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|The Infrared Ca II Triplet as Metallicity Indicator|
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
|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.
|Oscillating blue stragglers, γ Doradus stars and eclipsing binaries in the open cluster NGC 2506|
Context: This is the first step in a project to combine studies ofeclipsing binaries and oscillating stars to probe the interior of BlueStragglers (BS). This may imply a way to discriminate observationallybetween different birth mechanisms of BS stars. Aims: We study theopen cluster NGC 2506 which contains oscillating BS stars and detachedeclipsing binaries for which accurate parameters can be derived. Thiswill tightly constrain the cluster isochrone and provide an absolutemass, radius and luminosity-scale for the cluster stars along with thecluster age, metallicity and distance. The present work focuses onobtaining the light curves of the binaries and determine their orbitalperiods, on obtaining power spectra of the oscillating BS stars toselect targets for follow-up studies, and on searching for γDoradus type variables which are also expected to be present in thecluster. Methods: With a two-colour, dual-site photometric campaign weobtained 3120 CCD-images of NGC 2506 spread over four months. Weanalysed the BI time-series of the oscillating stars and usedsimulations to derive statistical uncertainties of the resultingfrequencies, amplitudes and phases. A preliminary mode-identificationwas performed using frequency ratios for the oscillating BS stars, andamplitude ratios and phase differences for a population of newlydetected γ Doradus stars. Results: We quadrupled the number ofknown variables in NGC 2506 by discovering 3 new oscillating BS stars,15 γ Doradus stars and four new eclipsing binaries. The orbitalperiods of 2 known, detached eclipsing binaries were derived. Wediscovered a BS star with both p-mode and g-mode variability and weconfronted our γ Doradus observations with state-of-the-artseismic models, but found significant discrepancy between theory andobservations. Conclusions: . NGC 2506 is an excellent target forasteroseismic tests of stellar models, as strong external constraintscan be imposed on the models of a population of more than 20 oscillatingstars of different types.Based on observations obtained at the Flemish Mercator telescope on LaPalma, Spain, the Danish 1.5-m telescope at ESO, La Silla, Chile, and onobservations collected at the European Southern Observatory, Paranal,Chile (ESO Programme 075.D-0206(B)). Catalog of individual photometrymeasurements for all variables is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/965
|Galactic Interstellar Gas Cloud Mass Functions: A Simple Quantitative Approach|
We present here a simple approach to understanding the gas cloud massdistribution function by simulating formation and destruction of gasclouds and gas clumps in the ISM. We include as relevant processescoagulation to form bigger clouds, as well as disruption by collisionsand the removal of gas by collapse to form stars. We evolve initial setsof preexisting gas clumps with a range of initial distribution functions(flat, Gaussian, fractal) for their physical parameters and withdifferent geometrical forms (spherical or elongated) for the individualclouds, and constrain them within an imaginary box representinggravitational bounding, applying the kinematic laws of nonelasticcollisions. The results agree well with observations of the massdistribution function of Galactic giant gas clouds if we choose aGaussian for the initial distribution function, and initial gas cloudswhich are quasi-spherical.
|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
|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.
|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.
|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.
|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.
|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.
|Pinpointing isochrones in clusters .|
Detached eclipsing binaries allow the determination of accurate stellarmasses and radii. Here we present the first results of a programme whichaims at using such systems for the determination of accurate stellarparameters in open star clusters and discuss this in relation to thestudy of pulsating stars. As an example we show results for a detachedeclipsing system in the old open cluster NGC 188 and briefly discuss thetwo intermediate age open clusters NGC 1817 and NGC 2506 which bothcontains pulsating stars and detached eclipsing binaries.
|Pulsating stars and EBs in clusters: NGC 2506 .|
Studying pulsating stars in clusters offers the advantage thatconstraints based on cluster membership can be imposed on the stellarmodels. This is a huge advantage in an asteroseismic analysis. Theconstraints become even stronger if the cluster contains detachedeclipsing binaries (dEBs) for which precise masses and radii can bederived, pinpointing the isochrones. We present new time-seriesphotometry, and VLT spectroscopy, of the open cluster NGC 2506, whichcontains a population of delta Scuti stars as well as two detachedeclipsing binary systems. Three delta Scuti stars were known prior toour observations, but our preliminary data analysis reveals the presenceof at least three more such variables. This makes NGC 2506 a key targetfor asteroseismology of ˜2Mȯ stars.
|High-resolution spectroscopy of the old open cluster Collinder 261: abundances of iron and other elements|
We present the analysis of high resolution spectra of six red giantstars in the old open cluster Collinder 261. Reddening values forindividual stars, derived from the relation between colours andtemperatures (deduced from our fully spectroscopic analysis) areconsistent with previous determinations based on photometry. For thiscluster we derive an iron abundance of [Fe/H] = - 0.03 ± 0.03. Wealso obtain the abundances of light metals (O, Na and Al),α-elements (Mg, Si, Ca, Ti), elements of the Fe-group (Sc, Cr, Mn,Co, Ni) and the neutron-capture element Ba. No intrinsic star-to-starscatter is present in any of these elements within our sample. Wecompare our findings with previous investigations on this cluster,discussing in detail differences in analysis methods and results.
|Astrophysical parameters of Galactic open clusters|
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
|A Dozen δ Scuti Stars in the Open Cluster NGC 1817|
We obtained high-precision time-series observations of stars in the opencluster NGC 1817 in order to find δ Scuti stars among the clustermembers. The detection of 12 δ Scuti stars, of which our datasuggest that 11 are cluster members, makes NGC 1817 a key target forasteroseismology of 1.5-2.5 Msolar stars. One of the clustermember δ Scuti stars is also an eclipsing binary, potentiallyoffering very strong constraints for theoretical modeling. In addition,we find one δ Scuti star candidate, a γ Dor candidate, twovariables of currently unknown type, and two eclipsing binary systems.We also describe a method for combining the oscillation frequencies ofseveral δ Scuti stars in a single cluster to obtain informationabout the order n of the excited modes. This method will eventually beused to constrain the theoretical models to be compared with preciseoscillation frequencies determined from a future multisite campaign(asteroseismology).
|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.
|Radial velocities and membership of stars in the old, distant open cluster Berkeley 29|
Multi slit spectroscopy at the Telescopio Nazionale Galileo was employedto measure radial velocities for 20 stars in the direction of the oldopen cluster Berkeley 29, the farthest known in our Galaxy. Membershipinformation was derived for stars all along the red giant branch, inparticular near its tip, and on the red clump. The sample of bona fidecluster members was used to revise the cluster distance to ˜ 15kpc, on the basis of an empirical comparison with the red clump in openclusters with known distances. A metallicity [Fe/H] = -0.74 ±0.18 was also estimated using the colours of spectroscopically confirmedred giant stars.Based on observations made with the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Centro Galileo Galileiof the INAF (Istituto Nazionale di Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
|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.
|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.
|Iron abundances from high-resolution spectroscopy of the open clusters NGC 2506, NGC 6134, and IC 4651|
This is the first of a series of papers devoted to deriving themetallicity of old open clusters to study the time evolution of thechemical abundance gradient in the Galactic disk. We present detailediron abundances from high resolution (R⪆ 40 000) spectra of severalred clump and bright giant stars in the open clusters IC4651, NGC 2506 and NGC6134. We observed 4 stars of NGC 2506, 3 stars of NGC 6134 and5 stars of IC 4651 with the FEROS spectrograph with the ESO 1.5 mtelescope; moreover, 3 other stars of NGC 6134 were observed with theUVES spectrograph on Kueyen (VLT UT2). After excluding the cool giantsnear the red giant branch tip (one in IC 4651 and one in NGC 2506), wefound overall [Fe/H] values of -0.20±0.01, rms = 0.02 dex (2stars) for NGC 2506, +0.15±0.03, rms = 0.07 dex (6 stars) for NGC6134 and +0.11±0.01, rms = 0.01 dex (4 stars) for IC 4651. Themetal abundances derived from a line analysis for each star wereextensively checked using spectrum synthesis of about 30 to 40 Fe Ilines and 6 Fe II lines. Our spectroscopic temperatures providereddening values in good agreement with literature data for theseclusters, strengthening the reliability of the adopted temperature andmetallicity scale. Also, gravities from the Fe equilibrium of ionizationagree quite well with expectations based on cluster distance moduli andevolutionary masses.Based on observations collected at ESO telescopes under programme65.N-0286, and in part 169.D-0473.
|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.
|Old Open Clusters|
Open clusters are among the best tracers of the Galactic disk properties(ages, distances, metallicities, etc. and their evolution in space andtime). In this framework, we are working on a long term programmededicated to the study of old Open Clusters. We are analyzingphotometric and spectroscopic data for a fair sample of Open Clusters,with the intent of determining in a precise and homogeneous way theirdistances, ages, reddenings, and detailed chemical abundances.
|High resolution spectroscopy of open clusters with SARG|
We present high resolution spectroscopic observations of open clustersof different ages and metallicities obtained with SARG at TNG. Wederived lithium abundances for solar-type stars in NGC 752 (˜2Gyr) and NGC 188 (˜6-8 Gyr) and for early-K stars in Praesepe(˜600 Myr). We found that (i) Li depletion in solar-type starsmight stop at an age of ˜2 Gyr; (ii) the spread in Li abundancesobserved in M 67 seems to be an exception rather than the rule in oldclusters; (iii) Li evolution does not seem to be affected by smallvariations in metallicity.Based on observations collected at TNG
|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.
|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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