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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.
| 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.
| 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.
| Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627
Charge-coupled device (CCD) photometry in the Johnson V, Kron-Cousins Iand Washington CMT1 systems is presented in the field of thepoorly known open cluster NGC 2627. Four independent Washingtonabundance indices yield a mean cluster metallicity of [Fe/H]=-0.12 +/-0.08, which is compatible with the existence of a radial gradient in theGalactic disc. The resultant colour-magnitude diagrams indicate that thecluster is an intermediate-age object of 1.4 Gyr. Based on the best fitsof the Geneva group's isochrones to the (V, V-I) and (T1,C-T1) diagrams, we estimate E(V-I) = 0.25 +/- 0.05 andV-MV= 11.80 +/- 0.25 for logt= 9.15, and E(C-T1) =0.23 +/- 0.07 and T1-MT1= 11.85 +/-0.25 for logt= 9.10, respectively, assuming solar metal content. Thederived reddening value E(C-T1) implies E(B-V) = 0.12 +/-0.07 and a distance from the Sun of 2.0 +/- 0.4 kpc. Using the WEBDAdata base and the available literature, we re-examined the overallproperties of all the open clusters with ages between 0.6 and 2.5 Gyr.We identified peaks of cluster formation at 0.7-0.8, 1.0-1.1, 1.6-1.7and 2.0-2.1 Gyr, separated by relative quiescent epochs of ~0.2-0.3 Gyr.We also estimated a radial abundance gradient of -0.08 +/- 0.02, whichis consistent with the most recent determinations for the Galactic disc,but no clear evidence for a gradient perpendicular to the Galactic planeis found.
| 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.
| 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.
| 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.
| A multicolour CCD photometric and mass function study of the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253
We derive cluster parameters and mass functions from new UBVRI CCDphotometric observations of ~3500 stars reaching down to V~20mag for thedistant southern open star clusters NGC 3105, NGC 3603, Melotte 105,Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCDdata are presented for the first time. The reddening is non-uniformacross the face of the young (age <300Myr) clusters NGC 3105, NGC3603, Melotte 105, Hogg 15 and Pismis 20, with average values ofE(B-V)=1.06, 1.44, 0.52, 1.15 and 1.20mag respectively, while it isuniform with average values of E(B-V)=0.72 and 0.20mag for the olderclusters NGC 4815 and 6253 respectively. The values of colour excessratios indicate the presence of normal interstellar reddening across thecluster regions studied here. Well-defined main sequences can be seen inall the clusters. However, main-sequence turn-off points and subgiantbranches are well defined only in the older clusters NGC 4815 and 6253.The distances to the clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15,NGC 4815, Pismis 20 and NGC 6253 are 9.5+/-1.5, 7.2+/-1.2, 2.3+/-0.2,3.0+/-0.3, 2.75+/-0.2, 3.55+/-0.35 and 1.8+/-0.12kpc respectively, whilethe corresponding ages derived using theoretical convective coreovershooting stellar evolutionary isochrones are 25+/-10, 3+/-2,250+/-30, 6+/-2, 400+/-50, 6+/-2 and 2500+/-600Myr respectively. In themass range ~1-75Msolar, the mass functions of all clustersexcept for NGC 6253 have been studied. The slopes of their mass spectraagree within errors with the Salpeter value (1.35). The slope of themass function for stars more massive than 10Msolar is almostthe same as for the lower mass stars. The mass function slopes ofclusters younger than 500Myr seem to have no dependence on Galacticlongitude, Galactocentric distance and cluster age. As the inherentuncertainties in the mass function determinations of young Galactic starclusters can produce internal scatter that is larger than the externalscatter, we conclude that, above 1Msolar, the initial massfunction is universal with a slope of Salpeter value.
| Structure and mass function of five intermediate/old open clusters
The UBVRI data of five northern open star clusters are used to study themass function (MF), structure and dynamical state of these clusters. Theclusters under discussion have ages ranging from ~ 0.6 to 5 Gyr. Acomparison of the density profiles indicates that the evolution of thecore of these clusters is almost the same, whereas the corona of theclusters are probably affected by the external environment and dynamicalevolution. For the entire cluster region, the slope of the MF of threeclusters (Be 64, Be 69 and King 5) has a value that agrees within theerror with the Salpeter value, whereas King 7 and Be 20 show a steeper(Gamma = -2.02+/- 0.24) and almost a flat MF respectively. We find thatthe slope of the MF of two clusters (King 5 and King 7) changessignificantly from the inner region to the outer region, becomingsteeper at larger radii. The dynamical relaxation time of the clustersunder discussion is less than the age of the clusters, which indicatesthat all of these clusters are dynamically relaxed. Thus the observedmass segregation in three clusters can be attributed to the dynamicalevolution of the clusters. The ratio of the clusters' present radius tothe limiting radius (determined from the relation given by King\cite{King62}) can be representated by an exponent law.
| 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].
| A study of the old galactic star cluster Berkeley 32
We present new CCD photometry of the distant old open star clusterBerkeley 32 in Johnson V and Cousins I passbands. A total of ~ 3200stars have been observed in a field of about 13? x 13?. Thecolour-magnitude diagram (CMD) in V, (V - I) has been generated down toV = 22 mag. A broad but well defined main sequence is clearly visible.Some blue stragglers, a well developed subgiant branch and a Red Clumpare also seen. By fitting isochrones to this CMD as well as to otherCMDs available in the literature, and using the Red Clump location, thereddening, distance and age of the star cluster have been determined.The cluster has a distance of ~ 3.3 kpc, its radius is about 2.4 pc; thereddening E(B-V) is 0.08 mag and the age is ~ 6.3 Gyr. By comparisonwith theoretical isochrones, a metallicity of [Fe/H] ??-0.2 dex has beenestimated. Theoretical isochrones have been used to convert the observed clusterluminosity function into a mass function in the mass range ~ 0.6-1.1 M?.We find a much flatter mass function than what has been found for youngclusters. If the mass function is a power law dN ~ m?dm, then we get ??=-0.5 ± 0.3. This may be seen as a signature of the highly evolveddynamical state of the cluster.
| 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.
| Mass functions of five distant northern open star clusters
We analyse BVI CCD data of five northern open star clusters in theGalaxy in order to determine their mass functions. The clusters areBerkeley 81, Berkeley 99, NGC 6603, NGC 7044 and NGC 7510. They aredistant (>= 3kpc) and compact (radius <=2.8 arcmin) objects.Except for NGC 7510 whose age is 10 Myr, all are intermediate-age andold star clusters with ages between 0.5 and 3.2 Gyr. The observedcluster luminosity function (LF) is corrected for both dataincompleteness and field star contamination. Theoretical stellarevolutionary isochrones are used to convert LFs into mass functions(MFs). The MF slope becomes flatter if a correction for dataincompleteness is not applied, while it becomes steeper if a correctionfor field star contamination is ignored; however, both correctionsincrease with decreasing stellar brightness. In the mass range ~1-14Msolar, the MF slope of NGC 7510 is 1.1+/-0.2. As the cluster is notdynamically evolved, its present-day MF can be considered as the initialMF. It is not too different from the Salpeter value (x=1.35). In anarrow mass range ~0.6-2 Msolar, the values of the MF slope in the fourintermediate-age and old clusters range from 0.3 to 2.5 and differsignificantly from each other. For Berkeley 99 and NGC 6603, the MFslopes are 1.4+/-0.6 and 1.1+/-0.4 respectively. They agree with theSalpeter value within the errors. However, significantly differentvalues of MF slopes, 2.5+/-0.2 and 0.3+/-0.2, are found for Berkeley 81and NGC 7044 respectively. We therefore conclude that the MF does varyamong this cluster sample. The effects of mass segregation are observedin all the four intermediate-age and old clusters; this segregation ismost probably due to dynamical evolution, as the ages of the clustersare much longer than the corresponding dynamical relaxation times. Thereis no obvious dependence of the MF slope on either Galactocentricdistance or age of the well-studied Galactic open clusters. Except forsome of the dynamically evolved older (age >=50 Myr) clusters, the MFslopes of the clusters are not too different from the Salpeter value.
| BVI CCD photometry of the distant open star clusters Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044
We present CCD observations for the distant northern open star clustersBerkeley 81, Berkeley 99, NGC 6603 and NGC 7044 in BVI photometricpassbands. A total of 9900 stars have been observed in fields of about6x6 arcmin^2 of the sky around the clusters. Colour-magnitude diagramsin V, (B-V) and V, (V-I) have been generated down to V=22 mag and, forthe first time, such diagrams have been produced for the clustersBerkeley 81 and Berkeley 99. The data serve as a base for the study ofmass functions and for comparison with theoretical models. Analysis ofthe radial distribution of stellar surface density indicates that theradius values for Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044 are2.7, 2.8, 2.8 and 2.2 arcmin respectively. By fitting the latestconvective core overshooting isochrones to the colour-magnitude diagramand using its morphological features, reddenings, distances and ages ofthe star clusters have been determined. Broad but well-defined mainsequences with stellar evolutionary effects in the brighter stars areclearly visible in colour-magnitude diagrams of all the clusters understudy. Some blue stragglers along with well-developed giant branches andred giant clumps are also clearly seen in all of them. The clustersstudied here are located at a distance of ~3 kpc, except for Berkeley 99which is located at a distance of 4.9 kpc. Their linear sizes liebetween 3.8 and 8.0 pc; E(B-V) values range from 0.3 to 1.0 mag, whiletheir ages are between 0.5 and 3.2 Gyr. Thus the star clusters studiedhere are of intermediate and high age but are compact and distantobjects.
| 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.
| Physical parameters of the intermediate age open cluster IC 1311: Clues for the theory of stellar evolution
The results of a UBVR photometric study in the field of the open clusterIC 1311 are presented. A comparison with previously published results onNGC 7044, based on measurements secured during the same observing run,leads to a self-consistent constraints to the parameters of the cluster.The location of selected samples of giant and unevolved dwarf members inthe U-B vs B-V diagram is used to simultaneously compute the colorexcess E(B-V) and the metallicity (Fe/H). Distance modulus and age areestimated by the Zero Age Main Sequence (ZAMS) fitting method, and bythe use of theoretical isochrones, based on models computed with andwithout the consideration of convective overshooting during the phasesof core nuclear burning. The adopted set of values is: E(B-V) = 0.28,(Fe/H) = 0.0, DM = 13.9, Age = 1.6 x 109 yr. The massfunction for the cluster main sequence has been estimated, with a slopex = -2.58, significantly steeper than the Salpeter initial mass function(IMF) (-1.35). The uncertainties in the adopted values for the clusterparameters, due to interstellar reddening, and to the calibrations oftheoretical luminosity and effective temperature in terms ofobservational quantities, are also addressed. The discrepancies betweenthe observational color-magnitude (CM) and the model isochrones, inparticular the difference between observed and predicted luminosity ofthe red giant clump (RGC), are sensibly reduced for the models withconvective core overshooting. The results indeed suggest the need ofincreasing the overshooting effectiveness beyond the values consideredup to now in evolutionary computations.
| 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).
| The Galactic system of old open clusters: age calibration and age-metallicity relation
In this paper, we present a new homogeneous compilation of ages for thesystem of intermediate age and old open clusters of the Galaxy, and theaccompanying age-metallicity relation (AMR). This study stands on theanalysis by Carraro et al. (1991, 1993a,b), who have obtained goodestimates for the color excess, distance modulus, and age for a sampleof ten clusters, for which modern color-magnitude diagrams (CMD) of goodphotometric quality and spectroscopic data on the metallicity (Friel& Janes 1991, 1993) were available. Firstly, we revise the resultsby Carraro et al. (1991, 1993a,b) to take into account recentdevelopments in the libraries of stellar models (Alongi et al. 1993;Bressan et al. 1993; Fagotto et al. 1993), and secondly we presentuseful age calibrations based on the correlation between metallicity,age, and magnitude difference between the turn-off and red clumpluminosities. The age calibration does not depend on the color excess,distance modulus, but only weakly on the metallicity. With the aid ofthe new age calibration, we assign the age to a more numerous sample ofclusters. The resulting ages span the range from 0.5x10^9^yr for NGC5822 to 8.0x10^9^yr for NGC 6791. With such a compilation, and adoptingan homogeneous source for the metal content, we propose a new AMR forthe family of Galactic open clusters. The AMR is also corrected for theeffect of the gradient in metallicity across the Galactic Disk. Althoughat any given age the spread in metallicity is high, the AMR togetherwith the distribution of clusters with different age and metallicityacross the Galactic Plane, confirms previous suggestions that themetallicity of a cluster is more related to the position than the age.
| 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.
| The intermediate age open cluster NGC 7044
A photometric analysis is conducted of the open cluster NGC 7044, on thebasis of UBVR CCD observations of 896 stars in the cluster field.Reddening and metallicity estimates are highly dependent on whetherreddening slopes and absorption coefficients vary with spectral type.The color-magnitude diagram obtained is compared with selected sets ofisochones, based on evolutionary models computed with and withoutconsideration of convective overshooting from the stellar core duringthe H and He phases of core burning. Better general agreement isobtained between predicted and observed features of the color-magnitudediagram with overshooting models.
| CCD BV photometry of three unstudied open clusters - NGC 7044, King 2 and K11
Results are reported from photometric observations of three rich openclusters, obtained using CCD cameras 1 and 3 on the 1.09-m telescope atKPNO in December 1986. The data are presented in extensive tables andgraphs and characterized in detail. The approximate age, reddening, andapparent distance modulus are found to be 1.5 Gyr, 0.74, and 15.3 magfor NGC 7044 and 4-6 Gyr, 0.23-0.50, and 15.0-16.0 mag for King 2; theage and apparent distance modulus of K11 are given as 5 Gyr and 15.3mag.
| 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.
| A list of clusters that may be old.
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