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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.
| A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings
As a follow-up to the automated cluster search carried out by Williams& Hodge, we have examined 39 Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) pointings to locate and study a comprehensivecollection of disk clusters. The Williams technique was effective infinding young clusters, but not intermediate-age or old clusters. Oursearches have shown that M31 has large numbers of these intermediate andolder open clusters, most of them undetected by both the Williams surveyand other ground-based searches. We present a catalog of 343 clustersdetected on the WFPC2 images. Extrapolation from our data indicates thatthe entire disk of M31 contains approximately 80,000 star clusters. Wehave carried out integrated multicolor photometry of these clusters toascertain their properties and to compare their properties with clustersystems of other galaxies. We show the cluster luminosity function, thecolor-magnitude diagram, the formation function, and the sizedistribution. Cluster densities and colors show trends with diskposition. An age distribution is derived and, although the ages are veryuncertain for the fainter clusters, there is evidence for clusterdynamical destruction at about the same rate as in our Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| 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.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| 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.
| Multicolour photometry and Coravel observations of stars in the southern open cluster IC 2488
We present new UBV photoelectric observations of 119 stars in the fieldof the southern open cluster IC 2488, supplemented by DDO and Washingtonphotometry and Coravel radial velocities for a sample of red giantcandidates. Nearly 50% of the stars sampled - including three red giantsand one blue straggler - are found to be probable cluster members.Photometric membership probabilities of the red giant candidates showgood agreement with those obtained from Coravel data. A mean radialvelocity of (-2.63 +/- 0.06) km s-1 is derived for thecluster giants. The reddening across the cluster is found to be uniform,the mean value being E(B-V) = 0.24 +/- 0.04. IC 2488, located at adistance of (1250 +/- 120) pc from the Sun and 96 pc below the Galacticplane, is most probably not related to the planetary nebula ESO166-PN21. A metal abundance [Fe/H] = 0.10 +/- 0.06 relative to the Sunis determined from DDO data of the red giant members, in good agreementwith the [Fe/H] values derived from five independent Washingtonabundance indices. An age of 180 Myr is determined from the fitting ofisochrones computed with convective overshooting for Z = 0.019. Theisochrone for log t = 8.25 reproduces remarkably well not only themorphology of the upper main sequence but also the observed red giantpattern.Based on observations made at Las Campanas Observatory (Chile) and CerroTololo Inter-American Observatory (Chile), National Optical AstronomyObservatories, operated by the Association of the Universities forResearch in Astronomy, Inc., under contract with the National ScienceFoundation and on observations collected with the Danish 1.54-mtelescope at the European Southern Observatory, La Silla (Chile).Tables 1 and 4b are only available in electronic from at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/543
| 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.
| Nonstellar objects in old star catalogues - Part 1.
Not Available
| 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
| 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
| Red giants in open clusters. VI. Orbits of ten spectroscopic binaries.
We present new orbits for ten (single-lined) spectroscopic binaries inseven open clusters: NGC 2489, 2567, 3033, 5822, 6134, 6664 and IC 2488based on 243 individual radial velocities obtained with the southernCORAVEL scanner. The orbital periods range from 98.5 to 3566 days. Theshortest-period orbit is circular, as expected. Seven of the binariesare confirmed cluster members, one is a possible member, and two areclearly non-members. Maximum masses from photometric separation andminimum masses from the spectroscopic orbits define the mass of thesecondaries within an interval of M_max_-M_min_=0.5Msun_. NGC6664 #54 seems to have a rather massive secondary(M>3Msun_), but the UBV colours appear normal for aluminous red giant, while significantly bluer (B-V) and (U-B) colourswould be expected. It could be a triple system, the secondary beingitself a short period binary. This paper brings the number of orbitspublished in this programme to 53, 45 of which are confirmed red-giantcluster members and 8 are non-members.
| Absolute proper motions of 181 young open clusters.
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.
| Photometric study of the southern open cluster IC 2488
A photometric study of the southern open cluster IC 2488 is presented. Adistance modulus V(o) - M(v) = 10.80 + or - 0.18 mag and a color excessE(B - V) = 0.26 + or - 0.02 are determined for the cluster by fittingthe observed main sequence to an empirical isochrone by means of apreviously-developed computer code. The location of a concentration ofpresumed red giant stars in the color-magnitude diagrams in conjunctionwith the empirical isochrones are used to estimate an age of 1.0 x 10 tothe 8th yr for the cluster. The cluster membership status of these redstars, along with a possible blue straggler and a nearby planetarynebula, are analyzed.
| Uniform survey of clusters in the southern Milky Way.
Abstract image available at:http://adsabs.harvard.edu/abs/1975AJ.....80...11V
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