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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.
| 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.
| 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.
| 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 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.
| 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].
| 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 peculiar red clump morphology in the open clusters NGC 752 and NGC 7789
The red clump stars in the open cluster NGC 752 present a peculiardistribution in the colour-magnitude diagram (CMD): the clump isobserved to present a faint extension, slightly to the blue of the mainconcentration of clump stars. We point out that a similar structure ispresent in the CMD of NGC 7789, and discuss their possible origins. Thisfeature may be understood as the result of having, at the same time,stars of low-mass which undergo the helium-flash, and those just massiveenough for avoiding it. The ages of both clusters are compatible withthis interpretation. Similar features can be produced in theoreticalmodels which assume a non-negligible mass spread for clump stars, ofabout 0.2 Msun. However, one can probably exclude that theobserved effect is due to the natural mass range of core helium burningstars found in single isochrones, although present models do not presentthe level of detail necessary to completely explore this possibility.Also the possibility of a large age spread among cluster stars can berefuted on observational grounds. We then suggest a few alternatives.This spread may be resulting either from star-to-star variations in themass-loss rates during the RGB phase. Alternatively, effects such asstellar rotation or convective core overshooting, could be causing asignificant spread in the core mass at He-ignition for stars of similarmass. Finally, we point out that similar effects could also help tounderstand the distribution of clump stars in the CMDs of the clustersNGC 2660 and NGC 2204.
| 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.
| Physical parameters of the intermediate age open cluster IC 1311: Clues for the theory of stellar evolution
The results of a UBVR photometric study in the field of the open clusterIC 1311 are presented. A comparison with previously published results onNGC 7044, based on measurements secured during the same observing run,leads to a self-consistent constraints to the parameters of the cluster.The location of selected samples of giant and unevolved dwarf members inthe U-B vs B-V diagram is used to simultaneously compute the colorexcess E(B-V) and the metallicity (Fe/H). Distance modulus and age areestimated by the Zero Age Main Sequence (ZAMS) fitting method, and bythe use of theoretical isochrones, based on models computed with andwithout the consideration of convective overshooting during the phasesof core nuclear burning. The adopted set of values is: E(B-V) = 0.28,(Fe/H) = 0.0, DM = 13.9, Age = 1.6 x 109 yr. The massfunction for the cluster main sequence has been estimated, with a slopex = -2.58, significantly steeper than the Salpeter initial mass function(IMF) (-1.35). The uncertainties in the adopted values for the clusterparameters, due to interstellar reddening, and to the calibrations oftheoretical luminosity and effective temperature in terms ofobservational quantities, are also addressed. The discrepancies betweenthe observational color-magnitude (CM) and the model isochrones, inparticular the difference between observed and predicted luminosity ofthe red giant clump (RGC), are sensibly reduced for the models withconvective core overshooting. The results indeed suggest the need ofincreasing the overshooting effectiveness beyond the values consideredup to now in evolutionary computations.
| The galactic system of old star clusters: The development of the galactic disk
The vast majority of open clusters persist as clusters for no more thana few hundred million years, but the few which survive for much longerperiods constitute a unique sample for probing the evolution of thegalactic disk. In a charge coupled device (CCD) photometric survey forpossible old open clusters combined with previously publishedphotometry, we have developed a list of 72 clusters of the age of theHyades or older (Phelps (1994). Using our version of parameters based onthe luminosity difference between the main sequence turnoff and thehorizontal branch and on the color difference between the turnoff andthe giant branch, we have calculated a 'Morphological Age Index' (MAI)for the clusters in our list and for a sample of globular clusters. Wefind that the MAI is well correlated with the logarithm of cluster ages,as determined by fitting to theoretical isochrones. We conclude that theindex is a good measure of the relative ages of both globular and openclusters, although uncertainties in the models and residual metallicityeffects prevent the use of the MAI as a definitive calibration of actualcluster ages. The age distribution of the open clusters overlaps that ofthe globular clusters, indicating that the galactic disk began todevelop toward the end of the period of star formation in the galactichalo. The open cluster age distribution can be fit approximately with atwo-component exponential decay function; one component can beidentified as the tail of the dominant population of thin disk openclusters with an age scale factor of 200 Myr, and the other consists oflonger-lived clusters with an age scale of 4 Gyr. The young openclusters are distributed on the galactic plane almost symmetricallyabout the Sun with a scale height perpendicular to the galactic plane of55 pc. The old population consists of rich clusters found only in theouter disk, nore than RGC = 7.5 kpc from the galactic center;this population has a scale height of 375 pc. After accounting for thetwo exponential distributions of cluster ages, there are indications ofan excess of clusters in the age range of 5-7 Gyr; there may have beenlarge bursts of star formation in that period, or perhaps a largerproportion of the clusters forming at that time had advantageous orbitsfor survival. Either circumstance is consistent with the idea that thegalactic disk has been repeatedly disturbed, possibly in collisions orother interactions with external systems, resulting in the occasionalformation of clusters with relatively large velocities perpendicular tothe plane; these are the clusters that have survived until the present.Finally, the repeated accretion of low angular momentum material ontothe disk from the halo or beyond would also explain the observed radialcomposition gradient and the lack of a correlation between open clustermetallicity and age found by Friel & Janes (1993).
| 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.
| 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.
| A project for investigation of the stellar population of the galactic disk.
Not Available
| CCD BVR photometry of the open cluster IC 1311
Six CCD frames for two fields in the area of the rich, presumably old,and unstudied open cluster IC 1311 have been obtained in the BVR bands.The photometric analysis leads to an estimated age of between 1 Gyr(standard models) and 2 Gyr (overshooting models), placing the clusterat a distance of about 4.8 kpc from the sun. Comparison of theobservational color-magnitude diagram (CMD) with model isochronesproduces a good fit for a metallicity of Z = 0.006-0.008. Somemorphological features displayed by the CMD of this cluster are betterunderstood in terms of convective overshooting rather than by classicaltheories. The location of IC 1311 on the age-metallicity diagramsuggests an ecological environment for its formation more similar tothat found in the LMC than in the solar vicinity.
| Photographie extrem schwacher H II-Regionen. Teil 2.
Not Available
| Catalogue of Eclipsing and Spectroscopic Binary Stars in the Regions of Open Clusters
Not Available
| Open clusters and galactic structure
A total of 610 references to 434 clusters are employed in thecompilation of a catalog of open clusters with color-magnitude diagramson the UBV or RGU systems. Estimates of reddening, distance modulus, ageand number of cluster members are included. Although the sample isconsidered representative of the discoverable clusters in the galaxy,the observed distribution is nonuniform because of interstellarobscuration. Cluster distribution in the galactic plane is found to bedominated by the locations of dust clouds rather than by spiralstructure. The distributions of clusters as a function of age andrichness class show that the lifetimes of poor clusters are much shorterthan rich ones, and that clusters in the outer disk survive longer thanthose in the inner disk. An outer disk age which is only about 50% theage of the globular clusters is indicated by cluster statistics. Thethickening of the galactic disk with increasing galactocentric distancemay be due to either a younger dynamical age or a lower gravitationalpotential in the outer regions.
| A list of clusters that may be old.
Not Available
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