|Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1|
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
|The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters|
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
|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.
|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.
|The CFHT Open Star Cluster Survey. I. Cluster Selection and Data Reduction|
We present this paper in conjunction with a companion paper as the firstresults in the Canada-France-Hawaii Telescope Open Star Cluster Survey.This survey is a large BVR imaging data set of 19 open star clusters inour Galaxy. This data set was taken with the CFH12K mosaic CCD(42'×28'), and the majority of the clusters were imaged underexcellent photometric, subarcsecond seeing, conditions. The combinationof multiple exposures extending to deep (V~25) magnitudes with short(<=10 s) frames allows for studies ranging from faint white dwarfstars to bright turnoff, variable, and red giant stars. The primary aimof this survey is to catalog the white dwarf stars in these clusters andestablish observational constraints on the initial-final massrelationship for these stars and the upper mass limit to white dwarfproduction. Additionally, we hope to better determine the properties ofthe clusters, such as age and distance, and also test evolution anddynamical theories by analyzing luminosity and mass functions. In orderto more easily incorporate these data in further studies, we haveproduced a catalog of positions, magnitudes, colors, and stellarityconfidence for all stars in each cluster of the survey. This reduction,along with the computed calibration parameters for all three nights ofthe observing run will encourage others to use these data in differentastrophysical studies outside of our goals. Additionally, the data setis reduced using the new TERAPIX photometric reduction package, PSFex,which is found to compare well with other packages. This paper isintended both as a source for the astronomical community to obtaininformation on the clusters in the survey and as a detailed reference ofreduction procedures for further publications of individual clusters. Wediscuss the methods employed to reduce the data and compute thephotometric catalog. We reserve both the scientific results for eachindividual cluster and global results from the study of the entiresurvey for future publications. The first of these further publicationsis devoted to the old rich open star cluster, NGC 6819, and appears as acompanion paper in the same issue of the Journal.
|Open clusters in the third galactic quadrant. I. Photometry|
We have performed a photometric survey of open clusters in the thirdGalactic quadrant in order to study the star formation history andspatial structure in the Canis Major-Puppis-Vela region. In this paperwe describe a catalogue of CCD U BV RI photometry of approximately 65000 stars in the fields of 30 open clusters. The data were obtained andreduced using the same telescope, the same reduction procedures, and thesame standard photometric system, which makes this catalogue the largesthomogeneous source of open cluster photometry so far. In subsequentpapers of this series, colour-colour and colour-magnitude diagrams willbe presented which, amongst other uses, will allow the determination ofan homogeneous set of cluster reddenings, distances, and ages that willconstitute the observational basis for our studies of the spatialstructure and star formation history in the third Galactic quadrant.
|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 (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
|Photometric study of the open cluster NGC2323|
UBV photoelectric photometry for 175 stars in the field of the southernopen cluster NGC2323, supplemented by DDO photometry of 5 probablegiants, is presented. The analysis of the photometric data yields 109probable members; one of them being a red giant, and 3 possible members.The reddening across the cluster is slightly variable and the mean valueE(B-V)=0.25. The apparent cluster distance modulus is 10.62,corresponding to a distance of 940 pc. The age, determined by fittingisochrones with core overshooting, turns out to be 100+/- 20 Myr. Otherfundamental cluster parameters are also determined. NGC2323 appears notto be physically connected to the CMa OB1 association. Based onobservations made at Cerro Tololo Inter-American Observatory (Chile) andat Las Campanas Observatory (Chile).
|Absolute proper motions of 181 young open clusters.|
|Chemical Evolution of the Galactic Disk: Evidence for a Gradient Perpendicular to the Galactic Plane|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2813P&db_key=AST
|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.
|Photoelectric Search for Peculiar Stars in Open Clusters - Part Fourteen - NGC1901 NGC2169 NGC2343 CR:132 NGC2423 and NGC2447|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102....1M&db_key=AST
|Stellar associations in the region of CMa|
Forty-three early-type stars brighter than 9 mag at 1640 A were observedin the region of CMa with the Glazar Space Telescope. The observed starsare shown to compose three groups situated at distances of 320, 570, and1100 pc. The first two groups are B-associations, and the third is anO-association. It is concluded that the dust matter situated atdistances up to 1000 pc yields an interstellar extinction parameter of0.7 mag at 1640 A.
|An observational age-metallicity relation for the Galaxy|
An observational Age-Metallicity Diagram for the Galaxy has beenconstructed on the base of star cluster data available in theliterature. This diagram exhibits two independent and simultaneousmetallicity enhancement processes identified with two dynamical galacticsubstructures: the thin and the thick disks. Within the observationaluncertainties both these processes can be satisfactory approximated byInfall-Inflow models of the chemical evolution with different high,although rapidly decaying with time, Star Formation and Infall Rates.The comparison of the galactic age-metallicity relations with that forthe Large Magellanic Cloud shows some similarities between processes ofthe chemical enhancement in the Large Magellanic Cloud and the Thin Disksubsystem in the Galaxy.
|Metallicities of open clusters|
An examination is conducted of available metallicity data on openclusters, gathering the largest possible set of homogeneous estimationsof this parameter and determining whether open cluster metallicities arerelated to the spatial and age distributions of such objects. The'composite' C-M diagrams for the two metallicity groups discussedexhibit interesting morphological differences which may be characterizedas the presence of numbered blue stragglers at the two metal-richclusters and a lack of such members in two metal-poor ones.
|Two populations of open clusters|
The paper proposes a division of the local open clusters into twodifferent dislike subpopulations on the base of the metallicityenhancement history, spatial distribution in the Galaxy and morphologyof their C-M diagrams. The thin disk population objects exhibit highermetallicities and are more closely concentrated to the center and thedisk of the Galaxy than the thick disk clusters. The former contain alsofrequently blue stragglers while the later exhibit the presence of Clumpgiant members, typical for the thick disk globular clusters. Kinematicalproperties of the two considered open cluster subpopulations remainunknown due to the lack of observational data.
|Component Analysis of Open Clusters|
|Young stellar-gas complexes in the Galaxy|
It is found that about 90 percent of OB-associations and o-b2 clusterssituated within 3 kpc of the sun can be united into complexes withdiameters of 150-700 pc. Almost all of these clusters contain giantmolecular clouds with a mass greater than about 100,000 solar masses. Anumber of complexes are associated with giant H I clouds; a few of thesmall complexes are situated in the HI caverns. The concentration ofOB-associations and young clusters in star complexes attests to theircommon origin in the supergiant gaseous clouds.
|A cluster analysis of open clusters|
The Galactic distribution of 361 open clusters is studied using acluster analysis method. It is shown that more than half of the clustersenter groups with characteristic dimensions of several hundred parsecs.To distinguish physical clusters from random condensations, criteriabased on age similarity, the color of the main-sequence blue end, andthe integrated color and radial velocity of the clusters are used. Theproximity of these values suggests a physical unity and common origin ofclusters in a group.
|Catalog of open clusters and associated interstellar matter.|
|A cluster analysis of young open clusters|
Cluster analysis methods are used to consider the galactic distributionof 224 open clusters with an age up to 10 to the 8th yrs. Most of theseclusters enter condensations with characteristic dimensions of a fewhundred parsecs. Some condensations are so similar in terms of the age,integrated color, and radial velocity of their components, that thiscannot be considered a coincidence. This suggests that each condensationis a physical entity consisting of clusters apparently linked by acommon origin.
|The large-scale brightness distribution of an ionized gas in the Canis Major OB1 association|
The northern sector of the RATAN-600 radio telescope was used to observethe northern part of the Canis Major OB1 association at wavelengths of7.6, 13, and 31 cm. At a wavelength of 7.6 cm, a radio luminositydistribution map was constructed. A comparison of data at differentwavelengths reveals that the emission from this association is thermal.It is also found that the known stars in the association do not sufficeto explain the ionization of all of the radiating gas. In the region ofthe NGC 2343 cluster, an increase in the gas abundance was found whichcan be explained by an influx of cold gas into a 'gravitational pit'formed by an old massive cluster and by its subsequent ionization due toexternal sources (young stars of the Canis Major OB1 association).
|Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)|
|Search for CP-Stars in Open Clusters|
|Membership and photometric abundances of red evolved stars in open clusters|
UBV and DDO photometric observations of 31 red evolved stars toward 11open clusters, obtained with the 41-cm and 91-cm telescopes at CerroTololo Interamerican Observatory on four nights in 1978, are reported.The data are compiled in tables and characterized for each cluster, andtwo criteria are used to determine the probability that a star is amember of a cluster. Six of the clusters are found to contain oneevolved red star, one has two, one has four, and three have none; theother 19 stars are seen as red field objects. The estimated cluster Fe/Habundance ratios range from -0.2 to +0.3, and the red giants in theclusters NGC 2335 and NGC 2567 are found to have CN strengths like thoseseen in Hyades giants. The unusual weakness of the 421.6-nm band of CNfor star 108 in NGC 2548 is attributed either to unusual composition andproper motion or to its nonmembership in the cluster despite meeting thephotometric criteria.
|Investigation of the initial mass spectrum of open star clusters|
The mass spectra of 228 open star clusters were derived by comparison ofcolor-magnitude diagrams with evolutionary tracks. The application tobinary stars showed the reliability of the mass determination. Thederived mass spectra were fitted by power laws as well as exponentiallaws. It could be shown that both approximate the mass spectra of openstar clusters on the same average significance level. The presentinvestigation revealed a correlation of the slope of the mass spectrawith the cluster age, whereas a detected correlation of the slope withgalactocentric distance is slight. The results suggest that the slope ofthe mass spectrum increases with increasing cluster and galactocentricdistance. These findings are discussed with respect to their reasons andprevious results concerning open clusters and field stars.
|Integrated Photometric Parameters of Open and Globular Clusters|
|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.