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New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is 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/463/789
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| High Spatial Resolution Near-Infrared Images of Protostars in Taurus
We present near-infrared K and L images of 15 embedded Class I objectsbelonging to the Taurus-Auriga molecular cloud. L-band images wereobtained with the COB, at the KPNO 4-m telescope, used in thediffraction limited mode (DLIRIM) while K-band data were observed withboth the COB and the STERLIMCam attached to the 1.2-m telescope of theFred L. Whipple observatory in Arizona. In this analysis we use theradiative transfer code of Whitney et al. (2003, ApJ, 598, 1099) tomodel these images. We adopt a geometric configuration that consists ofa central illuminating source surrounded by an accretion disk andinfalling envelope. Perpendicular to the disk, a bipolar outflowexcavates cavities along the rotational axis of the envelope. Weconstructed a grid of models for the centrifugal radius (R_c]),accretion rate ( Ṁ), opening angle (θ) and inclination angle(i). In addition we tried two grain models in the envelopes, a modelwith r_V = 4.3 and the ISM grains. We obtained physical and geometricparameters for each of the analyzed protostars. We compare our resultsderived from high and low resolution images with those obtained by otherauthors. We find that envelopes with grains corresponding to R_V =4.3reproduce better the real images than envelopes with ISM grains.
| 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.
| Separation of composite spectra: the spectroscopic detection of an eclipsing binary star
We present an iterative method that allows us to compute the individualspectra and radial velocities of the two stellar components ofspectroscopic binaries. We applied this method to HD143511 and detected the occurrence of eclipses at theconjunction phases. The orbital analysis provides minimum masses witherrors of 1%. Absolute masses and radii were derived in two differentways: a) by combining information provided by radial velocities,rotational velocities, and spectral line intensities; and b) using lightcurves from the ASAS-3 database.
| 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.
| Spectroscopic binaries in southern open clusters
This is a report on an ongoing program about binaries in southern openclusters. The long-term purpose of this project is to contribute tounderstanding the formation and evolution of spectroscopic binaries,providing observational constraints that will permit tests of some ofthe current theories on binary formation in open clusters.
| IC 2395 and BH 47: Only one open cluster in the Vela constellation
We report UBV photoelectric photometry for 273 stars in the fields ofthe open clusters IC 2395 and BH 47. Our postulate is that, rather thantwo different clusters in this region of Vela, there is only one, whichwe call IC 2395 = BH 47. The cluster is centered at about alpha =8{h} 42fm 5, delta = -48degr 06farcm 8 (2000), itsangular diameter being ~ 19'. The analysis of the photometric datayields 61 probable cluster members and 16 possible members. Photometricmembership probabilities show good agreement with those obtained from aproper motion study for 21 stars in common. The reddening across thecluster appears to be uniform, the mean E(B-V) value being 0.09 +/-0.02. The true distance modulus is (V-MV)0 = 9.52+/- 0.10, corresponding to a distance from the Sun of (800 +/- 40) pcand 48 pc below the Galactic plane. The cluster age, determined byfitting isochrones with core overshooting, turns out to be (6 +/- 2)Myr. There is a strong likelihood that IC 2395 = BH 47 is physicallyconnected to the Vela OB1C association.
| 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.
| 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.
| 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
| A spectroscopic survey for lambda Bootis stars. III. Final results
In the third paper of a series dedicated to the spectroscopic survey fornew lambda Bootis stars, we present all new and confirmed members of thegroup as well as a detailed analysis of the observed sample. The natureof this small group of chemically peculiar stars of the upper mainsequence still challenges our understanding of processes like diffusion,mass-loss and accretion. The typical abundances pattern (nearly solarvalues for C, N, O and S whereas the Fe-peak elements are moderate tostrong underabundant) can still not be explained by any proposed theory.Hence, the significant increase of new members gives the opportunity toinvestigate the group properties in more detail. We report the discoveryof 26 new members of the group and the confirmation of 18 candidatesfrom the literature. This almost triples the number of known lambdaBootis stars. The existence of one member in the young open cluster NGC2264 and four members in the Orion OB1 association proves that thelambda Bootis phenomenon already works at very early stages of stellarevolution. Recent results from the Hipparcos mission have shown that thewell established lambda Bootis stars of the Galactic field comprise thewhole area from the Zero Age Main Sequence to the Terminal Age MainSequence (~ 109 yr for an A-type star). There is a continuoustransition between very young and rather evolved evolutionary stages. Wefind that the overall percentage of lambda Bootis type among all normaltype stars in the spectral range from B8 to F4 is 2% in the Galacticfield as well as in open clusters. Furthermore, 44 metal-weak objectsare listed which might be connected with the lambda Bootis phenomenon.Our biased sample (chosen by photometric boxes) is not distinguishedfrom all A-type stars in the corresponding spectral region by therotational velocity distribution. Only for the luminosity classes IV andIII (especially for the cooler program stars) the determined mean v sini values are very high compared to those of the literature. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| Hipparcos Trigonometric Parallaxes and the Distance Scale for Open Star Clusters
Hipparcos trigonometric parallaxes are used to estimate the distances tothe maximum possible number of open star clusters (OSC); distance moduliare estimated for 45 clusters with maximum heliocentric distances ofabout 1000 pc. The latter value can serve as an estimate of the limit towhich it still makes sense to use Hipparcos trigonometric parallaxes todetermine the distances to small groups composed of 6-10 sufficientlybright stars. A systematic correction to the distance moduli of clustersfrom the homogeneous catalog of OSC parameters (Loktin et al. 1997,2000) is estimated, which turns out to be independent of the clusterage.
| 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 Relationship Between the Böhm-Vitense Gap and Stellar Activity in Open Clusters
Using published UBV photometry of nine open clusters and chromosphericactivity data in five clusters, we have performed the most completestudy to date of the supposed Böhm-Vitense gap in main-sequencelate-A and early-F stars. Using the χ2 test on thedistribution of B-V colors of the main-sequence stars of these clusters,we located gaps at the 0.90 probability level or greater in eight out ofthe nine clusters. Six of the nine clusters show a gap near B-V=0.35 atthis level. In four nearby clusters, Alpha Persei, Pleiades, Coma, andPraesepe, we compare the gaps with measurements of chromosphericactivity via the He I λ5876 (D3) line. If the Böhm-Vitensegap is related to the onset of surface convection zones, as has beensuggested, we would expect to find a direct relationship between theonset of activity and the gaps. However, in all four cases we find starson both sides of the gap showing strong activity. The observed gaps donot appear to correlate directly with any known physical mechanism andthus either point to an unknown mechanism or a statistical anomaly.
| Search for variable stars in the open cluster NGC 7654
Time-series CCD photometry of the open cluster NGC 7654 was performedover five nights from October 2nd to 18th, 1997 U.T., in order to searchfor new variable stars. We also carried out UBV photometric observationsto obtain physical parameters of variable stars in the cluster. We havederived the distance modulus of (V-M_V)deg = 10.9 +/- 0.2 andthe interstellar reddening of E(B-V) = 0.62 +/- 0.05 by fitting the ZAMSto the observed color-color and color-magnitude diagrams of the cluster.Using the theoretical isochrones with solar metal abundance by Bertelliet al. (1994), an age of the cluster, log t = 8.0, was estimated.Applying the ensemble normalization technique to the observed 1215time-series data, light variations of 284 stars, fainter than 11\fm0 inthe observed field, were carefully examined. As a result, three slowlypulsating B stars and one eclipsing binary were discovered in thecluster. We also confirmed the light variation of a low amplitude deltaSct star which has been recently discovered by Viskum et al. (1997).
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Search for Low-Instability Strip Variables in the Young Open Cluster NGC 2516
In this paper we revise and complete the photometric survey of theinstability strip of the southern open cluster NGC 2516 published byAntonello & Mantegazza. No variable stars with amplitudes largerthan 0.02 mag were found. However, by means of an accurate analysisbased on a new statistical method, two groups of small-amplitudevariables have been disentangled: one with periods of less than 0.25days (probably delta Scuti stars) and one with periods of greater than0.025 days. The position in the H-R diagram and the apparent timescalemay suggest that the stars of the second group belong to a recentlydiscovered new class of variables, named gamma Dor variables. Theycertainly deserve further study. We also present a comparison betweenthe results of the photometric survey and the available pointed ROSATobservations of this cluster.
| Composite M V versus (V-I) 0 Diagram for Template Open Clusters
New CCD VI photometric data for 10 template open clusters withaccurately determined fundamental parameters are presented. From theobserved V versus V - I diagrams of the clusters, fiducial sequenceshave been defined and transformed into the Mv versus (V - I)0 diagram byaveraging sequences of template open clusters with similar ages. Theresulting composite Mv versus (V - I)0 diagram presents a homogeneousset of empirical isochrones in the age range between 5 Myr and 4 Gyr.These empirical isochrones show an overall very good agreement withthose computed from stellar evolutionary models. Theoretical isochroneswith moderate overshooting fit the observed main sequences better thanthe canonical ones for clusters older than 600 Myr. The present set ofempirical isochrones will be useful for the study of faint reddened openclusters
| Geneva photometry in the young open cluster NGC 6231.
We present photoelectric (127 stars) and CCD (168 stars) Genevaphotometry for the very young open cluster NGC 6231. We have searchedfor new cluster members out to a distance of ~13['arc], extending theSeggewiss area (~8['arc]), and we found at least 64 new probable membersin this extended field. Differential reddening is clearly measuredacross the cluster area. We determine the cluster distance (1800 pc) andage (3.8+/-0.6x10^6^yr). The probable presence of PMS stars and theconsequence of this population on the cluster formation history isanalysed. We also found that the O8.5III star S161 is a long termvariable and we present its light curve extending over more than 20years. Finally we discuss the existence of Ap stars in the cluster.
| Parameters of open star clusters from uvby-beta photometry.
Not Available
| 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.
| New dating of galactic open clusters
A new set of isochrones for solar metallicity computed from modelstaking into account mass loss and moderate overshooting and using therecent opacity tables by Rogers and Iglesias (1992) has been used todetermine the ages of 30 galactic open clusters covering an interval ofage from 4 million to 9.5 billion years. The clusters collected hereconstitute an ensemble of homogeneous and accurate data, well discussedfor distance, reddening, membership and binarity, and thus they providea basic reference for age determinations. The present set of isochronesfits the observed color-magnitude diagrams of the clusters veryaccurately, and may thus be expected to provide reliable ages. Analyticrelations between several characteristics of the isochrones and the ageare given and can be used for further age calibrations consistent withthis homogeneous scale.
| Effects of rotation on the colours and line indices of stars. VI - The reality of the blue straggler phenomenon
The effect of rotation on the observed colors of stars has beenconsidered as a possible cause for the blue straggler phenomenon inclusters listed by Mermilliod (1982). It appears that this phenomenon isdefinitely not real in the case of the late B and early A spectral typeblue stragglers that are intrinsic slow rotators. Among clusterscontaining the early B type blue stragglers it is found that theanomalous position of the stragglers in NGC 6633, NGC 4675, and NGC 2516cannot be accounted for by rotation effects alone.
| Formation and evolutionary properties of the Galactic open cluster system
Results are reported from a statistical analysis of observational dataon 100 open clusters within 2 kpc of the sun, selected from the catalogof Lynga (1987). The selection criteria and the completeness of thesample are discussed; the data are compiled in a table; and the analysisresults are presented in a series of graphs and characterized in detail.A cluster formation rate of 0.45 clusters/kpc Myr is found,significantly lower than the rates determined previously (using clusterswithin 1 kpc of the sun) and corresponding to a cluster star-formationefficiency of 0.0063. The low average cluster lifetime (about 10 Myr)suggests that clusters are formed as unstable systems.
| The Gould plates
Most of the 1200 photographs of southern galactic star clusters anddouble stars taken by Benjamin A. Gould at Cordoba, Argentina, in1872-1882 are in the Harvard College Observatory plate collection. Arecent evaluation of these plates shows many to be in usable condition.Details of the characteristics of Gould's plates of galactic clusters,including limiting magnitudes, are presented.
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