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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.
| CCD photometric search for peculiar stars in open clusters. VI. NGC 1502, NGC 3105, Stock 16, NGC 6268, NGC 7235 and NGC 7510
In a sample of six young open clusters (NGC 1502, NGC 3105, Stock 16,NGC 6268, NGC 7235, and NGC 7510) we investigated 1753 objects using thenarrow band, three filter Δ a photometric system resulting in thedetection of eleven bona-fide magnetic chemically peculiar (CP) starsand five Be or metal-weak stars. The results for the distant cluster NGC3105 is most important because of the still unknown influence of theglobal metallicity gradient of the Milky Way. These findings confirmthat CP stars are present in open clusters of very young ages (log t≥ 6.90) at galactocentric distances up to 11.4 kpc. For all programmeclusters the age, reddening, and distance modulus were derived using thecorresponding isochrones. Some additional variable stars within Stock 16could be identified by comparing different photometric studies.
| 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 Study of Stellar Rotation in Seven Young Clusters
The distribution of rotational velocities among young stars may provideinsight into initial conditions in star-forming regions (e.g. Wolff,Strom, & Hillenbrand 2004). In this contribution, we reportprojected rotational velocities (vsini) for a large sample of starsdrawn from 7 clusters ranging in age from 1--20 Myr (IC 1805, Cyg OB2,NGC 2244, NGC 6611, NGC 6823, NGC 7235, NGC 7380). These data providethe basis for assessing (a) the cosmic dispersion in the distribution ofvsini values, N(vsini), for late-O and early- B stars among clusters ofsimilar age; and (b) evolution-driven changes in N(vsini) via comparisonof young and ``old'' clusters in our sample.Wolff, S.C., Strom, S.E. & Hillenbrand, L.A. 2004 ApJ 601, 979
| Structure and evolution of low-mass W UMa-type systems
The structure and evolution of low-mass W UMa-type contact binaries arediscussed by employing Eggleton's stellar evolution code. Assuming thatthese systems completely satisfy Roche geometry for contact binarieswith every kind of mass ratio (0.02-1.0) we calculate the relative radii(R1,2/A, where R1,2 are the radii of both starsand A is the orbital separation) of both components of contact binarieswith different contact depths between inner and outer Roche lobes. Weobtain a radius grid of contact binaries and can ensure the surfaces oftwo components lying on an equipotential surface by interpolation usingthis radius grid when we follow the evolution of the contact binaries.There are serious uncertainties concerning mainly the transfer of energyin these systems, i.e. it is unclear how and where the energy istransferred. We assume that the energy transfer takes place in differentregions of the common envelope to investigate the effects of the regionof energy transfer on the structure and evolution of contact binaries.We find that the region of energy transfer has a significant influenceon the structure and evolution of contact binaries, and conclude thatthe energy transfer may occur in the outermost layers of the commonconvective envelope for W-type systems, and that this transfer takesplace in the deeper layers of the common envelope for A-type systems.Meanwhile, if we assume that the energy transfer takes place in theoutermost layers for our model with low total mass, and find that ourmodel steadily evolves towards a system with a smaller mass ratio and adeeper envelope, suggesting that some A-type W UMa systems with lowtotal mass could be considered as the later evolutionary stages ofW-subtype systems, and that the surface temperature of the secondaryexceeds that of the primary during the time when the primary expandsrapidly, or the secondary contracts rapidly, suggesting that W-subtypesystems may be caused by expansion of the primary, or by the contractionof the secondary.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| Stellar tracers of the Cygnus Arm. I. Spectroscopic study of bright photometric candidates
We present medium-resolution spectroscopy of a sample of stars in thesecond Galactic quadrant selected from the literature because theircolours suggest that they are moderately-reddened early-type stars atvery large distances. From the derived spectral types and observedcolours, we calculate distances to all these objects. For a sizablefraction of our sample, we find distances well in excess of what isexpected for Perseus Arm objects, even allowing for rather generouserrors. In the interval l=150degr -180degr , there is a large number ofobjects with distances in excess of 4 kpc, which are likely tracing theOuter or Cygnus Arm. In particular, we find that the association Cam OB3is placed on this Arm. Based on our results, the extent and definitionof the associations Cas OB4 and Aur OB2 need to be reevaluated.Based on observations made at Observatoire de Haute Provence (CNRS),France.
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. V. NGC 2169
We present results of a search for variable stars in the field of theyoung open cluster NGC 2169. The General Catalogue of Variable Stars(GCVS, http://www.sai.msu.su/groups/cluster/gcvs/) lists four variablestars in the field we observed, viz., two beta Cep stars, V 916 and V917Ori, an alpha^2CVn variable, V 1356Ori, and an RRc variable, V1154Ori. We find V 916 and V 1154Ori to be constant in light. We confirmthe variability of V 917Ori, but not the period given in GCVS. For thechemically peculiar A0 V Si star V 1356Ori we definitely establish theperiod of 1.565 d, thus settling the uncertainty persisting in theliterature since the star was discovered to be variable. In addition, wefind two other stars to be variable in light. Both show irregularvariations.For V 917Ori, one of the two GCVS beta Cep variables, we determine aperiod of 0.267 d (frequency 3.7477d^-1). However, prewhitening withthis period leaves a significant amount of the star's light-variationunaccounted for. Since the star shows emission at Hα, wehypothesize that the unaccounted for variation is caused by an erratic,Be-type activity. As to the periodic term, we consider three hypotheses:(1) beta Cep-type pulsation, (2) rotational modulation of the lambda Eritype, and (3) ellipsoidal variation due to distorted primary componentin a close binary system. After deriving the star's effectivetemperature from Stromgren indices and the luminosity from the distancemodulus of the cluster, we show that while the third hypothesis isuntenable, the first two should be retained. However, neither isentirely satisfactory.For a number of stars we provide the V magnitudes. For 14 brighteststars in our field we also obtain the photometric alpha-index, a measureof the Hα equivalent width. From the alpha index, we detect mildemission at Hα in two stars, V 917Ori and NGC 2169-8.
| 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.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| The blue to red supergiant ratio in young clusters at various metallicities
We present new determinations of the blue to red supergiant ratio (B/R)in young open clusters at various metallicities. For this purpose, weexamine the HR diagrams of 45 clusters in the Galaxy and of 4 clustersin the Magellanic Clouds. The identification of supergiants is based onspectroscopic measurements (with photometric counts to check theresults). The new counts confirm the increase of the B/R ratio when themetallicity increases with the following normalized relation:(B/R)/((B/R)sun) =~ 0.05* e3(Z)/(Zsun)}, where Zsun=0.02 and(B/R)sun is the value of B/R at Zsun which dependson the definition of B and R and on the age interval considered (e.g.for spectroscopic counts including clusters with log age between 6.8 and7.5, (B/R)sun =~ 3 when B includes O, B and A supergiants).
| 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].
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IV. NGC663
We present results of the variability search in the field of the youngopen cluster NGC663. In addition to the one beta Cep-type variable knownin this cluster, we found another one. It is a mono-periodic pulsatorchanging brightness with a period of 0.27640 d. In total, 19 newvariables were discovered and the variability of 5 other ones wasconfirmed. Out of all 24 variables in the observed field, 21 areprobable cluster members. One SPB candidate and three eclipsing orellipsoidal variables could be classified. Moreover, ten out of fourteenBe stars we observed vary in brightness. Only one of them shows periodicvariations of the lambda Eri-type, while the remaining ones exhibitirregular changes with the range up to 0.4 mag in the I_C band. We alsoprovide new VI_C photometry of 477 stars in the field of the cluster andcheck the consistence of the present photometry with cluster parametersderived earlier. The average cluster E(V-I_C) color excess amounts toabout 0.92mag.
| 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.
| A CCD Search for Variable Stars of Spectral TypeB in the Northern Hemisphere Open Clusters. III. NGC 6823
We present the results of variability search in the field of a veryyoung open cluster NGC 6823. Two delta Sct stars, still at the pre-mainsequence (PMS) stage of evolution, were found. These objects could beused for future testing of the evolutionary period changes in this classof variable stars. In addition, we found 13 other variables including abright cluster eclipsing binary and an SPB candidate. A few othervariables could be PMS stars of the UX Ori type. The H alpha photometry,which we made for 69 brightest stars, revealed only one object withstrong emission, discovered recently by the photographic methods. Twoother stars announced to have H alpha emission, do not show any evidencefor its presence. For a number of stars in the search field, we alsoprovide the BV(RI)_C photometry. We explain how the distribution ofabsorbing matter along the line of sight results in the unusualmorphology of the cluster color-magnitude (CM) diagram. The dereddenedCM diagram is used in the calculation of the extinction map for theobserved field, in which the E(R-I)_C color excess varies from 0.54 to0.72 mag, with the average value equal to 0.62 mag. It is also shownthat all cluster stars with spectral types later than A0 are PMSobjects. We use these stars to estimate the cluster age: 3+/-1 Myr.Using the cluster CM diagram, we compare and discuss the position of thetwo discovered delta Sct stars with respect to the theoreticalinstability strip for PMS stars of this type.
| 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.
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters II. NGC 7235
We report the discovery of nine variable stars in a 4' times 6' fieldcovering NGC 7235. The variable stars include a beta Cep star, a Beshort-period variable of the lambda Eri type, a Mira, a W UMa eclipsingbinary, a candidate alpha Cyg variable, and a probable eclipsing binary.The remaining three variable stars could not be classified because ofinsufficient data. The beta Cep star, the lambda Eri variable, thecandidate alpha Cyg variable, and the probable eclipsing binary aremembers of the cluster. We found no SPB stars in NGC 7235. For a numberof stars in our field we also provide the I magnitudes and the R-I, V-I,and B-V color indices on the BV(RI)_C system of Johnson, Kron andCousins. We find large systematic effects for the faint photographic Vmagnitudes. We define an alpha index which is a measure of theequivalent width of the H alpha line. We give the alpha index for 64brightest stars in our field. Our alpha index revealed the Be nature ofthe above-mentioned lambda Eri variable.
| Absolute proper motions of 181 young open clusters.
Not Available
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters I. NGC 7128
In 1994 we started a program of looking for variable stars in young openclusters. Our main goal is finding pulsating stars of spectral type B.The equipment we use consists of a 60-cm reflecting telescope and a CCDcamera. In this paper, intended as the first one in a series, we detailthe equipment, describe the reductions and present results for NGC 7128.For stars with instrumental magnitudes corresponding to early B spectraltypes on the cluster's main-sequence, the photometric accuracy weachieved allows detection of short-period variables with amplitudesexceeding 5mmag. The results include confirmation of the variability oftwo and discovery of six variables in the cluster. The variables wediscovered comprise two eclipsing binaries, one irregular red variable,and three small-amplitude periodic variables. No B-type pulsators arepresent in NGC 7128.
| 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
| The Initial Mass Function and Massive Star Evolution in the OB Associations of the Northern Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...454..151M&db_key=AST
| Estimates of geometric and dynamic parameters of star-gas complexes in the Galaxy
Parameters of geometric models of 11 gas-star complexes (GSCs) wereobtained. We used information about GSC projections onto the celestialsphere and the Galactic plane and about GSC extension along the line ofsight. GSCs were represented as triaxial ellipsoids. To estimate thesemiminor axis of the GSC ellipsoidal model and GSC slope angle to theGalactic plane, we used data on spatial location of open stellarclusters (OSCs) entering GSCs. GSC slopes to the Galactic plane varybetween 2.5 and 20.5 deg. Their semiminor axes are between 11 and 164pc. GSC total masses are estimated from GSC tidal effect on OSCs thatare members of the corresponding GSCs. The effect manifests itself insmaller sizes of young OSCs as compared to their tidal sizes in theforce field of the Galaxy. We used studies of stability of an OSC movingin the joint force field of the Galaxy and spheroidal stationary GSC, aswell as studies of evolution of a virialized cluster located at thecenter of a nonstationary ellipsoidal GSC. Estimated total masses fordifferent GSCs lie between 0.65 x 10 exp 5 solar masses and 11.5 x 10exp 7 solar masses.
| 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.
| Topography of the Galactic disk - Z-structure and large-scale star formation
A 3D morphological description of the Galactic disk defined by the youngstellar population is delineated using a sample of young open clusterswith cataloged distances and treated with Kriging techniques. The valuesof the positional variable Z for the cluster sample are considered asprospectings of the displacement of the Galactic disk in respect to theequator in the plane. The Kriging technique is described with emphasison its application to the automatic cartography problem. A view of theGalactic disk in a region of about 3 kpc around the sun emerges fromthis treatment and shows a trough-peak structure with four maindepressions as the more striking features. The most prominentdepression, named Big Dent, is apparent at about 1.8 kpc in anapproximately 240-deg direction. It has an elliptical shape with axissizes of 1.5 and 3 kpc, reaching a Z value of 200 pc below the formalGalactic plane. Two-dimensional sections across some selected directionsof the topography show profiles in good agreement with previousobservational studies based on different object samples.
| Numerical taxonomy for open clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..305T
| An optical spiral arm beyond the Perseus arm
In the second galactic quadrant, optical spiral arm tracers have beencollected in a systematic literature search. A uniform reduction of thedata led to the detection of a distinct structure (probably a spiralarm) beyond the Perseus arm that is separated by a statisticallysignificant gap from the latter.
| Component Analysis of Open Clusters
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