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Публикации по объекту
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.
| Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy
Context: Recently, Carraro (2005) drew attention to the remarkable starcluster Whiting 1 by showing that it lies about 40 kpc from the Sun andis therefore unquestionably a member of the Galactic halo (b=-60.6 deg).Its Color Magnitude Diagram (CMD) indicated that Whiting 1 is very young(~5 Gyr) for a globular cluster. It is very likely that Whiting 1originated in a dwarf galaxy that has since been disrupted by the MilkyWay. Aims: The main goals of this investigation were to constrainbetter the age, metallicity, and distance of Whiting 1 and to assesswhether it belongs to a stellar stream from the Sagittarius dwarfspheroidal galaxy (Sgr dSph). Methods: Deep CCD photometry in the BVIpass-bands obtained with the VLT is used to improve the quality of theCMD and to determine the cluster's luminosity function and surfacedensity profile. High-resolution spectrograms obtained with Magellan areused to measure the cluster's radial velocity and to place limits on itspossible metallicity. The measurements of distance and radial velocityare used to test the cluster's membership in the stellar streams fromthe Sgr dSph. Results: From our CMD of Whiting 1, we derive newestimates for the cluster's age (6.5+1.0-0.5 Gyr),metallicity (Z=0.004±0.001, [Fe/H] = -0.65), and distance(29.4+1.8-2.0 kpc). From echelle spectrograms ofthree stars, we obtain -130.6±1.8 km s-1 for thecluster's radial velocity and show from measurements of two infra-redCaII lines that the [Fe/H] of the cluster probably lies in the range-1.1 to -0.4. Both the luminosity function and the surface densityprofile suggest that the cluster has undergone tidal stripping by theMilky Way. We demonstrate that the position of Whiting 1 on the sky, itsdistance from the Sun, and its radial velocity are identical to withinthe errors of both the theoretical predictions of the trailing stream ofstars from the Sgr dSph galaxy and the previous observations of the Mgiant stars that delineate the streams. Conclusions: With the additionof Whiting 1, there is now strong evidence that 6 globular clustersformed within the Sgr dSph. Whiting 1 is particularly interestingbecause it is the youngest and among the most metal rich. The relativelyyoung age of Whiting 1 demonstrates that this dwarf galaxy was able toform star clusters for a period of at least 6 Gyr, and the age andmetallicity of Whiting 1 are consistent with the age-metallicityrelationship in the main body of the Sgr dSph. The presence now ofWhiting 1 in the Galactic halo provides additional support for the viewthat the young halo clusters originated in dwarf galaxies that have beenaccreted by the Milky Way.Based on observations with the ESO VLT at the Paranal Observatory, underthe program 76.D-0128. This paper includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile. Tablesof the individual photometric measurements are available at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/181
| 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.
| 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
| Abundance gradients in the Milky Way for α elements, iron peak elements, barium, lanthanum, and europium
We model the abundance gradients in the disk of the Milky Way forseveral chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn,Cu, Mn, Cr, Ba, La and Eu), and compare our results with the most recentand homogeneous observational data. We adopt a chemical evolution modelable to well reproduce the main properties of the solar vicinity. Themodel assumes that the disk formed inside-out with a timescale for theformation of the thin disk of 7 Gyr in the solar vicinity, whereas thehalo formed on a timescale of 0.8 Gyr. We adopt new empirical stellaryields derived to best fit the abundances and the abundance ratios inthe solar vicinity. We compute, for the first time, the abundancegradients for all the mentioned elements in the galactocentric distancerange 4-22 kpc. Comparison with the observed data on Cepheids in thegalactocentric distance range 5-17 kpc gives a very good agreement formany of the studied elements. In addition, we well fit the data for theevolution of lanthanum in the solar vicinity, for which we presentresults here for the first time. We explore, also for the first time,the behavior of the abundance gradients at large galactocentricdistances by comparing our results with data relative to distant openclusters and red giants and select the best chemical evolution model onthat basis. We find a very good fit to the observed abundance gradients,as traced by Cepheids, for most of the elements, thus confirming thevalidity of the inside-out scenario for the formation of the Milky Waydisk as well as the adopted nucleosynthesis prescriptions. The flatgradients at large galactocentric distances (> 12 kpc), as traced bythe Cepheids, open cluster and red giant data, lead us to conclude thata model where the density of the halo stellar component is constant inthe inner 20 kpc should be preferred. Other models with differentdistributions of the halo stellar mass do not produce a good fit of thedata.Full Table 4 is only available in electronic form athttp://www.aanda.org
| 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.
| 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.
| Elemental Abundance Ratios in Stars of the Outer Galactic Disk. II. Field Red Giants
We summarize a selection process to identify red giants in the directionof the southern warp of the Galactic disk, employing VICphotometry and multiobject spectroscopy. We also present results fromfollow-up high-resolution, high signal-to-noise echelle spectroscopy ofthree field red giants, finding [Fe/H] values of about -0.5. The fieldstars, with galactocentric distances estimated at 10-15 kpc, support theconclusion of Yong and coworkers that the Galactic metallicity gradientdisappears beyond RGC values of 10-12 kpc for the older starsand clusters of the outer disk. We summarize the detailed abundancepatterns for 15 other elements for these stars and compare them withrecently obtained results for old open cluster red giants in the outerdisk. The field and cluster stars at such large distances show verysimilar abundance patterns, and, in particular, all show enhancements ofthe α-elements O, Mg, Si, Ca, and Ti and the r-process element Eu.These results suggest that Type II supernovae have been significantcontributors to star formation in the outer disk relative to Type Iasupernovae within the past few gigayears. We also compare our resultswith those available for much younger objects. The limited results forthe H II regions and B stars in the outer disk also suggest that theradial metallicity gradient in the outer disk is shallow or absent. Themuch more extensive results for Cepheids confirm these trends and thatthe change in slope of the metallicity gradient may occur at a largergalactocentric distance than for the older stars and clusters. However,the younger stars also show rising α-element enhancements withincreasing RGC, at least beyond 12 kpc. These trends areconsistent with the idea of a progressive growth in the size of theGalactic disk with time and episodic enrichment by Type II supernovae aspart of the disk's growth.This paper makes use of observations obtained at the National OpticalAstronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under contract fromthe National Science Foundation. We also employ data products from theTwo Micron All Sky Survey, which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center,California Institute of Technology, funded by the National Aeronauticsand Space Administration and the National Science Foundation.
| 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.
| Elemental Abundance Ratios in Stars of the Outer Galactic Disk. I. Open Clusters
We summarize radial velocity studies of selected stars in the old,distant clusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, andBerkeley 31. Cluster members are identified using optical and infraredcolor-magnitude diagrams, as well as radial velocities derived fromhigh-resolution echelle spectra. Three members of M67 were observedsimilarly, and those velocities compare extremely well with priormeasures. Mean cluster radial velocities are determined. We also employthe highest quality spectra to analyze the chemical compositions of allsix clusters for [Fe/H], as well as abundances of α-elements,iron-peak elements, and those synthesized in either the s-process or ther-process. In Be 21 our observed star is found to be rotating rapidlyand overabundant in lithium, the second Li-rich star found in thissparse cluster.We confirm the lack of correlation between abundance and age. For theouter disk, the abundance gradient for [Fe/H] deviates from the trenddefined near the solar neighborhood. Rather than declining withincreasing galactocentric distance, [Fe/H] appears to reach a``basement'' at [Fe/H]~-0.5 beyond RGC~10-12 kpc. Our radialabundance distribution for [Fe/H] is not inconsistent with the radialabundance discontinuity exhibited by Cepheids. We find enhanced [O/Fe],[α/Fe], and [Eu/Fe] in the outer disk, revealing a rapid starformation history. The outer disk also exhibits enhancements fors-process elements. We compare the open cluster compositions with thoseof the thin disk, thick disk, halo, bulge, and dwarf spheroidalgalaxies. None of these stellar populations perfectly matches theabundance ratios of the outer disk open clusters. Several key pointsarise from these comparisons: (1) [O/Fe] and [α/Fe] resemble thoseof the thick disk. (2) [Na/Fe] and [Al/Fe] are enhanced relative tothose of the thin disk. (3) [Ni/Fe] and [Mn/Fe] are in accord with thoseof the thin disk, while [Co/Fe] may be slightly enhanced. (4) Theneutron-capture elements indicate different ratios of s-process tor-process material, with no cluster showing a pure r-processdistribution. (5) An unusual pattern exists among the α-elements,with [/Fe] enhanced while [/Fe] is normal.Similar abundance ratios have been reported for Galactic bulge giantsand indicate a common but not necessarily shared nucleosynthetic historybetween the bulge and the outer disk. Enhanced ratios of [Al/Fe] and[Co/Fe] offer another possible similarity between the bulge and theouter disk.An intriguing but tentative conclusion is that the outer disk opencluster abundance ratios are consistent with the outer disk being formedvia a merger event or series of merger events. The basement in [Fe/H]and enhanced [α/Fe] suggest that the outer disk formed from areservoir of gas with a star formation history distinct from the solarneighborhood. That the open clusters may be associated with an accreteddwarf galaxy or galaxies is appealing, since the clusters are young andhave [α/Fe] ratios indicating a rapid star formation history.However, the high [α/Fe] ratios are unlike those seen in anycurrent dwarf galaxies at the same [Fe/H]. Therefore, the open clustersmay have formed as a result of star formation triggered by a mergerevent or series of mergers in the outer disk. The ages of the outer diskopen clusters would then be a measure of when the merger(s) occurred.However, Be 29 is a candidate merger member, while Be 31 is not. Oneproblem with the merger scenario is that open clusters with presumablyvery different origins have similar and unusual compositions.This paper makes use of observations obtained at the National OpticalAstronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under contract fromthe National Science Foundation. We also employ data products from theTwo Micron All Sky Survey, which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center,California Institute of Technology, funded by the National Aeronauticsand Space Administration and the National Science Foundation.
| Spectroscopy of Red Giants in the Large Magellanic Cloud Bar: Abundances, Kinematics, and the Age-Metallicity Relation
We report metallicities and radial velocities derived from spectra atthe near-infrared calcium triplet for 373 red giants in a 200arcmin2 area at the optical center of the LMC bar. These arethe first spectroscopic abundance measurements of intermediate-age andold field stars in the high surface brightness heart of the LMC. Themetallicity distribution is sharply peaked at the median value[Fe/H]=-0.40, with a small tail of stars extending down to[Fe/H]<=-2.1 10% of the red giants are observed to have[Fe/H]<=-0.7. The relative lack of metal-poor stars indicates thatthe LMC has a ``G dwarf'' problem, similar to the Milky Way. Theabundance distribution can be closely approximated by two Gaussianscontaining 89% and 11% of the stars, respectively: the first componentis centered at [Fe/H]=-0.37 with σ=0.15, and the second at[Fe/H]=-1.08 with σ=0.46. The dominant population has ametallicity distribution similar to that of the LMC's intermediate-agestar clusters. The mean heliocentric radial velocity of the sample is257 km s-1, corresponding to the same center-of-mass velocityas the disk (measured at larger radii). Because of the central locationof our field, kinematic constraints are not strong, but there is noevidence that the bar deviates from the general motion of the LMC disk.The velocity dispersion of the whole sample isσv=24.7+/-0.4 km s-1. When cut bymetallicity, the most metal-poor 5% of stars ([Fe/H]<-1.15) showσv=40.8+/-1.7 km s-1, more than twice thevalue for the most metal-rich 5%; this suggests that an old, thickerdisk or halo population is present. The age-metallicity relation (AMR)is almost flat during the period from 5 to 10 Gyr ago, with an apparentscatter of +/-0.15 dex about the mean metallicity for a given age.Comparing this to chemical evolution models from the literature, we findthat a burst of star formation 3 Gyr ago does not reproduce the observedAMR more closely than a steadily declining star formation rate. The AMRsuggests that the epoch of enhanced star formation, if any, must havecommenced earlier, ~6 Gyr ago-the exact time is model dependent. Wecompare the properties of the LMC and the Milky Way, and discuss ourresults in the context of models that attempt to use tidal interactionswith the Milky Way and Small Magellanic Cloud to explain the star andcluster formation histories of the LMC.
| 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 effects of age on red giant metallicities derived from the near-infrared CaII triplet
We have obtained spectra with a resolution of ~2.5 Å in the regionof ~7500-9500Å for 116 red giants in five galactic globularclusters and six old open clusters (five with published metallicitiesand one previously unmeasured). The signal-to-noise (S/N) ratio lieswithin the range 20 <= S/N <= 85. We measure the equivalent widthsof the infrared CaII triplet absorption lines in each star and comparethem with cluster metallicities taken from the literature. With globularcluster abundances on the Carretta & Gratton scale, and open clusterabundances taken from the compilation of Friel and collaborators, wefind a linear relation between [Fe/H] and CaII line strength spanningthe range -2 <~[Fe/H]<~-0.2 and 2.5 <~ (age/Gyr) <~ 13. Thereference abundance scales appear to be consistent with each other atthe ~0.1 dex level. Alternate choices for metallicity scales canintroduce curvature into the relation between the [Fe/H] and CaIIequivalent width. No evidence for an age effect on the metallicitycalibration is observed. Using this calibration, we find the metallicityof the massive, old, open cluster Trumpler 5 to be [Fe/H]=-0.56 +/-0.11. This is the first spectroscopic abundance measurement for Trumpler5, and is lower by ~0.3 dex than estimates based on the clustercolour-magnitude diagram. Considering the 10 clusters of knownmetallicity shifted to a common distance and reddening, we find that theadditional error introduced by the variation of horizontal branch/redclump magnitude with metallicity and age is of the order of +/-0.05 dex,which can be neglected in comparison with the intrinsic scatter in themethod. The results are discussed in the context of abundancedeterminations for red giants in Local Group galaxies.
| 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.
| 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.
| The relatively young and metal-poor Galactic open cluster NGC 2194
We present CCD VIKC photometry down to V~ 21 mag in the fieldof the rich open cluster NGC 2194, which is projected towards theGalactic anticentre direction. We measured V magnitudes and V-I coloursfor a total of 2515 stars in a field of 13.6 × 13.6arcmin2. These data are supplemented with CCD photometry inthe C, M and T1 filters of the Washington system andphotoelectric CMT1T2 photometry of 20 red giantcandidates. Based on the best fits of isochrones computed by the Genevaand Padova groups to the (V, V-I) colour-magnitude diagram, we derive acolour excess E(V-I) = 0.75, a cluster distance of 3.2 kpc and an age of400 Myr. Five independent Washington abundance indices yield a meancluster metallicity of [Fe/H]=-0.27 +/- 0.06, which is compatible withthe existence of both a radial and Z gradient in the Galactic disc. NGC2194 is a relatively young open cluster, whose deficiency in metalcontent can be accounted for by the fact that it was born not only farfrom the Galactic centre in the outer disc, but also at a relativelyhigh Z value.
| 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.
| Metallicities of Old Open Clusters
We present radial velocities and metallicities for a sample of 39 openclusters with ages greater than about 700 million years. For 24 clustersnew moderate-resolution spectroscopic data obtained with multiobjectspectrographs on the Kitt Peak National Observatory and the Cerro TololoInter-American Observatory 4 m telescopes are used to determine radialvelocities and mean cluster metallicities. These new results arecombined with data published previously by Friel & Janes to providea sample of 459 giants in 39 old open clusters, which are used toinvestigate radial abundance gradients in the Galactic disk. Based on anupdated abundance calibration of spectroscopic indices measuring Fe andFe-peak element blends, this larger sample yields an abundance gradientof -0.06+/-0.01 dex kpc-1 over a range in Galactocentricradius of 7 to 16 kpc. There is a slight suggestion of a steepening ofthe abundance gradient with increasing cluster age in this sample, butthe significance of the result is limited by the restricted distancerange for the youngest clusters. The clusters show no correlation ofmetallicity with age in the solar neighborhood. Consistent with theevidence for a steepening of the gradient with age, the clusters in theouter disk beyond 10 kpc show a suggestion at the 1.5 σ level of adependence of metallicity on age.
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are 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/388/168
| The intermediate-age open cluster NGC 2158
We report on UBVRI CCD photometry of two overlapping fields in theregion of the intermediate-age open cluster NGC 2158 down to V =21. Byanalysing colour-colour (CC) and colour-magnitude diagrams (CMDs) weinfer a reddening E (B -V )=0.55+/-0.10, a distance of 3600+/-400pc, andan age of about 2Gyr. Synthetic CMDs constructed with these parameters(but fixing E (B -V )=0.60 and [Fe/H]=-0.60), and including binaries,field contamination and photometric errors, yield a good description ofthe observed CMD. The elongated shape of the clump of red giants in theCMD is interpreted as resulting from a differential reddening of aboutΔE (B -V )=0.06 across the cluster, in the direction perpendicularto the Galactic plane. NGC 2158 turns out to be an intermediate-age opencluster with an anomalously low metal content. The combination of theseparameters, together with the analysis of the cluster orbit, suggeststhat the cluster belongs to the old thin disc population.
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
| Statistics and supermetallicity: The metallicity of NGC 6791
For the old galactic cluster NGC 6791, Peterson & Green(\cite{pg98a}) and Chaboyer et al. (\cite{cgl99}) have found that [Fe/H]~ +0.4 dex. A second look at that conclusion is taken in this paper.Zero-point problems are reviewed for a high-dispersion analysis done byPeterson & Green, and it is found that accidental errors have notbeen determined rigorously for the results of that analysis. It is alsonoted that in a color-magnitude analysis performed by Chaboyer et al.,the important metallicity range between 0.0 and +0.3 dex is not exploredand hence is not ruled out. Moreover, that analysis does not yieldstatistically rigorous results, and it appears that such results may notbe produced in color-magnitude analysis of clusters in general. Resultsin the two cited papers and elsewhere are re-evaluated statistically,with an allowance being made for uncertainty in the cluster reddening.Apparently the best that can be said at present is that the clustermetallicity lies in the range from +0.16 to +0.44 dex. This conclusionis stressed by reviewing the immaturity of the underlying data base. Thepremature conclusion for a high metallicity turns out to be due largelyto neglect of accidental errors, though a tendency to ascribe too muchweight to high derived metallicities may also play a role.
| An optical and near IR study of the old open cluster NGC 2141
We report CCD optical (B and V passbands) and near IR (J and K bands)observations in the region of the old open cluster NGC 2141. Bycombining the two sets of photometry (500 stars in common) we derive newestimates of the cluster's fundamental parameters. We confirm that thecluster is 2.5 Gyrs old, but, with respect to previous investigations,we obtain a slightly larger reddening (E(B-V) = 0.40), and a slightlyshorter distance (3.8 kpc) from the Sun. Finally, we present theLuminosity Function (LF) in the V band, which is another age indicator.We provide a good fit for the age range inferred from isochrones byassuming the Kroupa et al. (\cite{r18}) IMF up to MV = 5.0.We interpret the disagreement at fainter magnitudes as evidence of masssegregation. Based on observations taken at ESO La Silla and TIRGO.Table~2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/372/879
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| On the Galactic disc age-metallicity relation
A comparison is made between the age-metallicity relations obtained fromfour different types of studies: F and G stars in the solarneighbourhood, analysis of open clusters, galactic structure studieswith the stellar population synthesis technique and chemical evolutionmodels. Metallicities of open clusters are corrected for the effects ofthe radial gradient, which we find to be -0.09 dex kpc^-1 and mostlikely constant in time. We do not correct for the vertical gradient,because its existence and value are not firmly established. Stars andclusters trace a similar age-metallicity relation, showing an excess ofrather metal-rich objects in the age range 5-9 Gyr. Galactic structurestudies tend to give a more metal-poor relation than chemical evolutionmodels. Neither relation explains the presence of old, relativelymetal-rich stars and clusters. This might be caused by uncertainties inthe ages of the local stars, or pre-enrichment of the disc with materialfrom the bulge, possibly as a result of a merger event in the earlyphases of the formation of our Galaxy.
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