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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
| The Open Cluster NGC 7789. II. CCD VI Photometry
A (V, V-I)-diagram for the intermediate-age open cluster NGC 7789 hasbeen derived from CCD observations of more than 15,000 stars within ~18'of the cluster center. From the brightest giants and blue stragglers atV~11 to the faintest lower main-sequence stars that were observed (atV~21, M_V~9), the color-magnitude diagram is well defined. A prominentclump of core helium-burning stars is evident at V=13.0, and the upperend of the main sequence shows a fairly pronounced curvature to the red,which is indicative of significant convective core overshooting. Indeed,comparisons with up-to-date stellar models show that it is not possibleto explain the observed morphology in the vicinity of the turnoff unlessthe overshooting is quite extensive. Interestingly, if sufficientovershooting is assumed in order to match the main-sequence data, it isnot possible to reproduce the cluster's extended giant branch unless thecluster age is at least 1.6 Gyr (assuming a metallicity in the range-0.2<=[Fe/H]<=0.0). This, in turn, requires that the cluster havean apparent distance modulus (m-M)_V<=12.2. Thus, sometime within thepast few hundred million years, the ignition of helium burning in NGC7789 has switched from a quiescent to an explosive (``flash'')phenomenon, and the length of the cluster's red giant branch has beensteadily increasing with the passage of time since then. Frommain-sequence fits to models that have been carefully normalized to theSun, we infer a reddening 0.35<=E(V-I)<=0.38.
| Radial velocities of blue stragglers. 1: A catalog of candidates in six open clusters
We discuss the observational definition of blue stragglers and present acatalog of 58 blue-straggler candidates in six open clusters (NGC 752,2360, 2420, 2682, 6939, and 7789) chosen to cover the range of ages from1.6 to 5 Gyr. Candidates which are nonmembers based on proper motionsare not included in the catalog. New radial-velocity observations areused to identify additional candidates which are nonmembers. We findthat the blue-straggler samples in 6939 and 7789 are significantlycontaminated by nonmembers.
| Blue stragglers - A search for binaries in the infrared
Infrared photometry in the J, H and K bands, performed on a sample of 33blue stragglers (BS) belonging to 8 open clusters is presented. Thisphotometry has been combined with published Johnson UBV photometry tosearch for possible cold companions among the BS, as expected for Case Bclose binary evolution. Probable binaries were identified by comparisonwith standard mean color relations for single stars. The number ofbinary detections is discussed together with the results of a previousspectroscopic search for binaries among BS (Manteiga et al. 1989), andthe consequences for the hypothesis of binarity for the origin of BS areanalyzed.
| UVBY photometry of blue stragglers in NGC 7789
uvby photometry of 28 blue stragglers in the open cluster NGC 7789 ispresented and analyzed. A rediscussion of the fundamental clusterparameters is presented. Comparison of these results with those from theblue stragglers indicates that standard photometric calibrations doapply to the stragglers. It is found that the extended halo of bluestragglers around NGC 7789 is composed predominantly of field stars, andthat the radial distribution of the stragglers is similar to that of thered giants, implying that the progenitors of both classes of stars havethe same mass, i.e., single stars near the cluster turnoff. Whencombined with weak photometric evidence for a lower bound to theluminosity distribution of the stragglers in the C-M diagram, it isconcluded that the best current model for these enigmatic stars is thatof a single star which has mixed internally, as delineated in the workof Saio and Wheeler (1980). Possibilities for future research areoutlined.
| The Blue Stragglers in NGC7789 and the Binary POST Main Sequence Mass Exchange Hypothesis
Abstract image available at:http://adsabs.harvard.edu/abs/1980PASP...92..682M
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