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CCD BV survey of 42 open clusters
Aims.We present results of a photometric survey whose aim was to derivestructural and astrophysical parameters for 42 open clusters. While oursample is definitively not representative of the total open clustersample in the Galaxy, it does cover a wide range of cluster parametersand is uniform enough to allow for simple statistical considerations. Methods: BV wide-field CCD photometry was obtained for open clusters forwhich photometric, structural, and dynamical evolution parameters weredetermined. The limiting and core radii were determined by analyzingradial density profiles. The ages, reddenings, and distances wereobtained from the solar metallicity isochrone fitting. The mass functionwas used to study the dynamical state of the systems, mass segregationeffect and to estimate the total mass and number of cluster members. Results: This study reports on the first determination of basicparameters for 11 out of 42 observed open clusters. The angular sizesfor the majority of the observed clusters appear to be several timeslarger than the catalogue data indicate. The core and limiting clusterradii are correlated and the latter parameter is 3.2 times larger onaverage. The limiting radius increases with the cluster's mass, and boththe limiting and core radii decrease in the course of dynamicalevolution. For dynamically not advanced clusters, the mass functionslope is similar to the universal IMF slope. For more evolved systems,the effect of evaporation of low-mass members is clearly visible. Theinitial mass segregation is present in all the observed young clusters,whereas the dynamical mass segregation appears in clusters older thanabout log({age}) = 8. Low-mass stars are deficient in the cores ofclusters older than log({age}) = 8.5 and not younger than one relaxationtime.Tables 1-5 are only available in electronic form at http://www.aanda.org
| 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
| 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.
| Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters
New photometric data in the Washington system are presented for redgiant candidates in NGC 1817 and 2251, two open clusters located towardsthe Galactic anticentre direction. In the case of NGC 2251, theWashington data are supplemented with new UBV and David DunlapObservatory (DDO) photoelectric photometry. Published radial velocitiesare used to separate field stars from cluster giants. The photometricdata yield an effective temperature and metal abundance for each clustermember. Five independent Washington abundance indices yield meanmetallicities of [Fe/H]= 0.25 +/- 0.04 for NGC 1817 and 2251,respectively. From combined BV and DDO data, we also derive E(B-V) =0.21 +/- 0.03 and [Fe/H]DDO=-0.14 +/- 0.05 for NGC 2251. Bothobjects are then found to be on the metal-poor side of the distributionof open clusters, their metallicities being compatible with theexistence of a radial abundance gradient in the disc. Using the WEBDAOpen Cluster data base and the available literature, we re-examined theoverall properties of a sample of 30 clusters located towards theGalactic anticentre with the distances, ages and metallicitiesavailable. This cluster sample presents no evidence of an abundancegradient perpendicular to the Galactic plane, nor is an age-metallicityrelation found. However, a radial abundance gradient of -0.093 dexkpc-1 is derived over a Galactocentric distance of 14 kpc, agradient which is in keeping with most recent determinations. This valuepractically does not change when all clusters with basic parametersknown up to this date are considered.
| Collaborative Research of Open Star Clusters
Preliminary results on observations of open clusters are pre-sented. Theproject has been initiated in the framework of the Uzbek-Taiwan andTaiwan-Baltic collaboration, mainly to upgrade and make use offacilities at Maidanak Observatory. We present detailed,multi-wavelength studies of the young cluster NGC 6823 and theassociated complex nebulosity, to diagnose the young stellar populationand star formation history in the region. In addition, 7 compact openclusters have been monitored for stellar variability. We show howobservations like these could feasibly be used to look for exoplanettransit events. We also expect to join the Whole-Earth Telescope effortin future campaigns for asteroseismology.
| 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.
| 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.
| 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.
| 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.
| 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].
| Luminosity and mass function of galactic open clusters I. NGC 4815
We present deep V and I photometry for the open cluster NGC 4815 andfour surrounding Galactic fields down to a limiting magnitude V ~ 25.These data are used to study cluster spatial extension by means of starcounts, and to derive the luminosity (LF) and mass function (MF). Theradius turns out to be 3.6+/-0.3 arcmin at V=19.0, whereas the mass is880+/-230 msun down to V=20.8. From the color-magnitudediagram, we obtain the LFs in the V and I bands, using both the standardhistogram and an adaptive kernel. After correction for incompletenessand field star contamination, the LFs were transformed into the presentday mass functions (PDMF). The PDMFs from the V and I photometry can berepresented as a power-law with a slope alpha = 3.1+/-0.3 and alpha =2.9+/-0.3 (the (Salpeter \cite{salp55}) MF in this notation has a slopealpha = 2.35) respectively, in the mass range 2.5 <=(m)/(msun) <= 0.8. Below this mass, the MF cannot beconsidered as representative of the cluster IMF, as it is the result ofthe combined effect of strong irregularities in the stellar background,probable internal dynamical evolution of the cluster and/or interactionof the cluster with the dense Galactic field. Unresolved binaries andmass segregation can only flatten the apparent derived IMF, so we expectthat the real IMF must be steeper than the quoted slope by an unknownamount. Based on observations made at the European Southern Observatory,La Silla, Chile.
| 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.
| UBVI charge-coupled device photometry of two old open clusters NGC 1798 and 2192
We present UBVI charge-coupled device (CCD) photometry of two openclusters NGC 1798 and 2192, which have been little studied before.Colour-magnitude diagrams of these clusters show several features thatare typical of old open clusters: a well-defined main sequence, a redgiant clump, and a small number of red giants. The main sequence of NGC1798 shows a distinct gap at V ~ 16.2 mag. From the surface numberdensity distribution we have measured the size of the clusters,obtaining 8.3 arcmin (= 10.2 pc) for NGC 1798 and 7.3 arcmin (= 7.5 pc)for NGC 2192. Then we have determined the reddening, metallicity, anddistance of these clusters using the colour-colour diagrams andcolour-magnitude diagrams: E(B-V)=0.51 +/- 0.04, [Fe/H] = -0.47 +/- 0.15dex and (m-M)_0=13.1 +/- 0.2 (d=4.2 +/- 0.3 kpc) for NGC 1798, andE(B-V)=0.20 +/- 0.03, [Fe/H] = -0.31 +/- 0.15 dex and (m-M)_0=12.7 +/-0.2 (d=3.5 +/- 0.3 kpc) for NGC 2192. The ages of these clusters havebeen estimated using the morphological age indicators and the isochronefitting with the Padova isochrones: 1.4 +/- 0.2 Gyr for NGC 1798 and 1.1+/- 0.1 Gyr for NGC 2192. The luminosity functions of the main-sequencestars in these clusters are found to be similar to other old openclusters. The metallicity and distance of these clusters are consistentwith the relation between the metallicity and the Galactocentricdistance of other old open clusters.
| A report on the studies of star clusters with the UPSO 104-cm Sampurnanand telescope during last 25 years
Not Available
| The Old Open Clusters Of The Milky Way
The Galactic open clusters, in particular the oldest members, serve asexcellent probes of the structure and evolution of the Galactic disk.Individual clusters provide excellent tests of stellar and dynamicalevolution. Cluster spatial and age distributions provide insight intothe processes of cluster formation and destruction that have allowedsubstantial numbers of old open clusters to survive.Spectroscopic andphotometric data for the old clusters yield kinematic, abundance, andage information that clarifies the relationship between the old opencluster population and other Galactic populations. New samples of oldopen clusters, which span a large range in distance and age, are used todefine disk abundance gradients and the cluster age-metallicityrelationship, and they point to a complex history of chemical enrichmentand mixing in the disk.
| The galactic system of old star clusters: The development of the galactic disk
The vast majority of open clusters persist as clusters for no more thana few hundred million years, but the few which survive for much longerperiods constitute a unique sample for probing the evolution of thegalactic disk. In a charge coupled device (CCD) photometric survey forpossible old open clusters combined with previously publishedphotometry, we have developed a list of 72 clusters of the age of theHyades or older (Phelps (1994). Using our version of parameters based onthe luminosity difference between the main sequence turnoff and thehorizontal branch and on the color difference between the turnoff andthe giant branch, we have calculated a 'Morphological Age Index' (MAI)for the clusters in our list and for a sample of globular clusters. Wefind that the MAI is well correlated with the logarithm of cluster ages,as determined by fitting to theoretical isochrones. We conclude that theindex is a good measure of the relative ages of both globular and openclusters, although uncertainties in the models and residual metallicityeffects prevent the use of the MAI as a definitive calibration of actualcluster ages. The age distribution of the open clusters overlaps that ofthe globular clusters, indicating that the galactic disk began todevelop toward the end of the period of star formation in the galactichalo. The open cluster age distribution can be fit approximately with atwo-component exponential decay function; one component can beidentified as the tail of the dominant population of thin disk openclusters with an age scale factor of 200 Myr, and the other consists oflonger-lived clusters with an age scale of 4 Gyr. The young openclusters are distributed on the galactic plane almost symmetricallyabout the Sun with a scale height perpendicular to the galactic plane of55 pc. The old population consists of rich clusters found only in theouter disk, nore than RGC = 7.5 kpc from the galactic center;this population has a scale height of 375 pc. After accounting for thetwo exponential distributions of cluster ages, there are indications ofan excess of clusters in the age range of 5-7 Gyr; there may have beenlarge bursts of star formation in that period, or perhaps a largerproportion of the clusters forming at that time had advantageous orbitsfor survival. Either circumstance is consistent with the idea that thegalactic disk has been repeatedly disturbed, possibly in collisions orother interactions with external systems, resulting in the occasionalformation of clusters with relatively large velocities perpendicular tothe plane; these are the clusters that have survived until the present.Finally, the repeated accretion of low angular momentum material ontothe disk from the halo or beyond would also explain the observed radialcomposition gradient and the lack of a correlation between open clustermetallicity and age found by Friel & Janes (1993).
| Development of the Galactic disk: A search for the oldest open clusters
In an extensive charge coupled devices (CCD) photometric survey ofpotential old open clusters, we have identified a number of systems thatare indeed old; some of them are among the oldest of the open clusters.Using our versions of two well-known morphological age indices, onebased on the luminosity difference between the main sequence turnoff andthe horizontal branch and the other on the color difference between theturnoff and the giant branch, we have ranked the open clusters inapproximate order of age. Our data together with previously publishedphotometry of other old open clusters, yields a catalogue of 72 clustersof the age of Hyades or older with 19 of the clusters as old or olderthan M67 (about 5 Gyr). Among the oldest open clusters are Be 17, Cr261, NGC 6791, Be 54, and AM 2. Be 17 and another old cluster, Lynga 7,are possibly as old as the youngest globulars. The data also suggestthat the formation rate of open clusters may have been higher early inthe history of the disk than at intermediate times since numerousclusters have survived from that time.
| Open clusters as standard candles - The age-metallicity relation and metallicity gradients
An overview is presented of a series of investigations of the use of theopen clusters as probes of the history of the galactic disk. Noindications are found of a gradient in metallicity perpendicular to thegalactic plane; along the plane, there is a very definite radialgradient. There is also no evidence for a systematic trend inmetallicity with cluster age, and it appears that the metallicity of adisk star depends entirely on its place of formation. On the basis ofthe spectra considered, it is possible to derive a metallicity that isprimarily based on Fe lines and another based on the Mg b + Mg IIfeature.
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