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Oscillating blue stragglers, γ Doradus stars and eclipsing binaries in the open cluster NGC 2506
Context: This is the first step in a project to combine studies ofeclipsing binaries and oscillating stars to probe the interior of BlueStragglers (BS). This may imply a way to discriminate observationallybetween different birth mechanisms of BS stars. Aims: We study theopen cluster NGC 2506 which contains oscillating BS stars and detachedeclipsing binaries for which accurate parameters can be derived. Thiswill tightly constrain the cluster isochrone and provide an absolutemass, radius and luminosity-scale for the cluster stars along with thecluster age, metallicity and distance. The present work focuses onobtaining the light curves of the binaries and determine their orbitalperiods, on obtaining power spectra of the oscillating BS stars toselect targets for follow-up studies, and on searching for γDoradus type variables which are also expected to be present in thecluster. Methods: With a two-colour, dual-site photometric campaign weobtained 3120 CCD-images of NGC 2506 spread over four months. Weanalysed the BI time-series of the oscillating stars and usedsimulations to derive statistical uncertainties of the resultingfrequencies, amplitudes and phases. A preliminary mode-identificationwas performed using frequency ratios for the oscillating BS stars, andamplitude ratios and phase differences for a population of newlydetected γ Doradus stars. Results: We quadrupled the number ofknown variables in NGC 2506 by discovering 3 new oscillating BS stars,15 γ Doradus stars and four new eclipsing binaries. The orbitalperiods of 2 known, detached eclipsing binaries were derived. Wediscovered a BS star with both p-mode and g-mode variability and weconfronted our γ Doradus observations with state-of-the-artseismic models, but found significant discrepancy between theory andobservations. Conclusions: . NGC 2506 is an excellent target forasteroseismic tests of stellar models, as strong external constraintscan be imposed on the models of a population of more than 20 oscillatingstars of different types.Based on observations obtained at the Flemish Mercator telescope on LaPalma, Spain, the Danish 1.5-m telescope at ESO, La Silla, Chile, and onobservations collected at the European Southern Observatory, Paranal,Chile (ESO Programme 075.D-0206(B)). Catalog of individual photometrymeasurements for all variables is only available in electronic form atthe 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/465/965
| K-band magnitude of the red clump as a distance indicator
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
| Time-series photometry of the open cluster NGC1817
We present light curves of 321 stars in the open cluster NGC1817. Thedata were obtained in a search for pulsating stars, mainly of the deltaScuti type, using CCD observations collected at the Nordic OpticalTelescope at La Palma, Spain. 12 delta Scuti stars (11 of which could beconfirmed as cluster members), 2 variables of unknown type and 2eclipsing binary systems were found.We describe the observations, data reduction and colour-transformationprocedures, and give the resulting light curves in tabular form. We alsopresent sets of B and V stacked images obtained at low air masses.
| The Overall Properties of Open Clusters Located Towards The Galactic Anticenter Direction: Washington Photometry of NGC 1817 and NGC 2251
We present Washington photometry for red giant candidates in the openclusters NGC 1817 and NGC 2251. Coravel radial velocities are used toseparate field stars from cluster giants. Effective temperatures andmetallicities are derived for each giant star. From new UBV and DDOdata, we also derive reddening and metal content for NGC 2251. We find[Fe/H] = -0.33+/-0.08 and -0.20+/-0.05 for NGC 1817 and NGC 2251,respectively. We reexamine the overall properties of a sample of 30clusters in the Galactic anticenter direction with distances, ages andmetallicities available. No evidence for an age-metallicity relation isfound. However, a radial abundance gradient of -0.093 dex kpc^{-1} isderived over a Galactocentric distance of 14 kpc. This value practicallydoes not change when all clusters with basic parameters known up to thisdate are considered.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| 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.
| Pinpointing isochrones in clusters .
Detached eclipsing binaries allow the determination of accurate stellarmasses and radii. Here we present the first results of a programme whichaims at using such systems for the determination of accurate stellarparameters in open star clusters and discuss this in relation to thestudy of pulsating stars. As an example we show results for a detachedeclipsing system in the old open cluster NGC 188 and briefly discuss thetwo intermediate age open clusters NGC 1817 and NGC 2506 which bothcontains pulsating stars and detached eclipsing binaries.
| Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters
New photometric data in the Washington system are presented for redgiant candidates in NGC 1817 and 2251, two open clusters located towardsthe Galactic anticentre direction. In the case of NGC 2251, theWashington data are supplemented with new UBV and David DunlapObservatory (DDO) photoelectric photometry. Published radial velocitiesare used to separate field stars from cluster giants. The photometricdata yield an effective temperature and metal abundance for each clustermember. Five independent Washington abundance indices yield meanmetallicities of [Fe/H]= 0.25 +/- 0.04 for NGC 1817 and 2251,respectively. From combined BV and DDO data, we also derive E(B-V) =0.21 +/- 0.03 and [Fe/H]DDO=-0.14 +/- 0.05 for NGC 2251. Bothobjects are then found to be on the metal-poor side of the distributionof open clusters, their metallicities being compatible with theexistence of a radial abundance gradient in the disc. Using the WEBDAOpen Cluster data base and the available literature, we re-examined theoverall properties of a sample of 30 clusters located towards theGalactic anticentre with the distances, ages and metallicitiesavailable. This cluster sample presents no evidence of an abundancegradient perpendicular to the Galactic plane, nor is an age-metallicityrelation found. However, a radial abundance gradient of -0.093 dexkpc-1 is derived over a Galactocentric distance of 14 kpc, agradient which is in keeping with most recent determinations. This valuepractically does not change when all clusters with basic parametersknown up to this date are considered.
| 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.
| uvby Hβ CCD photometry and membership segregation of the open cluster NGC 2548; gaps in the Main Sequence of open clusters
Deep CCD photometry in the uvby-Hβ intermediate-bandsystem is presented for the cluster NGC 2548 (M 48). A completemembership analysis based on astrometric and photometric criteria isapplied. The photometric analysis of a selected sample of stars yields areddening value of E(b-y)= 0.06±0.03, a distance modulus ofV_0-M_V= 9.3±0.5 (725 pc) and a metallicity of [ Fe/H] = - 0.24±0.27. Through isochrone fitting we find an age of log t = 8.6±0.1 (400 Myr). Our optical photometry and JHK from 2MASS arecombined to derive effective temperatures of cluster member stars. Theeffective temperature distribution along the main sequence of thecluster shows several gaps. A test to study the significance of thesegaps in the main sequence of the HR diagram has been applied. The methodis also applied to several other open clusters (Pleiades, Hyades, NGC1817 and M 67) to construct a sequence of metallicities and ages. Thecomparison of the results of each cluster gives four gaps with highsignificance (one of them, centred at 4900 K, has not been previouslyreported).
| A Dozen δ Scuti Stars in the Open Cluster NGC 1817
We obtained high-precision time-series observations of stars in the opencluster NGC 1817 in order to find δ Scuti stars among the clustermembers. The detection of 12 δ Scuti stars, of which our datasuggest that 11 are cluster members, makes NGC 1817 a key target forasteroseismology of 1.5-2.5 Msolar stars. One of the clustermember δ Scuti stars is also an eclipsing binary, potentiallyoffering very strong constraints for theoretical modeling. In addition,we find one δ Scuti star candidate, a γ Dor candidate, twovariables of currently unknown type, and two eclipsing binary systems.We also describe a method for combining the oscillation frequencies ofseveral δ Scuti stars in a single cluster to obtain informationabout the order n of the excited modes. This method will eventually beused to constrain the theoretical models to be compared with preciseoscillation frequencies determined from a future multisite campaign(asteroseismology).
| Morphology of Galactic Open Clusters
We analyzed the shapes of Galactic open clusters by the star-countingtechnique with the Two Micron All Sky Survey star catalog database.Morphological parameters such as the ellipticity and size have beenderived via stellar density distribution, weighed by clusteringprobability. We find that most star clusters are elongated, even for theyoungest star clusters of a few million years old, which are locatednear the Galactic disk. The shapes of young star clusters must reflectthe conditions in the parental molecular clouds and during the clusterformation process. As an open cluster ages, stellar dynamics cause theinner part of the cluster to circularize, but the overall radius getslarger and the stellar density becomes sparser. We discuss how theinternal relaxation process competes with Galactic external perturbationduring cluster evolution.
| uvby-Hβ CCD photometry of NGC 1817 and NGC 1807
We have investigated the area of two NGC entries, NGC 1817 and NGC 1807,with deep CCD photometry in the uvby-Hβintermediate-band system. The photometric analysis of a selected sampleof stars of the open cluster NGC 1817 yields a reddening value of E(b-y)= 0.19±0.05, a distance modulus of V0-MV =10.9±0.6, a metallicity of [Fe/H] = -0.34±0.26 and an ageof log t = 9.05±0.05. Our measurements allow us to confirm thatNGC 1807 is not a physical cluster.Figure \ref{map} and Tables \ref{log} and \ref{chips} are only availablein electronic form at http://www.edpsciences.orgTables 4 and 5 are only available in electronic form 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/426/827
| New membership determination and proper motions of NGC 1817. Parametric and non-parametric approach
We have calculated proper motions and re-evaluated the membershipprobabilities of 810 stars in the area of two NGC objects, NGC 1817 andNGC 1807. We have obtained absolute proper motions from 25 plates in thereference system of the Tycho-2 Catalogue. The plates have a maximumepoch difference of 81 years; and they were taken with the doubleastrograph at Zo-Sèstation of Shanghai Observatory, which has anaperture of 40 cm and a plate scale of 30 arcsec mm-1. Theaverage proper motion precision is 1.55 mas yr-1. Theseproper motions are used to determine the membership probabilities ofstars in the region, based on there being only one very extendedphysical cluster: NGC 1817. With that aim, we have applied and comparedparametric and non-parametric approaches to cluster/field segregation.We have obtained a list of 169 probable member stars.Table 5 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/819Tables 1, 2 and 4 and Fig. 1 are only available in electronic form athttp://www.edpsciences.org
| Red giants in open clusters. XI. Membership, duplicity, and structure of NGC 2477
New, accurate radial velocities and photoelectric UBV photometry of 83red-giant candidates in the field of the rich, intermediate-age opencluster NGC 2477 ([Fe/H]= -0.05, age ≃1 Gyr)are presented and discussed. From 49 constant-velocity members we find amean cluster velocity of +7.32±0.13 km s-1 and confirmthe membership of 76 of the stars. Among the cluster members, weidentify 26 definite and 1 probable spectroscopic binaries and determineorbits for 13 of these systems, with periods ranging from 40 to 4578days. The binary frequency is thus rather high (27/76 = 36%). Theobserved internal radial velocity dispersion of the cluster, asdetermined from the single member stars, is 0.93 km s-1,corrected for the small average observational error of 0.22 kms-1. Fitting King-type models to the observed stellar densitydistribution and velocity dispersion, and assuming a distance of 1.25kpc, we find the core and tidal radii of NGC 2477 tobe 1.8 and 8.1 pc, respectively, and estimate that the mass of clusterstars down to V = 17, corresponding to 1 Mȯ, is atleast 5400 Mȯ. The substantial differential reddening ofNGC 2477 requires a more detailed study beforedefinitive isochrone fits can be made.Based on observations collected with the Danish 1.54-m and ESO 1-mtelescopes at the European Southern Observatory, La Silla, Chile, andwith the University of Toronto 0.6-m telescope at Las CampanasObservatory, Chile.Full Tables 1 and 2 are only available in electronic form 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/423/189
| 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.
| 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.
| WIYN Open Cluster Study - XVI. Optical/infrared photometry and comparisons with theoretical isochrones
We present combined optical/near-infrared photometry (BVIK) for six openclusters - M35, M37, NGC 1817, NGC 2477, NGC 2420 and M67. The openclusters span an age range from 150 Myr to 4 Gyr and have metalabundances from [Fe/H]=-0.27 to +0.09 dex. We have utilized these datato test the robustness of theoretical main sequences constructed byseveral groups as denoted by the following designations - Padova,Baraffe, Y2, Geneva and Siess. The comparisons of the modelswith the observations have been performed in the [MV,(B-V)0], [MV, (V-I)0] and[MV, (V-K)0] colour-magnitude diagrams as well asthe distance-independent [(V-K)0, (B-V)0] and[(V-K)0, (V-I)0] two-colour diagrams. We concludethat none of the theoretical models reproduces the observational data ina consistent manner over the magnitude and colour range of the unevolvedmain sequence. In particular, there are significant zero-point and shapedifferences between the models and the observations. We speculate thatthe crux of the problem lies in the precise mismatch between theoreticaland observational colour-temperature relations. These results underscorethe importance of pursuing the study of stellar structure and stellarmodelling with even greater intensity.
| 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.
| Red giants in open clusters. X. NGC 1817
Radial-velocity and BV CCD observations of 29 red-giant candidates inthe central part of the intermediate-age open cluster NGC 1817 haveconfirmed the membership of 21 stars. In addition, we have used radialvelocities and new UBVR photoelectric photometry to investigate themembership of 59 new candidates located in an outer corona surroundingNGC 1817, identified from the POSS plates. We have confirmed 18 newmembers out to a distance of 27arcmin from the centre, thus doublingthe previous cluster radius. Ten definite spectroscopic-binary membershave been discovered, corresponding to a binary frequency of 25.6% andfour orbits have been determined so far. Three additional stars havebeen identified as possible spectroscopic-binary members. We derive acluster mean velocity of +65.33 +/- 0.09 (0.52 s.e.) km s-1based on 29 members with no detected velocity variations plus the 3binary members with orbital solutions. The radial distribution of the 39red-giant members shows a strong concentration of spectroscopic binariesin the inner part of the cluster, similar to that observed in M 67,although NGC 1817 is much younger.Based on observations collected at the Haute-Provence Observatory(France), and with the Jacobus Kapteyn Telescope, operated on the islandof La Palma by PPARC at the Spanish Observatorio del Roque de losMuchachos. Some of the radial-velocity observations were obtained withthe Multiple Mirror Telescope, a joint facility of the SmithsonianInstitution and the University of Arizona.
| Search for variable stars in the open cluster NGC 2539
We present UBVI absolute and V-band time-series CCD photometric resultsfor the intermediate-age open cluster NGC 2539. From the empiricalzero-age main sequence and theoretical isochrone fitting in color-colorand color-magnitude diagrams, we estimated the physical parameters ofthis cluster as follows: the color excess of E(B-V)=0.06 +/- 0.03, thedistance modulus of (V-MV)0=10.2 +/- 0.1 and theage of log t = 8.8 ( ~ 630 Myr). By carefully examining the time-seriesimages, we discovered seven new variable stars in the observed clusterfield. Considering the light curves, periods and positions on thecolor-magnitude diagram, we classified them as five eclipsing binarystars, one delta Scuti star and one gamma Doradus candidate. The gammaDoradus candidate might not be a cluster member because it is locatedredder by about Delta (B-V) = 0.2 than the cool edge of gamma Doradusinstability strip.
| 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.
| Metallicities of Old Open Clusters
We present radial velocities and metallicities for a sample of 39 openclusters with ages greater than about 700 million years. For 24 clustersnew moderate-resolution spectroscopic data obtained with multiobjectspectrographs on the Kitt Peak National Observatory and the Cerro TololoInter-American Observatory 4 m telescopes are used to determine radialvelocities and mean cluster metallicities. These new results arecombined with data published previously by Friel & Janes to providea sample of 459 giants in 39 old open clusters, which are used toinvestigate radial abundance gradients in the Galactic disk. Based on anupdated abundance calibration of spectroscopic indices measuring Fe andFe-peak element blends, this larger sample yields an abundance gradientof -0.06+/-0.01 dex kpc-1 over a range in Galactocentricradius of 7 to 16 kpc. There is a slight suggestion of a steepening ofthe abundance gradient with increasing cluster age in this sample, butthe significance of the result is limited by the restricted distancerange for the youngest clusters. The clusters show no correlation ofmetallicity with age in the solar neighborhood. Consistent with theevidence for a steepening of the gradient with age, the clusters in theouter disk beyond 10 kpc show a suggestion at the 1.5 σ level of adependence of metallicity on age.
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/168
| WIYN Open Cluster Study. X. The K-Band Magnitude of the Red Clump as a Distance Indicator
In an effort to improve the utility of the helium-burning red clumpluminosity as a distance indicator, we explore the sensitivity of theK-band red clump absolute magnitude [MK(RC)] to metallicityand age. We rely upon JK photometry for 14 open clusters and twoglobular clusters from the Second Incremental Data Release of the TwoMicron All Sky Survey (2MASS) Point Source Catalog. The distances,metallicities, and ages of the open clusters are all on an internallyconsistent system, while the K(RC) values are measured from the 2MASSdata. For clusters younger than ~2 Gyr, MK(RC) is insensitiveto metallicity but shows a dependence on age. In contrast, for clustersolder than ~2 Gyr, MK(RC) is influenced primarily by themetallicity of the population and shows little or no dependence on theage. Theoretical red clump models based on the formalism of Girardi etal. reinforce this finding. Over comparable metallicity and age ranges,our average MK(RC) value is in accord with that based onsolar neighborhood red clump stars with Hipparcos parallaxes. Lastly, wecompute the distance to the open cluster NGC 2158 using our red clumpcalibration. Adopting an age of 1.6+/-0.2 Gyr and [Fe/H]=-0.24+/-0.06,our calibration yields a distance of (m-M)V=14.38+/-0.09.
| 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.
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| Statistics and supermetallicity: The metallicity of NGC 6791
For the old galactic cluster NGC 6791, Peterson & Green(\cite{pg98a}) and Chaboyer et al. (\cite{cgl99}) have found that [Fe/H]~ +0.4 dex. A second look at that conclusion is taken in this paper.Zero-point problems are reviewed for a high-dispersion analysis done byPeterson & Green, and it is found that accidental errors have notbeen determined rigorously for the results of that analysis. It is alsonoted that in a color-magnitude analysis performed by Chaboyer et al.,the important metallicity range between 0.0 and +0.3 dex is not exploredand hence is not ruled out. Moreover, that analysis does not yieldstatistically rigorous results, and it appears that such results may notbe produced in color-magnitude analysis of clusters in general. Resultsin the two cited papers and elsewhere are re-evaluated statistically,with an allowance being made for uncertainty in the cluster reddening.Apparently the best that can be said at present is that the clustermetallicity lies in the range from +0.16 to +0.44 dex. This conclusionis stressed by reviewing the immaturity of the underlying data base. Thepremature conclusion for a high metallicity turns out to be due largelyto neglect of accidental errors, though a tendency to ascribe too muchweight to high derived metallicities may also play a role.
| M2000: An astrometric catalog in the Bordeaux Carte du Ciel zone +11degr <= delta <= +18degr
During four years, systematic observations have been conducted in driftscan mode with the Bordeaux automated meridian circle in the declinationband +11degr <= delta <= +18degr . The resulting astrometriccatalog includes about 2.3x 106 stars down to the magnitudelimit VM=16.3. Nearly all stars (96%) have been observed atleast 6 times, the catalog being complete down to VM=15.4.The median internal standard error in position is ~ 35 mas in themagnitude range 11 < VM <15, which degrades to ~ 50 maswhen the faintest stars are considered. M2000 also provides one bandphotometry with a median internal standard error of ~ 0.04 mag.Comparisons with the Hipparcos and bright part of Tycho-2 catalogs haveenabled us to estimate external errors in position to be lower than 40mas. In this zone and at epoch 1998, the faint part of Tycho-2 is foundto have an accuracy of 116 mas in alpha instead of 82 mas deduced fromthe model-based standard errors given in the catalog. The catalog isdistributed on CD-ROM and through the CDS(http://cdsweb.u-strasbg.fr/Cats.html). Further information is given onthe website http://www.observ.u-bordeaux.fr/~soubiran/m2000.htm
| Multimode delta Scuti stars in the open cluster NGC 7062
The central field of NGC 7062 was observed intensively with the maingoal of finding delta Scuti stars suitable for use inasteroseismological tests of stellar structure and evolution theory. BVtime series photometry was obtained for this northern open cluster,which has a large population of stars inside the delta Scuti instabilitystrip, making it a probable host of several such variables. We reportfindings of 15 pulsating stars, including at least 13 delta Scuti stars.Ten variables oscillate in two or more frequencies. Only one of thesevariables was known before, for which we detected 9 frequencies. Fiveprobable variables are mentioned, and period analysis is given for all20 stars. Based on observations obtained at the Nordic OpticalTelescope, operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofisíca deCanarias.
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