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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.
| Old open clusters as key tracers of Galactic chemical evolution. I. Fe abundances in NGC 2660, NGC 3960, and Berkeley 32
Aims.We obtained high-resolution UVES/FLAMES observations of a sample ofnine old open clusters spanning a wide range of ages and Galactocentricradii. The goal of the project is to investigate the radial metallicitygradient in the disk, as well as the abundance of key elements (αand Fe-peak elements). In this paper we present the results for themetallicity of three clusters: NGC 2660 (age ~1 Gyr, Galactocentricdistance of 8.68 kpc), NGC 3960 (~1 Gyr, 7.80 kpc), and Be 32 (~6-7 Gyr,11.30 kpc). For Be 32 and NGC 2660, our study provides the firstmetallicity determination based on high-resolution spectra.Methods: .We performed equivalent width analysis with the spectral codeMOOG, which allows us to define a metallicity scale and build ahomogeneous sample. Results: .We find that NGC 3960 and NGC 2660have a metallicity that is very close to solar ([Fe/H] = +0.02 and+0.04, respectively), while the older Be 32 turns out to have [Fe/H] =-0.29.
| Survey for Transiting Extrasolar Planets in Stellar Systems. IV. Variables in the Field of NGC 1245
The Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS)project is a search for planetary transits in open clusters. In thispaper we analyze the STEPSS observations of the open cluster NGC 1245 todetermine the variable-star content of the cluster. Out of 6787 starsobserved with V<22, of which ~870 are cluster members, we find 14stars with clear intrinsic variability that are potential clustermembers and 29 clear variables that are not cluster members. None ofthese variables have been previously identified. We present lightcurves, finder charts, and stellar/photometric data on these variableobjects. Several of the interacting binaries have estimated distancesconsistent with the cluster distance determined from isochrone fits tothe color-magnitude diagram. Four stars at the main-sequence turnoff ofthe cluster have light curves consistent with γ Doradusvariability. If these γ Doradus candidates are confirmed, theyrepresent the oldest and coolest members of this class of variablediscovered to date.
| Inverting Color-Magnitude Diagrams to Access Precise Star Cluster Parameters: A Bayesian Approach
We demonstrate a new Bayesian technique to invert color-magnitudediagrams of main-sequence and white dwarf stars to reveal the underlyingcluster properties of age, distance, metallicity, and line-of-sightabsorption, as well as individual stellar masses. The advantages ourtechnique has over traditional analyses of color-magnitude diagrams areobjectivity, precision, and explicit dependence on prior knowledge ofcluster parameters. Within the confines of a given set of often-usedmodels of stellar evolution, a single mapping of initial to finalmasses, and white dwarf cooling, and assuming photometric errors thatone could reasonably achieve with the Hubble Space Telescope, ourtechnique yields exceptional precision for even modest numbers ofcluster stars. For clusters with 50-400 members and one to a few dozenwhite dwarfs, we find typical internal errors ofσ([Fe/H])<=0.03 dex, σ(m-MV)<=0.02 mag, andσ(AV)<=0.01 mag. We derive cluster white dwarf ageswith internal errors of typically only 10% for clusters with only threewhite dwarfs and almost always <=5% with 10 white dwarfs. Theseexceptional precisions will allow us to test white dwarf cooling modelsand standard stellar evolution models through observations of whitedwarfs in open and globular clusters.WIYN Open Cluster Study XXIII. The WIYN Observatory is a joint facilityof the University of Wisconsin-Madison, Indiana University, YaleUniversity, and the National Optical Astronomy Observatory.
| 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.
| FLAMES observations of old open clusters: Constraints on the evolution of the Galactic disc and mixing processes in stars.
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| 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.
| Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
| Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic openclusters are studies in this work, in which 27 Galactic open clusters ofages >=1 Gyr are selected as the working sample. Based on thephotometric observations of these open clusters, a synthetic integratedspectrum has been made for the stellar population of each cluster. Theeffects of blue straggler (BS) stars on the conventional simple stellarpopulation (SSP) model are analyzed on an individual cluster basis. Itis shown that the BSs, whose positions in the color-magnitude diagramscannot be predicted by the current single-star evolution theory, requiresignificant modifications to the integrated properties of theoreticalSSP model. The synthesized integrated spectral energy distributions(ISEDs) of our sample clusters are dramatically different from those ofSSPs based on an isochrone only. The BS-corrected ISEDs of stellarpopulations show systematic enhancements toward shorter wavelengths inthe spectra. When measured with broadband colors in unresolvableconditions, the age of a stellar population can be seriouslyunderestimated by the conventional SSP model. Therefore, considering thecommon existence of BS components in real stellar populations, we shouldexpect considerable alterations of the conventional ISEDs when we applythe technique of evolutionary population synthesis to more complicatedstellar systems.
| EXPLORE/OC: Variable Stars in the Southern Galactic Open Cluster NGC 2660
The EXPLORE/OC Project is a photometric monitoring survey of Galacticsouthern open clusters (OCs) with the aim to find transiting close-inextrasolar giant planets. A by-product of this search is the discoveryof a wide range of different variable stars. Our search for planets inthe open cluster NGC 2660 has produced several tens of variables in thefield of the OC. In this presentation, we show the light curves of thevariable stars, their locations in the field and color-magnitude diagramof the cluster, and give our estimates concerning cluster membership.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| UBVIJHK Photometric Study of the Open Cluster NGC 2849
We present UBVIJHK photometry of the open cluster NGC 2849.Color-magnitude diagrams (CMDs) of this cluster show a well-defined mainsequence and a sparse red giant branch. In spite of a relatively largecolor excess of E(B-V)=0.50+/-0.04, our color-color analysis shows thatthe interstellar reddening toward this cluster is compatible with thestandard extinction model. The cluster also has a broad main sequenceindicative of binary sequence rather than differential reddening orfield contamination, as suggested elsewhere. From the ultravioletexcess, we estimate the metallicity to be [Fe/H]=-0.24+/-0.12. Adistance modulus of (m-M)0=13.93+/-0.17 is obtained fromzero-age main-sequence fitting. A CMD comparison with the Paduatheoretical models published by Bertelli and coworkers gives an age oflogt=8.8+/-0.1.
| 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.
| Star cluster ecology - V. Dissection of an open star cluster: spectroscopy
We have modelled in detail the evolution of rich open star clusterssuch as NGC 2516, NGC 2287, Pleiades, Praesepe, Hyades, NGC 2660 and3680, using simulations that include stellar dynamics as well as theeffects of stellar evolution. The dynamics is modelled via direct N-bodyintegration, while the evolution of single stars and binaries isfollowed through the use of fitting formulae and recipes. The feedbackof stellar and binary evolution on the dynamical evolution of thestellar system is taken into account self-consistently.Our model clusters dissolve in the tidal field of the Galaxy in atime-span of the order of a billion years. The rate of mass loss israther constant, ~1 Msolar per million years. The binaryfraction at first is nearly constant in time, then increases slowly nearthe end of a cluster's lifetime. For clusters which are more than about108 yr old the fractions of stars in the form of binaries,giants and merger products in the inner few core radii are considerablyhigher than in the outer regions, beyond the cluster's half-mass radius.When stars with masses >~2 Msolar escape from the cluster,they tend to do so with velocities higher than average.The stellar merger rate in our models is roughly one per 30 millionyears. Most mergers are the result of unstable mass transfer in closebinaries (~70 per cent), but a significant minority are caused by directencounters between single and binary stars. While most mergers occurwithin the cluster core, even beyond the half-mass radius stellarmergers occasionally take place. We notice a significant birth rate ofX-ray binaries, most containing a white dwarf as the mass acceptor. Wealso find one high-mass X-ray binary with a neutron-star accretor. Ifformed and retained, black holes participate in many (higher-order)encounters in the cluster centre, resulting in a large variety of exoticbinaries. The persistent triple and higher-order systems formed in ourmodels by dynamical encounters between binaries and single stars are notrepresentative for the multiple systems observed in the Galactic disc.We conclude that the majority of multiples in the disc probably formedwhen the stars were born, rather than through later dynamicalinteractions.
| 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.
| 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.
| 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.
| Integrated spectral evolution of Galactic open clusters
We present a library of 47 open-cluster integrated spectra, mostlyobtained at CASLEO (San Juan, Argentina) in the range 3600 < λ< 7400 Å, which are made available at CDS. The data arecombined with previous spectra to obtain 10 high signal-to-noise ratiobasic templates in the young and intermediate-age domains, which arealso provided in the library. These Galactic disc templates representthe increased time resolution spectral evolution of a stellar populationunit around the Solar metallicity level. The improved signal-to-noiseratio of the present templates with respect to previous template lists,together with their increased time resolution, allowed us to improve thefundamental parameters of some open clusters. The present spectrallibrary will be useful for several astrophysical applications,particularly for population syntheses of star-forming giant galaxies.
| Brown dwarf populations in open clusters
We present the results of multiple simulations of open clusters,modelling the dynamics of a population of brown dwarf members. Weconsider the effects of a large range of primordial binary populations,including the possibilities of having brown dwarf members containedwithin a binary system. We also examine the effects of various clusterdiameters and masses. Our examination of a population of wide binarysystems containing brown dwarfs, reveals evidence for exchange reactionswhereby the brown dwarf is ejected from the system and replaced by aheavier main-sequence star. We find that there exists the possibility ofhiding a large fraction of the brown dwarfs contained within theprimordial binary population. We conclude that it is probable that themajority of brown dwarfs are contained within primordial binary systemswhich then hides a large proportion of them from detection.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| Search for star clusters close to the Galactic plane with DENIS
An automated search for star clusters close to the Galactic plane (|b|< 5o) was carried out on the Point Source Catalogue of theDENIS survey. 44% of the Galactic plane have been observed andcalibrated. The method allowed to retrieve 22 known star clusters and toidentify two new ones, not published yet although previously presentedin the 2MASS web site as embedded clusters in HII regions. Extinction inthe field and in front of the clusters are estimated using a model ofpopulation synthesis. We present the method and give the properties ofthese clusters. Based on observations collected at ESO La Silla.
| 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.
| Pismis 2: a poorly studied, intermediate-age open cluster
We present CCD BVI photometry of the intermediate-age open clusterPismis 2, covering from the brighter red giants to about 5mag below themain-sequence turn-off. The cluster is heavily reddened and is possiblyaffected by a differential reddening of ΔE(B-V)~=0.04-0.06. Usingthe synthetic colour-magnitude diagram method, we estimate the distancemodulus, (m-M)0~=12.5-12.7, and age, τ~=1.1-1.2Gyr, in aself-consistent way. The cluster probably contains a significantfraction of binary systems. The metallicity is most likely solar and thereddening E(B-V) ranges between 1.26 and 1.32 depending on the clusterregion.
| 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].
| Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259
We present accurate radial velocities and photoelectric UBV photometryfor 73 and 57 red-giant candidates, respectively, in theintermediate-age open clusters NGC 2324, 2818, 3960 and 6259. These dataconfirm the membership of 47 stars, 12 of which (26%) are spectroscopicbinaries; three preliminary orbits have been determined in NGC 3960.From Washington photometry of 8 red giant members, the metallicity ofNGC 6259 is found to be [Fe/H] = 0.06 +/- 0.08. At the age of theseclusters, most of the red giants are observed in the core-helium (clump)burning phase, the general morphology of which is well reproduced bytheoretical models with convective overshooting. However, a number ofbona fide cluster giant members are found significantly to the red ofthe isochrones fitting the rest of the CMD of these and a few otherclusters. Some of these stars are binaries, but others seem to besingle. In either case, their red colours and/or low luminosities remainunexplained by current stellar evolution theory. Based on observationscollected with the Danish 1.54-m telescope at the European SouthernObservatory, La Silla (Chile) and at Cerro Tololo Inter-AmericanObservatory, National Optical Astronomy Observatories, operated by theAssociation of the Universities for Research in Astronomy, Inc., undercontract with the National Science Foundation.
| Star cluster ecology - IV. Dissection of an open star cluster: photometry
The evolution of star clusters is studied using N-body simulations inwhich the evolution of single stars and binaries is takenself-consistently into account. Initial conditions are chosen torepresent relatively young Galactic open clusters, such as the Pleiades,Praesepe and the Hyades. The calculations include a realistic massfunction, primordial binaries and the external potential of the parentGalaxy. Our model clusters are generally significantly flattened by theGalactic tidal field, and dissolve before deep core collapse occurs. Thebinary fraction decreases initially because of the destruction of softbinaries, but increases later because lower mass single stars escapemore easily than the more massive binaries. At late times, the clustercore is quite rich in giants and white dwarfs. There is no evidence forpreferential evaporation of old white dwarfs. On the contrary, the whitedwarfs formed are likely to remain in the cluster. Stars tend to escapefrom the cluster through the first and second Lagrange points, in thedirection of and away from the Galactic Centre. Mass segregationmanifests itself in our models well within an initial relaxation time.As expected, giants and white dwarfs are much more strongly affected bymass segregation than main-sequence stars. Open clusters are dynamicallyrather inactive. However, the combined effects of stellar mass-loss andevaporation of stars from the cluster potential drive the dissolution ofa cluster on a much shorter time-scale than if these effects areneglected. The often-used argument that a star cluster is barely olderthan its relaxation time and therefore cannot be dynamically evolved isclearly in error for the majority of star clusters. An observation of ablue straggler in an eccentric orbit around an unevolved star or a bluestraggler of more than twice the turn-off mass might indicate pastdynamical activity. We find two distinct populations of blue stragglers:those formed above the main-sequence turn-off, and those which appear asblue stragglers as the cluster's turn-off drops below the mass of therejuvenated star.
| 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.
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