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| 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.
| 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.
| A Photometric Search for Variable Stars in NGC 6709
The open cluster NGC 6709 was observed in the V and R filters using the0.4-m David Derrick Telescope of the Orson Pratt Observatory at BrighamYoung University. The cluster was monitored over 5 nights for a total of25 hours. Observations were taken at a rate of approximately one frameevery 2 minutes. Exposure lengths were selected to obtain optimalexposure for stars in the A-F spectral range of the main sequence. Wewill present the results of our search for new variables in thiscluster.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| 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.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| 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
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| Search for Variable Stars in Two Old Open Clusters: NGC2506 and NGC2420
We present results of photometric observations of two old open clusters:NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCDphotometric observations were carried out to search for variable starsand to investigate their physical properties. From the UBVI photometryof NGC 2506, we obtain interstellar reddening of E(B-V)=0.04+/-0.03 mag,distance modulus (V-M_V)_0=12.5+/-0.1 mag, and age log(t/yr)=9.25. Frommonitoring of 590 stars with 304 CCD frames taken for ten nights, threedelta Sct stars and one eclipsing binary star are discovered in NGC2506. For the other cluster, NGC 2420, we have examined light variationsof 505 stars using 347 time-series data obtained for five nights,finding no variable star. It should be noted that we have found no gammaDor star among main-sequence stars near F0 in these two old openclusters, which is consistent with the suggestion that gamma Dor-typephenomenon occurs in stars younger than log(t/yr)=8.4. On the otherhand, it has been known that color-magnitude diagrams of these twoclusters show well-established binary sequences, implying high incidenceof binary systems: >=20% for NGC 2506 and approx 50% for NGC 2420.However, only one eclipsing binary star was found in these two clusters.
| 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
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| An Infrared and Radio Study of the Galactic Worm GW 46.4+5.5
In order to study the physical properties and origin of the Galacticworm GW 46.4+5.5, we have carried out high-resolution (~3') H I and CO(J=1-0) line observations and analyzed available infrared and radioemission survey data. GW 46.4+5.5 appears as a long (~8 deg),filamentary structure extending vertically from the Galactic plane inboth median-filtered far-infrared and radio continuum maps. TheI60/I100 ratio in GW 46.4+5.5 is estimated to be0.29+/-0.05, which is significantly higher than the value determined forthe solar neighborhood. The high ratio is consistent with a hypothesisthat the dust grains in the worms have been processed by interstellarshocks. The radio continuum emission from GW 46.4+5.5 has spectral indexα~=-0.47 and does not correlate with I60 except foremission at low (|b|<=0.5d) latitudes. Thus, most of the radiocontinuum emission is likely to be nonthermal. Our one-dimensional H Iobservations show that the H I gas associated with GW 46.4+5.5 is mainlyat vLSR~=15-40 km s-1. The H I gas is clumpy, andwe detected two molecular clouds associated with the H I peaks. Themolecular clouds have large internal velocity dispersions, 8.0 and 6.6km s-1, compared with their masses, 2.8x103 and1.7x103 Msolar, which implies that they are notgravitationally bound. Using the Leiden-Dwingeloo H I data, we identifyan expanding H I supershell associated with GW 46.4+5.5, which iscentered on (l,b)~=(42deg,5deg) with an angularsize of 14degx22deg (or 340x540 pc2 at1.4 kpc). The supershell appears between vLSR~=18 and 40 kms-1 and slowly decreases in size as the velocity increases.An averaged position-velocity diagram reveals that the supershell has acentral velocity of ~18 km s-1, giving a kinematic distanceof 1.4 kpc and an expansion velocity of ~15 km s-1. Assumingthat it has been created by multiple stellar winds and supernovaexplosions, we estimate its kinematic age and the energy required toproduce it to be about 5 Myr and 1.5x1052 ergs, respectively.The structure is also visible in median-filtered radio continuum maps,but not in the ROSAT maps. The observed molecular clouds might havecondensed out of shock-compressed gas in GW 46.4+5.5 because they areclosely associated with the H I gas in velocity as well as in position.Their altitudes are 80 and 100 pc, respectively, higher than the scaleheight of the thin molecular gas disk. The physical properties of theclouds are very similar to those of the high-altitude clouds observedrecently in sensitive wide-latitude CO surveys. Our results suggest thatat least some of the high-altitude clouds might have formed in Galacticworms (or swept-up H I shells and supershells).
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| Multicolor CCD Photometry and Stellar Evolutionary Analysis of NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 Using Synthetic Color-Magnitude Diagrams
We present the first CCD photometric observations of NGC 2383 and NGC2384 in B, V, R, and I, NGC 1912, NGC 6709 in B, V, and I and NGC 1907in B and V passbands, reaching down to a limiting magnitude of V~20 magfor ~3300 stars put together. The results of the spectroscopicobservations of 43 bright stars in the field of NGC 1912, NGC 2383, NGC2384, and NGC 6709 are also presented. The color-magnitude diagrams(CMDs) of the clusters in V versus B-V, V versus V-R, and V versus V-Iare presented. The distances and reddening to these clusters aredetermined using the cluster CMDs. The distances to the clusters NGC1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are 1785+/-260,1820+/-265, 3340+/-490, 2925+/-430, and 1190+/-175 pc, respectively.Some gaps in the cluster main sequence have been identified. We havecompared the observed color-magnitude diagrams of these four openclusters with the synthetic ones derived from one classical and twoovershoot stellar evolutionary models. Overshoot models estimate olderages for clusters when compared to the classical models. The age of theclusters estimated using the isochrones of Bertelli et al. are 400, 250,400, 20, and 315 Myr for the clusters NGC 1907, NGC 1912, NGC 2383, NGC2384, and NGC 6709, respectively. A comparison of the syntheticcolor-magnitude diagrams with the observed ones indicates that theovershoot models should be preferred. The comparison of integratedluminosity functions do not clearly indicate as to which model is to bepreferred. The values of the mass function slopes estimated for theclusters are x=1.7+/-0.15 for NGC 1912 (mass range: 1.7-3.9 M_solar) andNGC 6709 (1.7-3.4 M_solar), x=1.3+/-0.15 for NGC 2383 (1.7-3.1 M_solar),and x=1.0+/-0.15 for NGC 2384 (2.0-14.0). The present age estimates showthat the closely located cluster pair NGC 1912 + NGC 1907 have similarages, indicating that they may have born together, making them a goodcandidate to be a binary open cluster.
| Young star clusters of our galaxy and the Large Magellanic Cloud as test for stellar evolutionary models.
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| Spectral Classification of "Photometric Subgiants" in Open Clusters
Color-magnitude diagrams of several intermediate-age open clusterscontain apparent post-main-sequence stars which are fainter thanpredicted by standard evolutionary tracks. We have obtainedclassification spectra for such stars in eight clusters. Some of theseare reddened background early-type stars, and several of the late-typestars turn out to have luminosities indicative of being foreground tothe cluster. A few stars appear to be cluster members but do not fit thetheoretical tracks. We find that the Bahcall-Soneira Galaxy model,modified for low galactic latitudes, can give statistically usefulpredictions of the numbers of field stars.
| Determination of absolute proper motions for stars in 21 open clusters.
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| 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.
| Infrared photometry and spectrophotometry of Nova Herculis 1991 (= V838 Herculis): Observations of the formation of dust in the ejecta of a very fast nova
We present near-infrared photometry and spectrophotometry of NovaHerculis 1991 (N Her). Models of the near-infrared spectra of N Herreveal that shortly after visual maximum, the ejected gas was dense(Ne greater than or = 1010/cu cm), and cool(Te = 1000 K). Models of the spectra also showed that as thefirst week following visual maximum progressed, the gas became hotter,and its density decreased. Within two days of visual maximum dust wascondensing out of the ejecta. This limited dust formation began 5 daysbefore the major dust shell formation event which occurred on, or near,1991 March 31.8, 8 days after the initial outburst. This is the earliesttime at which a dust shell has been observed to have been formed by anova. The dust which formed in the ejecta of N Her was hotter(Td = 2000 K) than previously observed for a nova. We findthat the Clayton & Wickramasinghe (1976) model for the formation ofcarbon grains adequately explains tha formation and evolution of thedust shell of N Her. We derive values of the outburst luminosity, theejected gas and dust masses, and other parameters of the outburst of NHer, and compare these to published values.
| A CO survey of regions around 34 open clusters. II - Physical properties of cataloged molecular clouds
The physical properties of the 148 molecular clouds found in a CO surveyof regions around 34 young open clusters have been examined. Expressionsare given for the cloud size spectrum and the mass spectrum. Themass-radius relation implies that clouds of all size larger than a fewpc have about the same mean volume density. Power laws with slopes of0.6 and 3 describe, respectively, the relations of CO linewidth andcloud mass to cloud size. The clouds are distinctly nonspherical andappear to be randomly oriented with respect to the Galactic plane. Theobservations can be explained by a model for molecular clouds in whichclouds are ensembles of dense clumps of gas. Based on such a model, itis shown that molecular clouds are perturbed on a time scale shortcompared to the time required for them to reestablish virialequilibrium.
| Some characteristics of complexes of open star clusters
Mean coordinates and velocities, phase sizes, mean elements of galacticorbits, mean ages, and metal abundances are given for 11 complexes ofopen clusters, and correlations between these characteristics arediscussed. The possible existence of a supercomplex encompassing 9 or 10complexes, and probably a number of individual clusters, is discussed.This rotates at an angular velocity of 10 to 13 km/s kpc.
| A CO survey of regions around 34 open clusters
Results are presented from a systematic search for CO emission fromregions around 34 young open clusters in the outer Galaxy. The clustershave well-determined distances ranging from about 1 to 5 kpc and agesnot greater than about 100 Myr. It was found that some moderately youngclusters have no associated CO emission. All the surveyed clustersyounger than about 5 Myr have associated with them at least onemolecular cloud more massive than 10,000 solar mass, while the molecularclouds associated with clusters older than about 10 Myr are not moremassive than a few thousands solar masses. It was also found thatmolecular clouds are receding from young clusters at a rate of about 10km/sec, and that they seem to be destroyed by their interaction with thestars. Sites of ongoing star formation were identified in a number ofclouds associated with young clusters.
| Groups of open clusters with common motion in the Galaxy
A method of identifying star groups with common motion in space isapplied to a sample of 66 open clusters with known space velocitycomponents. Eight groups are obtained; out of these, five are probablyreal. It is shown that the motions of the cepheids and the open clustersare analogous in the vicinity of the sun.
| Theoretical color-magnitude diagrams of open clusters
Theoretical isochrones were constructed for clusters with ages between10 to the 7th and 10 to the 9th yrs. The isochrones are transformed toM(v)-(B-V) coordinates. The ages of 40 open clusters are obtained on thebasis of these isochrones.
| 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.
| Catalog of open clusters and associated interstellar matter.
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