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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 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.
| A BVRI CCD Photometric Study of Open Clusters IC 361 and NGC 6802
We present a CCD photometric survey of two relatively unstudied openclusters, IC 361 and NGC 6802, on the BVRI system. Color-magnitudediagrams of both clusters reveal fairly populous red giant clumps andwell-defined main sequences, extending about 4 to 5 magnitudes below themain sequence turnoff to V{ =~ }21. Comparison with evolutionary modelsallows us to estimate cluster properties such as metallicity, reddening,age, and distance. Using VandenBerg's (1985) theoretical isochrones, wefind a best fit to the color-magnitude diagram sequences for IC 361 withparameters [Fe/H]=-0.23, E(B-V)=0.66, true distance modulus (m-M)=11.98,and an age of 1.25 - 1.5 Gyr. Likewise, for NGC 6802, we find a best fitwith [Fe/H]=-0.45, true distance modulus (m-M)=10.78, and age of 1.0Gyr. The reddening for NGC 6802 is calculated to be E(B-V)=0.94, but thewidth of the main sequence relative to expected observational errorssuggests the influence of differential reddening to the cluster. Ourrevised ages for these clusters make them attractive targets forin-depth studies of both stellar evolution and the structure andchemical evolution of the Galactic disk.
| 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.
| 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.
| Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)
Not Available
| Red giants in old open clusters - A test for stellar evolution
A group of 38 among the oldest open clusters has been analyzed usingtheoretical isochrones derived by Ciardullo and Demarque (1977) andevolutionary tracks for central He-burning of Sweigart and Gross (1976).This analysis has been performed in two steps: - comparison of HRdiagrams with isochrones statistical check of the fit between observedluminosity distributions of red giant stars and the theoretical ones.The following conclusions have been reached: - clusters older than 2 or3 billion years (B-V) equal to or greater than 0.35 at MS turnoff pointgenerally agree with theoretical description and allow the determinationof reliable ages from the distribution of red giants, - for clustersyounger than 2 or 3 billion years, the theory fails in theinterpretation of their red giant stars and the statistical analysisrejects all the tested distributions. In fact, their behavior is similarto that of even younger clusters, in which the evolved stars, duringshell H-burning, have non-degenerate cores and therefore do not undergothe helium flash. It is suggested that such behavior could be attributedto overshooting from the convective core during central H-burning in themass range 1.3-2.2 solar masses.
| 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.
| Preliminary photometry of the open cluster IC 361
A preliminary photometric investigation of the open star cluster IC 361is described which demonstrates that the cluster is not exceptionallyold. The color-magnitude diagram for stars within 1 arcmin of thecluster center is found to resemble those of the intermediate-ageclusters IC 166 and NGC 752. A cluster color excess of E(B-V) = 0.55 isestimated, a distance modulus of 13.80 and a ratio of total to selectiveabsorption equal to 3.3 are adopted, and a distance of 2.5 kpc isobtained along with a linear cluster diameter of 6.5 pc. Themain-sequence turn-off point is shown to suggest that IC 361 is anintermediate-age cluster that is 500 million to 1 billion years old.
| A list of clusters that may be old.
Not Available
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