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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.
| Survey for Transiting Extrasolar Planets in Stellar Systems. IV. Variables in the Field of NGC 1245
The Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS)project is a search for planetary transits in open clusters. In thispaper we analyze the STEPSS observations of the open cluster NGC 1245 todetermine the variable-star content of the cluster. Out of 6787 starsobserved with V<22, of which ~870 are cluster members, we find 14stars with clear intrinsic variability that are potential clustermembers and 29 clear variables that are not cluster members. None ofthese variables have been previously identified. We present lightcurves, finder charts, and stellar/photometric data on these variableobjects. Several of the interacting binaries have estimated distancesconsistent with the cluster distance determined from isochrone fits tothe color-magnitude diagram. Four stars at the main-sequence turnoff ofthe cluster have light curves consistent with γ Doradusvariability. If these γ Doradus candidates are confirmed, theyrepresent the oldest and coolest members of this class of variablediscovered to date.
| Survey for Transiting Extrasolar Planets in Stellar Systems. III. A Limit on the Fraction of Stars with Planets in the Open Cluster NGC 1245
We analyze a 19 night photometric search for transiting extrasolarplanets in the open cluster NGC 1245. An automated transit searchalgorithm with quantitative selection criteria finds six transitcandidates; none are bona fide planetary transits. We characterize thesurvey detection probability via Monte Carlo injection and recovery ofrealistic limb-darkened transits. We use this to derive upper limits onthe fraction of cluster members with close-in Jupiter radii,RJ, companions. The survey sample contains ~870 clustermembers, and we calculate 95% confidence upper limits on the fraction ofthese stars with planets by assuming that the planets have an evenlogarithmic distribution in semimajor axis over the Hot Jupiter (HJ;3.0
| Toward the Detection of Transiting Hot Earths and Hot Neptunes in Open Clusters
Radial velocity searches for extrasolar planets have recently detectedseveral very low mass (7-20 M_oplus) planets in close orbits withperiods less than 10 days. We consider the prospects for detecting theanalogs of these planets in Galactic open clusters via transits. Weoutline the requirements for constructing a transit survey that wouldallow one to probe such ``Hot Earths'' and ``Hot Neptunes.''Specifically, we present a simple criterion for detection that definesthe minimum aperture required to detect planets of a given radius in acluster at a given distance. Adopting photometric precisions that havebeen demonstrated in state-of-the-art variability surveys, we thenpredict the number of planets one could potentially detect withambitious transit surveys toward several open clusters. Dedicatedsurveys lasting more than 20 nights with Pan-STARRS toward the Hyadesand Praesepe could detect a handful of Hot Earths, if the majority ofstars host such planets. Similar surveys with larger aperture telescopes(eg CFHT, MMT), toward M67, M35, M50, and M37 could detect Hot Neptunes,provided that their frequency is greater than 1%. The majority ofplanets will be detected around M dwarfs; detecting Hot Neptunes aroundsuch primaries requires photometric precisions of approx 1%, whereas HotEarths require approx 0.1 %. We discuss potential hurdles in detectingand confirming small planets in ground-based surveys, includingcorrelated noise, false positives, and intrinsic stellar variability.
| 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.
| Survey for Transiting Extrasolar Planets in Stellar Systems. II. Spectrophotometry and Metallicities of Open Clusters
We present metallicity estimates for seven open clusters based onspectrophotometric indices from moderate-resolution spectroscopy.Observations of field giants of known metallicity provide a correlationbetween the spectroscopic indices and the metallicity of open clustergiants. We use χ2 analysis to fit the relation ofspectrophotometric indices to metallicity in field giants. The resultingfunction allows an estimate of the target-cluster giants' metallicitieswith an error in the method of +/-0.08 dex. We derive the followingmetallicities for the seven open clusters: NGC 1245, [M/H]=-0.14+/-0.04NGC 2099, [M/H]=+0.05+/-0.05 NGC 2324, [M/H]=-0.06+/-0.04 NGC 2539,[M/H]=-0.04+/-0.03 NGC 2682 (M67), [M/H]=-0.05+/-0.02 NGC 6705,[M/H]=+0.14+/-0.08 NGC 6819, [M/H]=-0.07+/-0.12. These metallicityestimates will be useful in planning future extrasolar planet transitsearches, since planets may form more readily in metal-richenvironments.
| From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main-Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory andmain-sequence stellar evolution theory to accurately determine stellarpopulation ages by comparing ages derived using both techniques for openclusters ranging from 0.1 to 4 Gyr. I find good agreement between whitedwarf and main-sequence evolutionary ages over the entire age rangecurrently available for study. I also find that directly comparingmain-sequence turnoff ages to white dwarf ages is only weakly sensitiveto realistic levels of errors in cluster distance, metallicity, andreddening. Additional detailed comparisons between white dwarf andmain-sequence ages have tremendous potential to refine and calibrateboth of these important clocks, and I present new simulations ofpromising open cluster targets. The most demanding requirements forthese white dwarf studies are very deep (V>=25-28) clusterobservations made necessary by the faintness of the oldest white dwarfs.
| Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
| Fraction of Stars with Planets in the Open Cluster NGC 1245
Based on our 19 night photometric search for transiting extrasolarplanets in the open cluster NGC 1245, we place an upper limit on thefraction of stars in the cluster that harbor close-in Jupiter radiicompanions. Monte-carlo injection and boxcar recovery of transits in theobserved light curves provide the basis for calculating the efficiencyfor transit detection. This cluster is the first target of the Searchfor Transiting Extrasolar Planets in Stellar Systems (STEPSS) projectwhose goal is to assess the frequency of close-in extrasolar planets inseveral open clusters of known metallicity, stellar density, and age inorder to constrain the impact of these parameters on the planetformation process.
| 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.
| 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.
| On the recent star formation history of the Milky Way disk
We have derived the star formation history of the Milky Way disk overthe last 2 Gyr from the age distribution diagram of a large sample ofopen clusters comprising more than 580 objects. By interpreting the agedistribution diagram using numerical results from an extensive libraryof N-body calculations carried out during the last ten years, wereconstruct the recent star formation history of the Milky Way disk.Under the assumption that the disk has never been polluted by anyextragalactic stellar populations, our analysis suggests thatsuperimposed on a relatively small level of constant star formationactivity mainly in small-N star clusters, the star formation rate hasexperienced at least five episodes of enhanced star formation lastingabout 0.2 Gyr with production of larger clusters. This cyclic behaviourshows a period of 0.4+/-0.1 Gyr and could be the result of density wavesand/or interactions with satellite galaxies. On the other hand, the starformation rate history from a volume-limited sample of open clusters inthe solar neighbourhood appears to be consistent with the overall starformation history obtained from the entire sample. Pure continuous starformation both in the solar neighbourhood and the entire Galactic diskis strongly ruled out. Our results also indicate that, in the Milky Waydisk, about 90% of open clusters are born with N<=150 and the slopein the power-law frequency distribution of their masses is about -2.7when quiescent star formation takes place. If the above results arere-interpreted taking into consideration accretion events onto the MilkyWay, it is found that a fraction of the unusually high number of openclusters with ages older than 0.6 Gyr may have been formed in disruptedsatellites. Problems arising from the selection effects and the ageerrors in the open cluster sample used are discussed in detail.
| Survey for Transiting Extrasolar Planets in Stellar Systems. I. Fundamental Parameters of the Open Cluster NGC 1245
We derive fundamental parameters for the old, rich, open cluster NGC1245 using BVI photometry from the MDM 1.3 m and 2.4 m telescopes. Basedon detailed isochrone fitting, we find that NGC 1245 has a slightlysubsolar metallicity,[Fe/H]=-0.05+/-0.03(statistical)+/-0.08(systematic) and an age of1.04+/-0.02+/-0.09 Gyr. In contrast to previous studies, we find noevidence of significant differential reddening. We determine anextinction of AV=0.68+/-0.02+/-0.09 mag and a distancemodulus of (m-M)0=12.27+/-0.02+/-0.12 mag, which correspondsto a distance of 2.8+/-0.2kpc. We derive a logarithmic mass functionslope for the cluster of α=-3.12+/-0.27, where a Salpeter slope isα=-1.35. Fits to the radial surface density profile yield a coreradius of rc=3.10m+/-0.52m(2.57+/-0.47pc). NGC 1245is highly relaxed and contains a strongly mass-segregated population.The mass function for the inner cluster has a very shallow slope,α=-0.56+/-0.28. In contrast, the outer periphery of the cluster isenriched with low-mass members and devoid of high-mass members out tothe tidal radius, rt=20' (16.5 pc). Based on the observedsurface density profile and an extrapolated mass function, we derive atotal cluster mass, M=1300+/-90+/-170Msolar.
| The age of the oldest Open Clusters
We determine ages of 71 old Open Clusters by a two-step method: we usemain-squence fitting to 10 selected clusters, in order to obtain theirdistances, and derive their ages from comparison with our own isochronesused before for Globular Clusters. We then calibrate the morphologicalage indicator δ(V), which can be obtained for all remainingclusters, in terms of age and metallicity. Particular care is taken toensure consistency in the whole procedure. The resulting Open Clusterages connect well to our previous Globular Cluster results. From theOpen Cluster sample, as well as from the combined sample, questionsregarding the formation process of Galactic components are addressed.The age of the oldest open clusters (NGC 6791 and Be 17) is of the orderof 10 Gyr. We determine a delay by 2.0±1.5 Gyr between the startof the halo and thin disk formation, whereas thin and thick disk startedto form approximately at the same time. We do not find any significantage-metallicity relationship for the open cluster sample. The cumulativeage distribution of the whole open cluster sample shows a moderatelysignificant (˜ 2σ level) departure from the predictions foran exponentially declining dissolution rate with timescale of 2.5 Gyr.The cumulative age distribution does not show any trend withgalactocentric distance, but the clusters with larger height to theGalactic plane have an excess of objects between 2-4 and 6 Gyr withrespect to their counterpart closer to the plane of the Galaxy.
| Analysing the database for stars in open clusters. I. General methods and description of the data
We present an overview and statistical analysis of the data included inWEBDA. This database includes valuable information such as coordinates,rectangular positions, proper motions, photometric as well asspectroscopic data, radial and rotational velocities for objects of openclusters in our Milky Way. It also contains miscellaneous types of datalike membership probabilities, orbital elements of spectroscopicbinaries and periods of variability for different kinds of variablestars. Our final goal is to derive astrophysical parameters (reddening,distance and age) of open clusters based on the major photometric systemwhich will be presented in a follow-up paper. For this purpose we havechosen the Johnson UBV, Cousins VRI and Strömgren uvbybetaphotometric systems for a statistical analysis of published data setsincluded in WEBDA. Our final list contains photographic, photoelectricand CCD data for 469 820 objects in 573 open clusters. We have checkedthe internal (data sets within one photometric system and the samedetector technique) and external (different detector technique) accuracyand conclude that more than 97% of all investigated data exhibit asufficient accuracy for our analysis. The way of weighting and averagingthe data is described. In addition, we have compiled a list of deviatingmeasurements which is available to the community through WEBDA.
| The relatively young and metal-poor Galactic open cluster NGC 2194
We present CCD VIKC photometry down to V~ 21 mag in the fieldof the rich open cluster NGC 2194, which is projected towards theGalactic anticentre direction. We measured V magnitudes and V-I coloursfor a total of 2515 stars in a field of 13.6 × 13.6arcmin2. These data are supplemented with CCD photometry inthe C, M and T1 filters of the Washington system andphotoelectric CMT1T2 photometry of 20 red giantcandidates. Based on the best fits of isochrones computed by the Genevaand Padova groups to the (V, V-I) colour-magnitude diagram, we derive acolour excess E(V-I) = 0.75, a cluster distance of 3.2 kpc and an age of400 Myr. Five independent Washington abundance indices yield a meancluster metallicity of [Fe/H]=-0.27 +/- 0.06, which is compatible withthe existence of both a radial and Z gradient in the Galactic disc. NGC2194 is a relatively young open cluster, whose deficiency in metalcontent can be accounted for by the fact that it was born not only farfrom the Galactic centre in the outer disc, but also at a relativelyhigh Z value.
| 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.
| NGC 1245 - an intermediate age open cluster.
The B,V CCD photometry of 916 stars in the field of the high galacticlatitude intermediate age open cluster NGC 1245 is presented. Thecluster parameters were estimated with the help of V vs (B-V)colour-magnitude diagram (CMD). After correcting for intra-clusterreddening, the mean reddening towards the cluster was found to be0.29$\pm$0.05 mag, with a distance modulus of 12.4$\pm$0.3 mag. Thecluster is found to be located at a distance of 3 Kpc. The isochronefits using Girardi et al. (2000) isochrones to the cluster CMD estimatedan age of 890$\pm$100 Myr. The synthetic CMDs obtained using Girardi etal. (2000) models incorporating photometric errors and 10% binary starsestimated an age of 1 Gyr for the cluster. The luminosity function (LF)of the main-sequence shows dips, which might arise due to some knowngaps in the main-sequence, including the Bohm-Vitense gap. The LF andthe integrated LF computed from the observed CMD is compared with thosecomputed from the sythetic CMDs for five values of mass function slope.The estimation of the slope of the present day mass function indicates aflatter value compared to the Salpeter value, though an accurateestimation is not possible due to the large dips found in the LF. Theapparent paucity of stars seen near the cluster center is estimated tohave a statistical significance of 2 sigma.
| Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS): The Frequency of Planets in NGC 1245
We present first results from the Survey for Transiting ExtrasolarPlanets in Stellar Systems (STEPSS). Our goal is to assess the frequencyof close-in extrasolar planets around main-sequence stars in severalopen clusters. By concentrating on main-sequence stars in clusters ofknown (and varied) age, metallicity, and stellar density, we will gaininsight into how these various properties affect planet formation,migration, and survival. We show preliminary results from our 19 nightphotometric campaign of the old, solar metallicity cluster NGC 1245.Taking into account the photometric precision, observational windowfunction, transit probability, and total number of cluster membersmonitored, if no transits are detected, we can rule out > 5% ofmain-sequence stars in NGC 1245 have Jupiter-sized companions withseparations of a < 0.09 AU. If 1% of the stars in the cluster haveJupiter-sized companions evenly distributed in log(a) between 0.03 and0.3 AU, we expect to find about 2 transits. A preliminary search of ourlight curve data has revealed a transit with a depth of about 4%. Basedon its shape, it is likely to be a grazing binary eclipse rather than aplanetary transit, emphasizing the need for high temporal resolution intransit surveys.
| 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.
| The intermediate-age open cluster NGC 2112
We report on BVI charge-coupled device photometry of a field centred onthe region of the intermediate-age open cluster NGC 2112 down to V= 21.Owing to the smaller field coverage, we are able to limit the effect offield star contamination that, in the past, hampered precisedeterminations of the cluster age and distance. In this way, we provideupdated estimates of the fundamental parameters for NGC 2112. Havingextended the photometry to the I passband, we are able to construct acolour-colour diagram, from which we infer a reddening E(B-V) = 0.63 +/-0.14 mag. The comparison of the colour-magnitude diagram withtheoretical isochrones leads to a distance of 850 +/- 100 pc and an ageof 2.0 +/- 0.3 Gyr. While the distance is in agreement with previousdeterminations, the age turns out to be much better constrained andsignificantly lower than previous estimates.
| 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 intermediate-age open cluster NGC 2158
We report on UBVRI CCD photometry of two overlapping fields in theregion of the intermediate-age open cluster NGC 2158 down to V =21. Byanalysing colour-colour (CC) and colour-magnitude diagrams (CMDs) weinfer a reddening E (B -V )=0.55+/-0.10, a distance of 3600+/-400pc, andan age of about 2Gyr. Synthetic CMDs constructed with these parameters(but fixing E (B -V )=0.60 and [Fe/H]=-0.60), and including binaries,field contamination and photometric errors, yield a good description ofthe observed CMD. The elongated shape of the clump of red giants in theCMD is interpreted as resulting from a differential reddening of aboutΔE (B -V )=0.06 across the cluster, in the direction perpendicularto the Galactic plane. NGC 2158 turns out to be an intermediate-age opencluster with an anomalously low metal content. The combination of theseparameters, together with the analysis of the cluster orbit, suggeststhat the cluster belongs to the old thin disc population.
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
| 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.
| 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].
| 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
| 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.
| The STACC Open Cluster Target List
Observations of variable stars offer a potential to test stellarstructure and evolution. The observations can be either of single,isolated stars, or of variable stars in clusters. The STACC group(Frandsen 1992) has for the last several years searched for openclusters with a population of delta Scuti stars. To make this searchmore efficient, we have produced a target list with a number ofpromising open clusters. The list includes parameters, finding charts,Colour-Magnitude diagrams (CM diagrams) and references for the clusters.This target list is presented here, and is thus made available toobservers interested in participating in the search for variable starsin open clusters. In this paper we describe the motivation, contents anduse of the STACC Open Cluster Target List. We also give some guidelineson how to make CCD observations of open clusters in order to search forvariable stars.
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