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Na, Al, and O Abundances of Open Clusters NGC 7142, NGC 6939, and IC 4756
We present an analysis of echelle spectra of stars in three openclusters obtained with the Hydra multiobject spectrograph on the WIYN3.5 m telescope. Abundances of Fe, O, Si, Ca, Na, Al, and Ni have beendetermined via equivalent width analysis and spectrum synthesis. Meanabundances for each cluster are compared to those of previous studiesand of other clusters in the literature, with emphasis on exploring theenhancements of Na and Al seen in many open clusters. All three clustersshow enhanced values of [Na/Fe] and [Al/Fe], while the abundances of Fe,O, Si, and Ca are consistent with their ages and locations in theGalactic disk.
| Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model
The presence of blue straggler stars (BSs) as secure members of Galacticopen clusters (OCs) poses a major challenge to the conventional pictureof simple stellar population (SSP) models. These are based on thestellar evolution theory of single stars, whereas the major formationmechanisms of BSs are all correlated with stellar interactions. We haveillustrated this in a previous study based on a small sample of old (age>=1 Gyr) Galactic OCs. However, for the purpose of demonstrating thecontributions of BSs to the conventional SSP models statistically andsystematically, a large database with sufficient coverage of age andmetallicity is definitely needed. The working sample now includes 100Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions ofBSs to the integrated light of their host clusters are calculated on anindividual cluster basis. The general existence of BSs in our starcluster sample dramatically alters the predictions of conventional SSPmodels in terms of their integrated properties. Neglecting theconsequences of nonstandard evolutionary products, such as BSs, instellar populations, very large uncertainties can be made in analyzingtheir integrated spectral energy distributions at unresolvableconditions. The current work strongly suggests that when evolutionarypopulation synthesis technique is used to study the properties ofunresolved stellar populations in galaxies, the contributions of BSsshould be taken into account.
| 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
| 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.
| An Examination of Main Sequence Stars within the Instability Strip for Open Clusters Over a Range of Ages
We are conducting a study of 14 open clusters which have an approximateage range of 7 to 9 Gyr. The observations for this project were takenwith the 1.8-m telescope at the Dominion Astrophysical Observatory usinga y filter and with the 0.4-m David Derrick telescope of the Orson PrattObservatory using a set of VRI filters. The exposure lengths wereoptimized for main sequence stars in the A-F range in order to bracketthe instability strip. We plan to correlate the cluster ages with 1) thenumber of pulsating variables in the instability strip, 2) thepercentage of variable stars in the instability strip, and 3) theamplitude of the pulsations. We will present preliminary results for 4of the 14 clusters: NGC 6811, NGC 6940, NGC 7142, and NGC 7160.We wish to thank the Dominion Astrophysical Observatory for access tothe 1.8-m telescope. We also acknowledge funding from an AAS SmallResearch Grant that helped make the telescope time at DAO possible.
| 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.
| 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.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| 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.
| 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.
| Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| A far-infrared survey of molecular cloud cores
We present a catalogue of molecular cloud cores drawn fromhigh-latitude, medium-opacity clouds, using the all-sky IRAS Sky SurveyAtlas (ISSA) images at 60 and 100μm. The typical column densities ofthe cores are N(H2)~3.8x1021cm-2 andthe typical volume densities aren(H2)~2x103cm-3. They are thereforesignificantly less dense than many other samples obtained in other ways.Those cloud cores with IRAS point sources are seen to be already formingstars, but this is found to be only a small fraction of the total numberof cores. The fraction of the cores at the protostellar stage is used toestimate the pre-stellar time-scale - the time until the formation of ahydrostatically supported protostellar object. We argue, on the basis ofa comparison with other samples, that a trend exists for the pre-stellarlifetime of a cloud core to decrease with the mean column density andnumber density of the core. We compare this with model predictions andshow that the data are consistent with star formation regulated by theionization fraction.
| 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.
| Corridors in the regions of young open stellar clusters.
Not Available
| Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2556T&db_key=AST
| Three new variable stars in Cygnus.
Not Available
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| The Old Open Clusters Of The Milky Way
The Galactic open clusters, in particular the oldest members, serve asexcellent probes of the structure and evolution of the Galactic disk.Individual clusters provide excellent tests of stellar and dynamicalevolution. Cluster spatial and age distributions provide insight intothe processes of cluster formation and destruction that have allowedsubstantial numbers of old open clusters to survive.Spectroscopic andphotometric data for the old clusters yield kinematic, abundance, andage information that clarifies the relationship between the old opencluster population and other Galactic populations. New samples of oldopen clusters, which span a large range in distance and age, are used todefine disk abundance gradients and the cluster age-metallicityrelationship, and they point to a complex history of chemical enrichmentand mixing in the disk.
| The Galactic system of old open clusters: age calibration and age-metallicity relation
In this paper, we present a new homogeneous compilation of ages for thesystem of intermediate age and old open clusters of the Galaxy, and theaccompanying age-metallicity relation (AMR). This study stands on theanalysis by Carraro et al. (1991, 1993a,b), who have obtained goodestimates for the color excess, distance modulus, and age for a sampleof ten clusters, for which modern color-magnitude diagrams (CMD) of goodphotometric quality and spectroscopic data on the metallicity (Friel& Janes 1991, 1993) were available. Firstly, we revise the resultsby Carraro et al. (1991, 1993a,b) to take into account recentdevelopments in the libraries of stellar models (Alongi et al. 1993;Bressan et al. 1993; Fagotto et al. 1993), and secondly we presentuseful age calibrations based on the correlation between metallicity,age, and magnitude difference between the turn-off and red clumpluminosities. The age calibration does not depend on the color excess,distance modulus, but only weakly on the metallicity. With the aid ofthe new age calibration, we assign the age to a more numerous sample ofclusters. The resulting ages span the range from 0.5x10^9^yr for NGC5822 to 8.0x10^9^yr for NGC 6791. With such a compilation, and adoptingan homogeneous source for the metal content, we propose a new AMR forthe family of Galactic open clusters. The AMR is also corrected for theeffect of the gradient in metallicity across the Galactic Disk. Althoughat any given age the spread in metallicity is high, the AMR togetherwith the distribution of clusters with different age and metallicityacross the Galactic Plane, confirms previous suggestions that themetallicity of a cluster is more related to the position than the age.
| Berkeley 93: A distant star cluster nestled in a dust cloud
While performing galactic and extragalactic studies near the galacticplane in Cepheus Charge Coupled Device (CCD) frames in B, V, and R weretaken of the faint (V greater than or equal to 16m)previously unstudied open star cluster Berkeley 93 (Be 93). Our resultsindicate that this object is the core of a larger aggregate, is slightlyevolved, strongly reddened (EB - V approximately equal to1.5), and shows a pronounced variable reddening that is probably due tothe location of the cluster inside (near the border) a dust cloud. Byfar the reddest, and obviously most evolved star is a (variable) carbonstar that -- because of its reddening and location -- appears to be acluster member. We present arguments in favor of a large distance ofmore than 5 kpc for Be 93 which possibly belongs to the galactic warp.As an addendum, we present six star concentrations discovered on thePOSS or European Southern Observatory (ESO) SERC atlas that mightrepresent hitherto uncatalogued open star clusters of 'Berkeley type'.
| Development of the Galactic disk: A search for the oldest open clusters
In an extensive charge coupled devices (CCD) photometric survey ofpotential old open clusters, we have identified a number of systems thatare indeed old; some of them are among the oldest of the open clusters.Using our versions of two well-known morphological age indices, onebased on the luminosity difference between the main sequence turnoff andthe horizontal branch and the other on the color difference between theturnoff and the giant branch, we have ranked the open clusters inapproximate order of age. Our data together with previously publishedphotometry of other old open clusters, yields a catalogue of 72 clustersof the age of Hyades or older with 19 of the clusters as old or olderthan M67 (about 5 Gyr). Among the oldest open clusters are Be 17, Cr261, NGC 6791, Be 54, and AM 2. Be 17 and another old cluster, Lynga 7,are possibly as old as the youngest globulars. The data also suggestthat the formation rate of open clusters may have been higher early inthe history of the disk than at intermediate times since numerousclusters have survived from that time.
| Properties of the Galactic Old Open Clusters System
Not Available
| Photographic Photometry of V350 Cep
Not Available
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