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Red giants in open clusters. XII. Six old open clusters NGC 2112, 2204, 2243, 2420, 2506, 2682
Aims.We studied the membership and binarity of 123 red giants in six oldopen clusters, NGC 2112, 2204, 2243, 2420, 2506 and 2682, to define moreprecisely the evolutionary path on the red-giant branch. Methods: Theanalysis is based on 185 radial-velocity observations with the Coravelspectrographs and available photometric data. Results: The membershipof 93 red giants was confirmed on the basis of the radial velocities.Seven definitive spectroscopic binaries were identified and 11additional stars are suspected of being binaries. The binary frequency(19%) is slightly lower than average. This is partly due to the smallnumber of observations secured for each star. Orbital elements have beendetermined for the first time for the BaII star NGC 2420-173 (D) andthose of the other BaII star NGC 2420-250 (X) have been improved. Thevalues of the cluster mean velocities have been significantly improved. Conclusions: With the new membership estimates and binary detections,the existing CCD data allow precise definition of the red-giant loci. Anumber of stars in NGC 2506, 2420 and 2204 appear to define anasymptotic branch, the position of which differs significantly from thatpredicted by the models.Based on observations collected with the Danish 1.54-m and ESO 1-mtelescopes at the European Southern Observatory, La Silla, Chile.
| Chemical Homogeneity in Collinder 261 and Implications for Chemical Tagging
This paper presents abundances for 12 red giants of the old open clusterCollinder 261 based on spectra from the Very Large Telescope UVES.Abundances were derived for Na, Mg, Si, Ca, Mn, Fe, Ni, Zr, and Ba. Wefind that the cluster has a solar-level metallicity of [Fe/H]=-0.03 dex.However, most α- and s-process elements were found to be enhanced.The star-to-star scatter was consistent with the expected measurementuncertainty for all elements. The observed rms scatter is as follows:Na=0.07, Mg=0.05, Si=0.06, Ca=0.05, Mn=0.03, Fe=0.02, Ni=0.04, Zr=0.12,and Ba=0.03 dex. The intrinsic scatter was estimated to be less than0.05 dex. Such high levels of homogeneity indicate that chemicalinformation remains preserved in this old open cluster. We use thechemical homogeneity we have now established in Cr 261, the Hyades, andthe HR 1614 moving group to examine the uniqueness of the individualcluster abundance patterns, i.e., chemical signatures. We demonstratethat the three studied clusters have unique chemical signatures anddiscuss how other such signatures may be searched for in the future. Ourfindings support the prospect of chemically tagging disk stars to commonformation sites in order to unravel the dissipative history of theGalactic disk.Based on observations collected during ESO VLT-UT2 Program 73.D-0716A atthe European Southern Observatory, Paranal, Chile.
| K-band magnitude of the red clump as a distance indicator
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
| Eclipsing Binaries in the Open Cluster NGC2243. II. Absolute Properties of NVCMa
We present echelle spectroscopic data for five eclipsing binary starsand two giant stars in the field of the open cluster NGC 2243. Theaverage cluster velocity is determined to be +60.4+/-0.6 km/s. Four ofthe eclipsing binaries are very likely members of the cluster based ontheir observed radial velocities. The absolute parameters of clustermember NV CMa are determined by analyzing photometric and radialvelocity data. We obtain 1.089+/-0.010 M_odot and 1.221+/-0.031 R_odotfor the primary, and 1.069+/-0.010 M_odot and 1.178+/-0.037 R_odot forthe secondary. Both components of the binary are located on the MainSequence, about 1 mag below the turn-off point on the clustercolor-magnitude diagram. Using model age-luminosity and age-radiusrelations we obtain 4.35+/-0.25 Gyr for the age of NV CMa. The derivedage is, however, very sensitive to the adopted metallicity of thecluster. We demonstrate that a meaningful determination of the ages ofobjects like NV CMa based on evolutionary models is possible only iftheir metallicity is known with a relative accuracy of a few percent.The distance moduli calculated for the components of NV CMa agreeclosely with each other, and imply an apparent distance modulus of thecluster of (m-M)_V=13.24+/-0.08 mag.
| uvbyCaHβ CCD Photometry of Clusters. VII. The Intermediate-Age Anticenter Cluster Melotte 71
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the anticenter, intermediate-age open cluster Melotte 71.Restricting the data to probable single members of the cluster using thecolor-magnitude diagram and the photometric indices alone generates asample of 48 F dwarfs on the unevolved main sequence. The averageE(b-y)=0.148+/-0.003 (standard error of the mean [s.e.m.]) orE(B-V)=0.202+/-0.004 (s.e.m.), where the errors refer to internal errorsalone. With this reddening, [Fe/H] is derived from both m1and hk, using Hβ and b-y as the temperature index, with excellentagreement among the four approaches and a final weighted average of[Fe/H]=-0.17+/-0.02 (s.e.m.) for the cluster, on a scale in which theHyades has [Fe/H]=+0.12. When adjusted for the higher reddeningestimate, the previous metallicity estimates from Washington photometryand from spectroscopy are now in agreement with the intermediate-bandresult. From comparisons to isochrones of appropriate metallicity, thecluster age and distance are determined as 0.9+/-0.1 Gyr and(m-M)=12.2+/-0.1 or (m-M)0=11.6+/-0.1. At this distance fromthe Sun, Mel 71 has a galactocentric distance of 10.0 kpc on a scale inwhich the Sun is 8.5 kpc from the Galactic center. Based on its age,distance, and elemental abundances, Mel 71 appears to be a less populousanalog to NGC 3960.
| 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.
| The core of the Canis Major galaxy as traced by red clump stars
The recently discovered stellar system in Canis Major is analysed usingHe-burning red clump stars as tracers. Canis Major turns out to be thestrongest and the most spatially confined overdensity of the wholeGalactic disc, in terms of both number density and statisticalsignificance. When projected on to the Galactic plane, it appears as anelongated and compact overdensity extending from l~ 200° to 280°with a roundish core towards l~ 240°. We find that the main body ofthe system has an integrated absolute magnitude MV=-14.4 +/-0.8, a central surface brightness μV,0~= 24.0 +/- 0.6 anda line-of-sight (LOS) profile peaked at Dsolar= 7.2 +/-1.0kpc with half width at half maximum ~ 2.0kpc, in excellent agreementwith the results obtained with widely different tracers (M giants andmain-sequence stars) in previous analyses. The mean distance to the mainbody of Canis Major is observed to increase with increasing Galacticlongitude, from Dsolar~= 6.3kpc at l~= 225° toDsolar~= 9.3kpc at l~= 265°, in good agreement with thepredictions of our more recent N-body simulation that models Canis Major(CMa) as a dwarf galaxy being accreted in a planar orbit on to the discof the Milky Way. We confirm that the Canis Major system has all thecharacteristics of the relic of a dwarf galaxy seen on top of alarge-scale overdensity that we detect all over the third and fourthGalactic quadrants (180°<=l<= 360°, with a strong maximumaround l= 290° and b>~-5°) that is identified as the stellarcomponent of the Southern Galactic warp. On the other hand, thepossibility that a peculiar deformation/asymmetry of the outer Galacticdisc may be at the origin of the observed distribution of overdensitiestowards CMa cannot be definitely ruled out with the data presented inthis paper. We also address a recent claim that Canis Major is on theoutskirts of a larger `Argo' structure centred at l~= 290°. Ouranalysis shows that the stellar populations in the latter aredistributed over a very large distance range along the LOS, and do notgive rise to a narrow peak in density in this direction, contrary towhat is observed in Canis Major. This suggests that the Argo structureis likely due to Galactic asymmetries such as the warp.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| Diameters of Open Star Clusters
The present paper presents a tabulation of data on all 600 Galactic openclusters for which it is currently possible to calculate lineardiameters. As expected, the youngest ``clusters,'' with ages <15 Myr,contain a significant (>=20%) admixture of associations. Amongintermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, themedian cluster diameter is found to increase with age. Small, compactclusters are rare among objects with ages >1.5 Gyr. Open clusterswith ages >1 Gyr appear to form what might be termed a ``clusterthick disk,'' part of which consists of objects that were probablycaptured gravitationally by the main body of the Galaxy.
| The Bologna Open Cluster Chemical Evolution Project: Midterm Results from the Photometric Sample
We describe a long-term project aimed at deriving information on thechemical evolution of the Galactic disk from a large sample of openclusters. The main property of this project is that all clusters areanalyzed in a homogeneous way to guarantee the robustness of the rankingin age, distance, and metallicity. Special emphasis is devoted to theevolution of the earliest phases of the Galactic disk evolution, forwhich clusters have superior reliability with respect to other types ofevolution indicators. The project is twofold: on one hand we derive theage, distance, and reddening (and indicative metallicity) byinterpreting deep and accurate photometric data with stellar evolutionmodels, and on the other hand, we derive the chemical abundances fromhigh-resolution spectroscopy. Here we describe our overall goals andapproaches and report on the midterm project status of the photometricpart, with 16 clusters already studied, covering an age interval from0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. Theimportance of quantifying the theoretical uncertainties by deriving thecluster parameters with various sets of stellar models is emphasized.Stellar evolution models assuming overshooting from convective regionsappear to better reproduce the photometric properties of the clusterstars. The examined clusters show a clear metallicity dependence on thegalactocentric distance and no dependence on age. The tight relationbetween cluster age and magnitude difference between the main-sequenceturnoff and the red clump is confirmed.
| Eclipsing Binaries in the Open Cluster NGC 2243 - I. Photometry
We obtained BV time series photometry for 12 variable stars from thefield of the old open cluster NGC 2243. The sample includes 3 newlyidentified detached/semi-detached binaries. There are now four detachedeclipsing binaries which are likely members of the cluster. Determination of the absolute parameters of the components would providea valuable check on evolutionary models of low-mass stars. An accurateephemeris and orbital period analysis are presented for thepreviously-known detached binary NV CMa. We also provide ephemerides forseven other periodic variables. We show that three contact binaries arelikely members of the cluster.
| The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting
Seventy-two grids of stellar evolutionary tracks, along with the meansto generate isochrones and luminosity/color functions from them, arepresented in this investigation. Sixty of them extend (and encompass)the sets of models reported by VandenBerg et al. for 17 [Fe/H] valuesfrom -2.31 to -0.30 and α-element abundances corresponding to[α/Fe]=0.0, 0.3, and 0.6 (at each iron abundance) to the solarmetallicity and to sufficiently high masses (up to ~2.2Msolar) that isochrones may be computed for ages as low as 1Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0Msolar and 12 [Fe/H] values between -0.60 and +0.49 (assumingsolar metal-to-hydrogen number abundance ratios): in this case,isochrones may be calculated down to ~0.2 Gyr. The extent of convectivecore overshooting has been modeled using a parameterized version of theRoxburgh criterion, in which the value of the free parameter at a givenmass and its dependence on mass have been determined from analyses ofbinary star data and the observed color-magnitude diagrams for severalopen clusters. Because the calculations reported herein satisfy manyempirical constraints, they should provide useful probes into theproperties of both simple and complex stellar populations.All of the model grids may be obtained from the Canadian Astronomy DataCenter(http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cvo/community/VictoriaReginaModels/).Included in this archive are (1) the interpolation software (FORTRAN 77)to produce isochrones, isochrone probability functions, luminosityfunctions, and color functions, along with instructions on how toimplement and use the software, (2) BVRI (VandenBerg & Clem 2003)and uvby (Clem et al. 2004) color-temperature relations, and (3)zero-age horizontal branch loci for all of the chemical compositionsconsidered.
| 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.
| vbyCaHβ CCD Photometry of Clusters. VI. The Metal-deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHβ system is presentedfor the metal-deficient open cluster NGC 2420. Restricting the data toprobable single members of the cluster using the CMD and the photometricindices alone generates a sample of 106 stars at the cluster turnoff.The average E(b-y)=0.035+/-0.003 (s.e.m.) or E(B-V)=0.050+/-0.004(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk, using b-yand Hβ as the temperature index. The agreement among the fourapproaches is reasonable, leading to a final weighted average of[Fe/H]=-0.37+/-0.05 (s.e.m.) for the cluster, on a scale where theHyades has [Fe/H]=+0.12. When combined with the abundances from DDOphotometry and from recalibrated low-resolution spectroscopy, the meanmetallicity becomes [Fe/H]=-0.32+/-0.03. It is also demonstrated thatthe average cluster abundances based on either DDO data orlow-resolution spectroscopy are consistently reliable to 0.05 dex orbetter, contrary to published attempts to establish an open clustermetallicity scale using simplistic offset corrections among differentsurveys.
| 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.
| 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.
| uvbyCaHβ CCD Photometry of Clusters. V. The Metal-Deficient Open Cluster NGC 2243
CCD photometry on the intermediate-band uvbyCaHβ systemis presented for the metal-deficient open cluster NGC 2243. Restrictingthe data to probable single members of the cluster using thecolor-magnitude diagram (CMD) and the photometric indices alonegenerates a sample of 100 stars at the cluster turnoff. The averageE(b-y)=0.039+/-0.003 (s.e.m.) or E(B-V)=0.055+/-0.004 (s.e.m.), wherethe errors refer to internal errors alone. With this reddening, [Fe/H]is derived from both m1 and hk, using b-y and Hβ as thetemperature index. The agreement among the four approaches is excellent,leading to a final weighted average of [Fe/H]=-0.57+/-0.03 (s.e.m.) forthe cluster, on a scale where the Hyades has [Fe/H]=+0.12. Using acombination of photometric and spectroscopic data, 27 probable clustermembers are identified and used to delineate the red giant branch and awell-defined clump at V=13.70, while eliminating the so-called secondclump at V=14.1. Interpolation between isochrones of appropriate [Fe/H]leads to an apparent modulus of m-M=13.15+/-0.1 and an age of 3.8+/-0.2Gyr. A differential CMD comparison with Ber 29, a cluster with agalactocentric distance almost twice that of NGC 2243, constrains Ber 29to be at least as young and as metal-rich as NGC 2243.
| Detection of the Canis Major galaxy at (l;b) = (244° -8°) and in the background of Galactic open clusters
We report on the detection of main-sequence stars belonging to therecently identified Canis Major (CMa) galaxy in a field located at~=4.°2 from the centre of the stellar system. With main-sequencefitting we obtain a distance modulus (m-M)0= 14.5 +/- 0.3 tothe dwarf, corresponding to a distance of Dsolar~= 8.0 +/-1.2 kpc, in full agreement with previous estimates based on thephotometric parallax of M-giants. From the comparison with theoreticalisochrones, we constrain the age of the main population of the CMasystem in the range ~4-10 Gyr. A blue plume of likely younger stars (age<1-2 Gyr) is also identified. The available colour-magnitude diagramsof open clusters that may be projected on to the main body of CMa arealso briefly analysed. The position, distance and stellar population ofthe old open clusters Arp-Madore 2 and Tombaugh 2 strongly suggest thatthey are physically associated with the CMa galaxy. Using our ownphotometry and data from the Two-Micron All-Sky Survey and the GuideStar Catalogue 2.2 we demonstrate that the claim by Momany et al. thatthe CMa overdensity is entirely due to the Galactic warp is notsupported by the existing observations, once all the available pieces ofinformation are taken into account. It is shown that the CMa overdensityclearly emerges at a heliocentric distance of ~8 kpc above anyoverdensity possibly produced by the Galactic warp.
| Metal Abundances in Extremely Distant Galactic Old Open Clusters. I. Berkeley 29 and Saurer 1
We report on high-resolution spectroscopy of four giant stars in theGalactic old open clusters Berkeley 29 and Saurer 1 obtained with HIRESat the Keck Telescope. These two clusters possess the largestGalactocentric distances yet known for open star clusters and thereforeare crucial objects to probe the chemical pattern and evolution of theoutskirts of the Galactic disk. We find that [Fe/H]=-0.38+/-0.14 and-0.44+/-0.18 for Saurer 1 and Berkeley 29, respectively. On the basis ofthese data, we first revise the fundamental parameters of the clustersand then discuss them in the context of the Galactic disk radialabundance gradients. Both clusters seem to significantly deviate fromthe general trend, suggesting that the outer part of the Galactic diskunderwent a completely different evolution compared with the inner disk.In particular, Berkeley 29 is clearly associated with the Monocerosstream, whereas Saurer 1 exhibits very different properties. Theabundance ratios suggest that the chemical evolution of the outer diskwas dominated by the Galactic halo.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.
| 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.
| Abundance Variations Within Globular Clusters
Abundance variations within globular clusters (GCs), and of GC starswith respect to field stars, are important diagnostics of a variety ofphysical phenomena, related to the evolution of individual stars, masstransfer in binary systems, and chemical evolution in high densityenvironments. The broad astrophysical implications of GCs as buildingblocks of our knowledge of the Universe make a full understanding oftheir history and evolution basic in a variety of astrophysical fields.We review the current status of the research in this field, comparingthe abundances in GCs with those obtained for field stars, discussing indepth the evidence for H-burning at high temperatures in GC stars,describing the process of self-enrichment in GCs with particularreference to the case of the most massive Galactic GC ( Cen), anddiscussing various classes of cluster stars with abundance anomalies.Whereas the overall pattern might appear very complex at first sight,exciting new scenarios are opening where the interplay between GCdynamical and chemical properties are closely linked with each other.
| 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.
| Lithium abundances of the local thin disc stars
Lithium abundances are presented for a sample of 181 nearby F and Gdwarfs with accurate Hipparcos parallaxes. The stars are on circularorbits about the Galactic centre and, hence, are identified as belongingto the thin disc. This sample is combined with two published surveys toprovide a catalogue of lithium abundances, metallicities ([Fe/H]),masses, and ages for 451 F-G dwarfs, almost all belonging to the thindisc. The lithium abundances are compared and contrasted with publishedlithium abundances for F and G stars in local open clusters. The fieldstars span a larger range in [Fe/H] than the clusters for which [Fe/H]~=0.0 +/- 0.2. The initial (i.e. interstellar) lithium abundance of thesolar neighbourhood, as derived from stars for which astration oflithium is believed to be unimportant, is traced from logɛ(Li) =2.2 at [Fe/H]=-1 to logɛ(Li) = 3.2 at +0.1. This form for theevolution is dependent on the assumption that astration of lithium isnegligible for the stars defining the relation. An argument is advancedthat this latter assumption may not be entirely correct, and, theevolution of lithium with [Fe/H] may be flatter than previouslysupposed. A sharp Hyades-like Li dip is not seen among the field starsand appears to be replaced by a large spread among lithium abundances ofstars more massive than the lower mass limit of the dip. Astration oflithium by stars of masses too low to participate in the Li dip isdiscussed. These stars show little to no spread in lithium abundance ata given [Fe/H] and mass.
| 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.
| Abundances of Red Giants in the Old Open Cluster Collinder 261
We present an analysis of high-dispersion echelle spectra of four giantstars in the 8 Gyr open cluster Cr 261 obtained with the Cerro TololoInter-American Observatory's 4 m telescope. Abundances were determinedrelative to the bright, slightly metal-poor disk giant Arcturus. Cr 261is found to have a mean [Fe/H]=-0.22+/-0.05 (mean error). Oxygenabundances, determined from the forbidden [O I] lines, show roughlysolar or slightly depleted abundance ratios. The α-elements Mg,Ca, and Ti also show scaled solar abundance ratios, with suggestionsthat the Si abundance is slightly enhanced. The odd-Z elements Na and Alare significantly enhanced relative to scaled solar abundances. Theabundance results for Cr 261 are compared with those fromhigh-dispersion studies of other old open clusters in the literature,which, overall, show similar behavior in all these elements. None of theabundance ratios show significant trends with overall cluster [Fe/H] orwith cluster age.
| Open Cluster LW 55 in the Large Magellanic Cloud
The field of the moderately old open cluster LW 55 in the outer parts ofthe Large Magellanic Cloud disk was monitored for detection ofshort-period variable stars over two nights. Eight variables were found;all but one (a detached eclipsing binary) are small-amplitude(ΔV<=0.1 mag) pulsating stars of the δ Scuti, SXPhoenicis, or γ Doradus type. The BV color-magnitude diagramextends down to the solar-type stars at V=24 (MV~=5.2). Thecluster age is estimated at 1.5 Gyr for Z=0.004 ([Fe/H]=-0.67) thesurrounding stellar field is older, with an age greater than 4 Gyr.Based on data obtained at Las Campanas Observatory, operated by theCarnegie Institution of Washington, during the University of Torontotime allocation.
| Variable stars in the globular cluster NGC 3201 - I. Multimode SX Phe type variables
We report on the discovery of 11 multimode SX Phoenicis type bluestragglers in the field of the southern globular cluster NGC 3201. Inthese variables both radial and non-radial modes are excited. For threevariables the derived period ratio is close to that observed in SXPhoenicis itself, suggesting that these stars are pulsating in thefundamental and the first-overtone radial modes. Using the McNamaraperiod-luminosity relation we have estimated the apparent distancemodulus to NGC 3201 to be 14.08 +/- 0.06 +/- 0.1 mag.
| 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.
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