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A new search for planet transits in NGC 6791
Context: Searching for planets in open clusters allows us to study theeffects of dynamical environment on planet formation and evolution. Aims: Considering the strong dependence of planet frequency on stellarmetallicity, we studied the metal rich old open cluster NGC6791 and searched for close-in planets using the transittechnique. Methods: A ten-night observational campaign was performedusing the Canada-France-Hawaii Telescope (3.6 m), the San PedroMártir telescope (2.1 m), and the Loiano telescope (1.5 m). Toincrease the transit detection probability we also made use of theBruntt et al. (2003, A&A, 410, 323) eight-nights observationalcampaign. Adequate photometric precision for the detection of planetarytransits was achieved. Results: Should the frequency and properties ofclose-in planets in NGC 6791 be similar to thoseorbiting field stars of similar metallicity, then detailed simulationsforesee the presence of 2-3 transiting planets. Instead, we do notconfirm the transit candidates proposed by Bruntt et al. (2003, A&A,410, 323). The probability that the null detection is simply due tochance coincidence is estimated to be 3%-10%, depending on themetallicity assumed for the cluster. Conclusions: Possible explanationsof the null-detection of transits include: (i) a lower frequency ofclose-in planets in star clusters; (ii) a smaller planetary radius forplanets orbiting super metal rich stars; or (iii) limitations in thebasic assumptions. More extensive photometry with 3-4 m class telescopesis required to allow conclusive inferences about the frequency ofplanets in NGC 6791.Based on observation obtained at theCanada-France-Hawaii Telescope (CFHT) which isoperated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the CentreNational de la Recherche Scientifique of France, and theUniversity of Hawaii and on observations obtained at San PedroMártir 2.1 m telescope (Mexico),and Loiano 1.5 m telescope (Italy).
| 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.
| 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.
| 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.
| New galactic open cluster candidates from DSS and 2MASS imagery
An inspection of the DSS and 2MASS images of selected Milky Way regionshas led to the discovery of 66 stellar groupings whose morphologies,color-magnitude diagrams, and stellar density distributions suggest thatthese objects are possible open clusters that do not yet appear to belisted in any catalogue. For 24 of these groupings, which we consider tobe the most likely to be candidates, we provide extensive descriptionson the basis of 2MASS photometry and their visual impression on DSS and2MASS. Of these cluster candidates, 9 have fundamental parametersdetermined by fitting the color-magnitude diagrams with solarmetallicity Padova isochrones. An additional 10 cluster candidates havedistance moduli and reddenings derived from K magnitudes and (J-K) colorindices of helium-burning red clump stars. As an addendum, we alsoprovide a list of a number of apparently unknown galactic andextragalactic objects that were also discovered during the survey.
| Planets in Stellar Clusters Extensive Search. IV. A Detection of a Possible Transiting Planet Candidate in the Open Cluster NGC 2158
We have undertaken a long-term project, Planets in Stellar ClustersExtensive Search, to search for transiting planets in open clusters. Inthis paper we present the results for NGC 2158, an intermediate-age,populous cluster. We have monitored the cluster for over 260 hours,spread over 59 nights. We have detected one candidate transitinglow-luminosity object, with an eclipse depth of 3.7% in the R band. Ifthe host star is a member of the cluster, the eclipse depth isconsistent with a 1.7RJ object. Cluster membership of thehost is supported by its location on the cluster main sequence and itsclose proximity to the cluster center (2'). We have discovered two otherstars exhibiting low-amplitude (4%-5%) transits, V64 and V70, but theyare most likely blends or field stars. Given the photometric precisionand temporal coverage of our observations and the current best estimatesfor the frequency and radii of short-period planets, the expected numberof detectable transiting planets in our sample is 0.13. We have observedfour outbursts for the candidate cataclysmic variable V57. We havediscovered 40 new variable stars in the cluster, bringing the totalnumber of identified variables to 97, and present for themhigh-precision light curves, spanning 13 months.Based on data from the Fred Lawrence Whipple Observatory 1.2 mtelescope.
| 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.
| The young open cluster NGC 2129
The first charge-coupled device UBV(RI)C photometric study inthe area of the doubtful open cluster NGC 2129 is presented. Photometryof a field offset 15 arcmin northwards is also provided, to probe theGalactic disc population towards the cluster. Using star counts, propermotions from the UCAC2 catalogue, colour-magnitude and colour-colourdiagrams, we demonstrate that NGC 2129 is a young open cluster. Thecluster radius is 2.5 arcmin, and across this region we find evidence ofsignificant differential reddening, although the reddening law seems tobe normal towards its direction. Updated estimates of the clusterfundamental parameters are provided. The mean reddening is found to beE(B-V) = 0.80 +/- 0.08 and the distance modulus is (m-M)0=11.70 +/- 0.30. Hence, NGC 2129 is located at 2.2 +/- 0.2 kpc from theSun inside the Local spiral arm. The age derived from 37 photometricallyselected members is estimated to be approximately 10 Myr. These starsare used to provide new estimates of the cluster absolute proper-motioncomponents.
| 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.
| Discovery of 13 New Variable Stars in the Field of the Open Cluster NGC 2168 (M35)
A wide-field time-series CCD photometric survey of variable stars in thefield of the open cluster NGC 2168 was carried out using the BATCSchmidt telescope. In total 13 new variable stars are discovered withthree W UMa systems, one EA type and two EB type eclipsing binaries (oneof them could be a W UMa system), and seven pulsating stars includingthree candidates of δ Scuti stars.
| Metal Abundances in Extremely Distant Galactic Old Open Clusters. II. Berkeley 22 and Berkeley 66
We report on high-resolution spectroscopy of four giant stars in theGalactic old open clusters Berkeley 22 and Berkeley 66 obtained withHIRES at the Keck telescope. We find that [Fe/H]=-0.32+/-0.19 and-0.48+/-0.24 for Be 22 and Be 66, respectively. Based on these data, wefirst revise the fundamental parameters of the clusters and then discussthem in the context of the Galactic disk radial abundance gradient. Wefound that both clusters nicely obey the most updated estimate of theslope of the gradient from the work of Friel and coworkers and aregenuine Galactic disk objects.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Planets in Stellar Clusters Extensive Search. III. A Search for Transiting Planets in the Metal-rich Open Cluster NGC 6791
We have undertaken a long-term project, Planets in Stellar ClustersExtensive Search (PISCES), to search for transiting planets in openclusters. In this paper we present the results for NGC 6791, a very old,populous, metal-rich cluster. We have monitored the cluster for over 300hr, spread over 84 nights. We have not detected any good transitingplanet candidates. Given the photometric precision and temporal coverageof our observations and the current best estimates for the frequency andradii of short-period planets, the expected number of detectabletransiting planets in our sample is 1.5. We have discovered 14 newvariable stars in the cluster, most of which are eclipsing binaries, andpresent high-precision light curves spanning 2 years for these newvariables and also the previously known variables.Based on data from the Fred Lawrence Whipple Observatory 1.2 mtelescope.
| Star counts in the Galaxy. Simulating from very deep to very shallow photometric surveys with the TRILEGAL code
We describe TRILEGAL, a new populations synthesis code for simulatingthe stellar photometry of any Galaxy field. The code attempts to improveupon several technical aspects of star count models, by: dealing withvery complete input libraries of evolutionary tracks; using a stellarspectral library to simulate the photometry in virtually any broad-bandsystem; being very versatile allowing easy changes in the inputlibraries and in the description of all of its ingredients - like thestar formation rate, age-metallicity relation, initial mass function,and geometry of Galaxy components. In a previous paper (Groenewegen etal. 2002, Paper I), the code was first applied to describe the very deepstar counts of the CDFS stellar catalogue. Here, we briefly describe itsinitial calibration using EIS-deep and DMS star counts, which, as weshow, are adequate samples to probe both the halo and the disccomponents of largest scale heights (oldest ages). We then present thechanges in the calibration that were necessary to cope with someimprovements in the model input data, and the use of more extensivephotometry datasets: now the code is shown to successfully simulate alsothe relatively shallower 2MASS catalogue, which probes mostly the discat intermediate ages, and the immediate solar neighbourhood as sampledby Hipparcos - in particular its absolute magnitude versus colourdiagram -, which contains a somewhat larger fraction of younger starsthan deeper surveys. Remarkably, the same model calibration canreproduce well the star counts in all the above-mentioned data sets,that span from the very deep magnitudes of CDFS (16
| Cluster Ages Experiment (CASE): Detection of a dwarf nova in the globular cluster M55
We report the detection of a dwarf nova (DN) in the core region of theglobular cluster M55. Six outbursts were observed during eight observingseasons spanning the period 1997-2004. The variable has an X-raycounterpart detected on images taken with the ROSAT telescope. Althoughwe cannot offer proof of cluster membership, one can see that both theposition on the Hertzsprung-Russell diagram and the X-ray flux areconsistent with a bright DN at the cluster distance. According to ouroutburst statistics, no more than one similar DN could remain undetectedin our field of view, centred at the cluster core.
| 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.
| Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic openclusters are studies in this work, in which 27 Galactic open clusters ofages >=1 Gyr are selected as the working sample. Based on thephotometric observations of these open clusters, a synthetic integratedspectrum has been made for the stellar population of each cluster. Theeffects of blue straggler (BS) stars on the conventional simple stellarpopulation (SSP) model are analyzed on an individual cluster basis. Itis shown that the BSs, whose positions in the color-magnitude diagramscannot be predicted by the current single-star evolution theory, requiresignificant modifications to the integrated properties of theoreticalSSP model. The synthesized integrated spectral energy distributions(ISEDs) of our sample clusters are dramatically different from those ofSSPs based on an isochrone only. The BS-corrected ISEDs of stellarpopulations show systematic enhancements toward shorter wavelengths inthe spectra. When measured with broadband colors in unresolvableconditions, the age of a stellar population can be seriouslyunderestimated by the conventional SSP model. Therefore, considering thecommon existence of BS components in real stellar populations, we shouldexpect considerable alterations of the conventional ISEDs when we applythe technique of evolutionary population synthesis to more complicatedstellar systems.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| 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.
| HATNET Variability Survey in the High Stellar Density ``Kepler Field'' with Millimagnitude Image Subtraction Photometry
The Hungarian-made Automated Telescope Network (HATnet) is an ongoingproject to detect transiting extrasolar planets using small-aperture (11cm diameter) robotic telescopes. In this paper, we present the resultsfrom using image subtraction photometry to reduce a crowded stellarfield observed with one of the HATnet telescopes (HAT-5). This field waschosen to overlap with the planned Kepler mission. We obtained I-bandlight curves for 98,000 objects in a 67 square degree field of viewcentered at J2000 (α,δ) =(19h44m00s0, +37°32'00.0"), near theGalactic plane in the constellations Cygnus and Lyra. These observationsinclude 788 exposures of 5 minutes' length over 30 days. For thebrightest stars (I~8.0) we achieved a precision of 3.5 mmag, falling to0.1 mag at the faint end (I~14). From these light curves we identify1617 variable stars, of which 1439 are newly discovered. The fact thatnearly 90% of the variables were previously undetected furtherdemonstrates the vast number of variables yet to be discovered evenamong fairly bright stars in our Galaxy. We also discuss some of themost interesting cases. These include V1171 Cyg, a triple system withthe inner two stars in a P=1.462 day period eclipsing orbit and theouter star a P=4.86 day Cepheid; HD 227269, an eccentric eclipsingsystem with a P=4.86 day period that also shows P=2.907 day pulsations;WW Cyg, a well-studied eclipsing binary; V482 Cyg, an R Coronae Borealisstar; and V546 Cyg, a PV Telescopii variable. We also detect a number ofsmall-amplitude variables, in some cases with full amplitude as low as10 mmag.
| 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.
| Planets in Stellar Clusters Extensive Search. II. Discovery of 57 Variables in the Cluster NGC 2158 with Millimagnitude Image Subtraction Photometry
We have undertaken a long-term project, Planets in Stellar ClustersExtensive Search (PISCES), to search for transiting planets in openclusters. NGC 2158 is one of the targets we have chosen-an intermediateage, populous, rather metal-poor cluster. In this paper we present theresults of a search for variable stars in the data from the first seasonof monitoring at the FLWO 1.2 m telescope. This is the first variabilitysearch ever conducted in this cluster. We present a catalog of 57variable stars, most with low-amplitude variability. Among the variablesis a cataclysmic variable (CV) that underwent a 2.5 mag outburst. If itis a member of NGC 2158, this would be the fourth CV known in an opencluster. We have also found five δ Scuti stars, three of whichhave two or more detectable modes of pulsation. Of the 57 variablesdiscovered, 28 have R-band amplitudes of 5% or below. Six of those varyat or below the 2% level, including one with 0.08% variability.
| Integrated spectral energy distributions and absorption-feature indices of single stellar populations
Using evolutionary population synthesis, we present integrated spectralenergy distributions and absorption-line indices defined by the LickObservatory image dissector scanner (referred to as Lick/IDS) system,for an extensive set of instantaneous burst single stellar populations(SSPs). The ages of the SSPs are in the range 1 Gyr <=τ<= 19Gyr and the metallicities are in the range -2.3 <=[Fe/H]<=+0.2.Our models use the rapid single stellar evolution algorithm of Hurley,Pols and Tout for the stellar evolutionary tracks, the empirical andsemi-empirical calibrated BaSeL-2.0 model of Lejeune, Cuisinier andBuser for the library of stellar spectra and the empirical fittingfunctions of Worthey, Faber, Gonzalez and Burstein for the Lick/IDSspectral absorption-feature indices.Applying our synthetic Lick/IDS absorption-line indices to the meritfunction, we obtain the age and the metallicity of the central region ofM32, which can be well explained by an instantaneous SSP with an age of~6.5 Gyr and a metallicity similar to solar. Applying the derived ageand the metallicity from the merit function to a number of index-indexdiagrams, we find that the plots of Hβ-Fe5015 andHβ-Fe5782 are the best index-index diagrams from whichwe can directly obtain reasonable age and metallicity.
| 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.
| The old open clusters Saurer A, B and C revisited
We report deep (V~ 24.0) VI CCD photometry of three fields centred inthe regions of the old open clusters Saurer A, B and C. In the case ofSaurer A, which is considered one of the oldest known open clusters, wealso provide a comparison field. From the analysis of the photometry weclaim that Saurer A is as old as M67 (~5 Gyr), but more metal-poor (Z=0.008). Moreover, it turns out to be the open cluster with the largestgalactocentric distance so far detected.As for Saurer B, it closely resembles NGC 2158, and indeed is ofintermediate age (1.8-2.2 Gyr) and significantly reddened. In this casewe revise both the age and the distance with respect to previousstudies, but we are not able to establish clearly the cluster metalabundance.Finally, Saurer C has an age of about 2 Gyr, but we emphasize that theprecise determination of its properties is hampered by the heavy fieldstar contamination.All of the photometry is available at the WEBDA data base:http://obswww.unige.ch/webda/navigation.html.
| 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.
| 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.
| 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.
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Observation and Astrometry data
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