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On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| 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.
| New variable stars in open clusters. I. Methods and results for 20 open clusters
We present high precision CCD photometry of 1791 objects in 20 openclusters with an age of 10 Myr to 1 Gyr. These observations wereperformed within the Δ a photometric system which is primarilyused to detect chemically peculiar stars of the upper main sequence.Time bases range from 30 min up to 60 days with data from severalnights. We describe the time series analysis reaching a detection limitof down to 0.006 mag for apparent variability. In total, we havedetected 35 variable objects of which four are not members of theircorresponding clusters. The variables cover the entireHertzsprung-Russell diagram, hence they are interesting targets forfollow-up observations.Based on observations obtained at Complejo Astronómico elLeoncito (CASLEO), operated under the agreement between the ConsejoNacional de Investigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba y San Juan; ESO-La Silla and UTSO-Las Campanas.
| Theoretical isochrones for the Delta a photometric system
We have calculated theoretical isochrones for the photometric Delta asystem to derive astrophysical parameters such as the age, reddening anddistance modulus for open clusters. The Delta a system samples the fluxdepression at 520 nm which is highly efficient to detect chemicallypeculiar (CP) objects of the upper main sequence. The evolutionarystatus of CP stars is still a matter of debate and very important totest, for example, the dynamo and diffusion theories. In fact, thedynamo or fossil origin of the magnetic fields present in this kind ofstar is still not clear. Using the stellar evolutionary models by Claret(\cite{Cla95}), a grid of isochrones with different initial chemicalcompositions for the Delta a system was generated. The published dataof 23 open clusters were used to fit these isochrones with astrophysicalparameters (age, reddening and distance modulus) from the literature. Asan additional test, isochrones with the same parameters for Johnson UBVdata of these open clusters were also considered. The fits show a goodagreement between the observations and the theoretical grid. We findthat the accuracy of fitting isochrones to Delta a data without theknowledge of the cluster parameters is between 5 and 15%.
| 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.
| CCD photometric search for peculiar stars in open clusters. IV. Collinder 272, Pismis 20, Lyng\aa 14, NGC 6396 and NGC 6611
The time scale on which chemical peculiarity in upper main sequencestars begins is unknown. Results from the Hipparcos measurementsindicate that the classical chemically peculiar (CP) stars occupy thewhole area from the zero age main sequence to the terminal age mainsequence (Gómez et al. \cite{Gom98}). However, Hubrig et al.(\cite{Hub00}) reported that the magnetic fields appear only in starsthat have already completed at least approximately 30% of their mainsequence life time. Previous observations in the very young Orion OB1and Scorpio-Centaurus associations (Joncas & Borra 1981; Borra etal. 1982) reveal a percentage of chemically peculiar stars that issignificantly smaller (less than 5%) than that of the galactic field (upto 25%). This already indicates that the phenomenon needs at leastseveral 106 yr to appear. Since no further observations havebeen published since then, we have observed the five very young (1 Myr<= age <= 25 Myr) open clusters Collinder 272, Pismis 20, Lyng\aa14, NGC 6396 and NGC 6611. Photometry from 250 CCD frames in the threefilter Delta a-system results in very low detection limits (0.008 to0.015 mag). We find no single photometrically chemically peculiarcandidate in approximately 460 observed cluster members in the fiveprogram clusters. From the number of observed objects we deduce upperlimits between 0.6% and 2.4% for the incidence of CP stars for the fourobserved aggregates. Based on observations obtained at ComplejoAstronómico el Leoncito (CASLEO), operated under the agreementbetween the Consejo Nacional de Investigaciones Científicas yTécnicas de la República Argentina and the NationalUniversities of La Plata, Córdoba y San Juan; ESO-La Silla andUTSO-Las Campanas.
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| Absolute proper motions of 181 young open clusters.
Not Available
| 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.
| Integrated photometric properties of open clusters
Galactic open clusters provide an abundant sample of stellar aggregatesof various sizes, ages and metal abundances, apt to constitute atemplate for comparison with star systems in other galaxies. In thispaper we present and discuss a standard methodology to synthesize U,B,Vfluxes and colours, and apply it to a set of 138 open clusters. Resultsare compared with previous ones available in the literature. We wereable to calibrate a mass-luminosity relation by which we evaluated themass of ~400 open clusters, leading to a well defined present-day massfunction. The number-complete sample of galactic open clusters presentedin Battinelli & Capuzzo-Dolcetta (1991) is enlarged of a 15%.
| 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.
| A photometric study of the bright cloud B in Sagittarius. VI - 1592 new variable stars and 30 diffuse objects
Abstract image available at:http://adsabs.harvard.edu/abs/1988A&AS...76..205T
| 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.
| 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.
| Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)
Not Available
| Integrated Photometric Parameters of Open and Globular Clusters
Not Available
| 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.
| Analysis of the results of MK classification of 176 stars in 37 southern open clusters
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979A&AS...37..345F&db_key=AST
| A catalogue of galactic clusters observed in three colours
This catalogue contains photometric data for 190 galactic clusters, allobserved in UBV or RGU. The distances of the young clusters (with spless than b3) have been calculated or recalculated according to method Aof Becker (1963). The galactic distribution of these clusters confirmstheir role of being good spiral tracers.
| Southern open star clusters VI. UBV-Hbeta photometry of 18 clusters from Centaurus to Sagittarius.
Abstract image available at:http://adsabs.harvard.edu/abs/1975A&AS...20..155M
| Uniform survey of clusters in the southern Milky Way.
Abstract image available at:http://adsabs.harvard.edu/abs/1975AJ.....80...11V
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