extinction in the area starts at about the cluster'sdistance. At larger distances the observed extinction values arescattered between zero and 0.7 mag. Among the suspected cluster membersonly two K dwarfs are present. It is possible that NGC 2395 has sufferedsignificantly from Galactic tidal distortion on its low-mass starcontent.
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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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The Parker Instability in Three Dimensions: Corrugations and Superclouds along the Carina-Sagittarius Arm
Here we present three-dimensional MHD models for the Parker instabilityin a thick magnetized disk, including the presence of a spiral arm. TheB field is assumed parallel to the arm, and the model results areapplied to the optical segment of the Carina-Sagittarius arm. Thecharacteristic features of the undular and interchange modes are clearlyapparent in the simulations. The interchange mode appears first andgenerates small interstellar structures in the interarm regions, but itsdevelopment inside the arm is hampered by the acceleration of the spiralwave. In contrast, the undular mode follows its normal evolution insidethe spiral wave, creating large gas concentrations distributed along thearm. This results in a clear arm/interarm difference; the instabilitytriggers the formation of large interstellar clouds (with masses in therange of 106-107 Msolar) inside thearms but generates only small structures with slight densityenhancements in the interarm regions. The resulting clouds aredistributed in an antisymmetric way with respect to the midplane, andtheir masses are similar to those inferred for H I superclouds in ourGalaxy. Such a cloud distribution results in an azimuthal corrugationalong the arm, and for conditions similar to those of the opticalsegment of the Carina-Sagittarius arm, it has a wavelength of about 2.4kpc. This structuring, then, can explain the origin of both H Isuperclouds and the azimuthal corrugations in spiral arms. Inparticular, the wavelength of the fastest growing undular mode matchesthe corrugation length derived with the young stellar groups located inthe optical segment of the Carina-Sagittarius arm.
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CCD Photometry of the Open Cluster NGC 2395
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Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
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Hipparcos Trigonometric Parallaxes and the Distance Scale for Open Star Clusters
Hipparcos trigonometric parallaxes are used to estimate the distances tothe maximum possible number of open star clusters (OSC); distance moduliare estimated for 45 clusters with maximum heliocentric distances ofabout 1000 pc. The latter value can serve as an estimate of the limit towhich it still makes sense to use Hipparcos trigonometric parallaxes todetermine the distances to small groups composed of 6-10 sufficientlybright stars. A systematic correction to the distance moduli of clustersfrom the homogeneous catalog of OSC parameters (Loktin et al. 1997,2000) is estimated, which turns out to be independent of the clusterage.
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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
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ω Centauri - a former nucleus of a dissolved dwarf galaxy? New evidence from Strömgren photometry
CCD vby Strömgren photometry of a statistically complete sample ofred giants and stars in the main sequence turn-off region in omegaCentauri has been used to analyse the apparently complex star formationhistory of this cluster. From the location of stars in the (b-y),m_1diagram metallicities have been determined. These have been used toestimate ages of different sub-populations in the color-magnitudediagram and to investigate their spatial distributions. We can confirmseveral earlier findings. The dominating metal-poor population around-1.7 dex is the oldest population found. More metal-rich stars between[Fe/H]=-1.5 and -1.0 dex tend to be 1-3 Gyr younger. These stars aremore concentrated towards the cluster center than the metal-poor ones.The most-metal rich stars around -0.7 dex might be up to 6 Gyr youngerthan the oldest population. They are asymmetrically distributed aroundthe center with an excess of stars towards the South. We argue that theStrömgren metallicity in terms of element abundances has anothermeaning than in other globular clusters. From a comparison withspectroscopic element abundances, we find the best correlation with thesum C+N. The high Strömgren metallicities, if interpreted by strongCN-bands, result from progressively higher N and perhaps C abundances incomparison to iron. The large scatter of Strömgren abundances maycome from a variety of evolutionary effects, including C-depletion andN-enrichment. We see an enrichment already among the metal-poorpopulation, which is difficult to explain by self-enrichment alone. Anattractive speculation (done before) is that omega Cen was the nucleusof a dwarf galaxy. We propose a scenario in which omega Cen experiencedmass inflow over a long period of time, until the gas content of itshost galaxy was so low that star formation in omega Cen stopped, oralternatively the gas was stripped off during its infall in the MilkyWay potential. This mass inflow could have occurred in a clumpy anddiscontinuous manner, explaining the second peak of metallicities, theabundance pattern, and the asymmetric spatial distribution of the mostmetal-rich population. Moreover, it explains the kinematic differencesfound between metal-poor and metal-rich stars.
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Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
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Revised Strömgren metallicity calibration for red giants
A new calibration of the Strömgren (b-y),m_1 diagram in terms ofiron abundance of red giants is presented. This calibration is based ona homogeneous sample of giants in the globular clusters omega Centauri,M 22, and M 55 as well as field giants from the list of Anthony-Twarog& Twarog (\cite{anth98}). Towards high metallicities, the newcalibration is connected to a previous calibration by Grebel &Richtler (\cite{greb92}), which was unsatisfactory for iron abudanceslower than -1.0 dex. The revised calibration is valid for CN-weak/normalred giants in the abundance range of -2.0 <[Fe/H]< 0.0 dex, and acolor range of 0.5 < (b-y) < 1.1 mag. As shown for red giants inomega Centauri, CN-weak stars with Strömgren metallicities higherthan -1.0 dex cannot be distinguished in the (b-y),m_1 diagram fromstars with lower iron abundances but higher CN band strengths. Based ondata collected at the European Southern Observatory, La Silla, Chile
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Remembering Gemini.
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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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Formation and evolutionary properties of the Galactic open cluster system
Results are reported from a statistical analysis of observational dataon 100 open clusters within 2 kpc of the sun, selected from the catalogof Lynga (1987). The selection criteria and the completeness of thesample are discussed; the data are compiled in a table; and the analysisresults are presented in a series of graphs and characterized in detail.A cluster formation rate of 0.45 clusters/kpc Myr is found,significantly lower than the rates determined previously (using clusterswithin 1 kpc of the sun) and corresponding to a cluster star-formationefficiency of 0.0063. The low average cluster lifetime (about 10 Myr)suggests that clusters are formed as unstable systems.
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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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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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Investigation of the initial mass spectrum of open star clusters
The mass spectra of 228 open star clusters were derived by comparison ofcolor-magnitude diagrams with evolutionary tracks. The application tobinary stars showed the reliability of the mass determination. Thederived mass spectra were fitted by power laws as well as exponentiallaws. It could be shown that both approximate the mass spectra of openstar clusters on the same average significance level. The presentinvestigation revealed a correlation of the slope of the mass spectrawith the cluster age, whereas a detected correlation of the slope withgalactocentric distance is slight. The results suggest that the slope ofthe mass spectrum increases with increasing cluster and galactocentricdistance. These findings are discussed with respect to their reasons andprevious results concerning open clusters and field stars.
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Open clusters and galactic structure
A total of 610 references to 434 clusters are employed in thecompilation of a catalog of open clusters with color-magnitude diagramson the UBV or RGU systems. Estimates of reddening, distance modulus, ageand number of cluster members are included. Although the sample isconsidered representative of the discoverable clusters in the galaxy,the observed distribution is nonuniform because of interstellarobscuration. Cluster distribution in the galactic plane is found to bedominated by the locations of dust clouds rather than by spiralstructure. The distributions of clusters as a function of age andrichness class show that the lifetimes of poor clusters are much shorterthan rich ones, and that clusters in the outer disk survive longer thanthose in the inner disk. An outer disk age which is only about 50% theage of the globular clusters is indicated by cluster statistics. Thethickening of the galactic disk with increasing galactocentric distancemay be due to either a younger dynamical age or a lower gravitationalpotential in the outer regions.
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A catalogue of galactic star clusters observed in three colours
Abstract image available at:http://adsabs.harvard.edu/abs/1971A&AS....4..241B
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An atlas of open cluster colour-magnitude diagrams
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The ages of open clusters
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Colour-magnitude diagrams of the galactic clusters NGC 2286 and NGC 2395
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Die räumliche Verteilung von 156 galaktischen Sternhaufen in Abhängigkeit von ihrem Alter. Mit 7 Textabbildungen
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Colour - magnitude diagrams of the galactic clusters NGC 2286 and NGC 2395.
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On some southern galactic clusters
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