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NGC 3766 (Pearl Cluster)



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Four-colour photometry of eclipsing binaries. XL. uvby light curves for the B-type systems DW Carinae, BF Centauri, AC Velorum, and NSV 5783
Aims.In order to increase the limited number of B-stars with accuratelyknown dimensions, and also the number of well studied eclipsing binariesin open clusters, we have undertaken observations and studies of foursouthern double-lined eclipsing B-type binaries; DW Car, BF Cen, AC Vel,and NSV 5783. Methods: .Complete {uvby} light curves were observedbetween January 1982 and April 1991 at the Danish 0.5 m telescope at ESOLa Silla, since 1985 known as the Strömgren Automatic Telescope(SAT). Standard indices for the systems and the comparison stars, aswell as additional minima observations for AC Vel, have been obtainedlater at SAT. For DW Car and AC Vel, high-resolution spectra fordefinitive spectroscopic orbits have also been obtained; they arepresented as part of the detailed analyses of these systems. A fewspectra of NSV 5783 are included in the present paper. Results:.For all four systems, the first modern accurate light curves have beenestablished. DW Car is a detached system consisting of two nearlyidentical components. It is member of the young open cluster Cr228. Adetailed analysis, based on the new light curves and 29 high-resolutionspectra, is published separately. BF Cen is semidetached and is memberof NGC 3766. Modern spectra are needed for a detailed study. AC Vel is adetached system with at least one more star. A full analysis, based onthe new light curves and 18 high-resolution spectra, is publishedseparately. NSV 5783 is discovered to be an eclipsing binary consistingof two well-detached components in an 11-day period eccentric (e = 0.18)orbit. Secondary eclipse is practically total. From the light curves anda few high-resolution spectra, accurate photometric elements andpreliminary absolute dimensions have been determined. The quite similarcomponents have masses of about 5 Mȯ and radii of about3.5 Rȯ, and they seem to have evolved just slightly offthe ZAMS. The measured rotational velocities (≈150 kms-1) are about 6 times those corresponding topseudosynchronization.

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.

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.

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.

The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.

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 Method to Search for Be Stars in Open Clusters
We describe a technique to identify Be stars in open clusters usingStrömgren b, y, and narrowband Hα photometry. We firstidentify the B-type stars of the cluster using a theoretical isochronefit to the (b-y, y) color-magnitude diagram. The strongest Be stars areeasily identified in a (b-y, y-Hα) color-color diagram, but thosewith weaker Hα emission (classified as possible Be stardetections) may be confused with evolved or foreground stars. Here wepresent such photometry plus Hα spectroscopy of members of thecluster NGC 3766 to demonstrate the accuracy of our technique.Statistical results on the relative numbers of Be and B-type stars inadditional clusters will be presented in a future paper.

Leading Wave as a Component of the Spiral Pattern of the Galaxy
The spiral pattern of the Galaxy, identified by analyzing the kinematicsof young stars within 3 kpc of the Sun, is Fourier decomposed intospiral harmonics. The spiral pattern of the Galaxy is shown to berepresentable as a superposition of trailing and leading waves withinterarm distances of λ = 1.8 ± 0.4 and 4 ± 2 kpc,respectively. Shock waves are probably present only in the portions ofthe trailing spiral pattern where it crosses the crest of the leadingwave. The small interarm distance of the trailing spiral wave (λ= 1.8 kpc) can be explained by its evolution—by the decrease inthe interarmd istance as the wave is displaced toward the inner Lindbladresonance. The Carina arm may be part of this resonance ring.

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.

uvbyCaHβ CCD Photometry of Clusters. IV. Solving the Riddle of NGC 3680
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the open cluster, NGC 3680. Restricting the data toprobable cluster members using the color-magnitude diagram (CMD) and thephotometric indices alone defines a sample of 34 stars at the clusterturnoff that imply E(b-y)=0.042+/-0.002 (s.e.m.) or E(B-V)=0.058+/-0.003(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk using bothb-y and Hβ as the temperature indices. The agreement among the fourapproaches is excellent, leading to final value of [Fe/H]=-0.14+/-0.03for the cluster and removing the apparent discrepancy between the pastuvby analyses and extensive results from the red giants. The primarysource of the photometric anomaly appears to be a zero-point offset inthe original m1 indices. Using the homogenized and combinedV, b-y data from a variety of studies transformed to B-V, the clusterCMD is compared with NGC 752, IC 4651, and the core-convective-overshootisochrones of Girardi et al. By interpolation to the proper metallicity,it is found that the E(B-V), m-M, and age for NGC 752, IC 4651, and NGC3680 are (0.03, 8.30, 1.55 Gyr), (0.10, 10.20, 1.7 Gyr), and (0.06,10.20, 1.85 Gyr), respectively. The revised age and metallicity sequenceand the color distribution of the giants provide evidence for thesuggestion that the giants defining the apparent clump in NGC 3680 arepredominantly first-ascent giants, as indicated by their Li abundance,while the clump stars in NGC 752, 0.1 mag bluer in B-V, areHe-core-burning stars. When combined with the color distribution in IC4651, it is suggested that over this modest age range where He-coreflash becomes important, the distribution of so-called clump starsswitches from being dominated by He-core-burning stars to first-ascentgiants in the bump phase.

Rotation of Early B-type Stars in the Large Magellanic Cloud: The Role of Evolution and Metallicity
I present measurements of the projected rotational velocities of asample of 100 early B-type main-sequence stars in the Large MagellanicCloud (LMC). This is the first extragalactic study of the distributionof stellar rotational velocities. The sample is drawn from two sources:from the vicinity of the main-sequence turnoff of young clusters (ages1-3×107yr) and from the general field. I find that thecluster population exhibits significantly more rapid rotation than thatseen in the field. I have drawn analogous Galactic cluster and fieldsamples from the literature. Comparison of these samples reveals thesame effect. I propose that the observed difference between cluster andfield populations can be explained by a scenario of evolutionaryenhancement of the surface angular momentum over the main-sequencelifetime. A comparison is made between the cluster and field populationsof the LMC and the Galaxy in order to explore the effects ofmetallicity. This shows that the stars of the LMC are more rapidrotators than their Galactic counterparts.

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.

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.

The very reddened open clusters Pismis 23 (Lyngå 10) and Stephenson 2
Pismis 23 and Stephenson 2 are two very reddened open clusters,projected close to the Galactic plane. We present VI and JH photometryfor Pismis 23 and I and Gunn z for Stephenson 2. We derive a relativelyold age of ~ 700 Myr for Pismis 23, with a reddening value of E(B-V) =1.73 and a distance dsun~ 2.6 kpc from the Sun. Stephenson 2has a clump of red supergiants, indicating a young age of about 20 Myr,somewhat younger and more distant (dsun~ 6 kpc) than recentlyreported in the literature. The age estimate for such a young object iscrucial for the distance estimate. The high reddening of these twoclusters relative to predictions from the diffuse interstellar medium attheir distances is due to discrete intervening clouds. Observationscollected at the European Southern Observatory - ESO, Chile. Tables 2and 3 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr ( viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/931

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.

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).

NGC 2571: An intermediate-age open cluster with a White Dwarf candidate
CCD UBVI imaging photometry was carried out in the field of the opencluster NGC 2571. From the analysis of our data we state the cluster isat a distance of 1380 +/- 130 pc and its age is 50 +/- 10 x106 yr. The cluster mass function has a slope larger than atypical Salpeter's law. There are two notorious features in NGC 2571:the cluster contains a high proportion of stars located below thereference line that are serious candidates to be metallic line stars(probably Am-Fm), and shows also a sharp gap along its main sequencethat cannot be explained by a random process nor by a biased rejectionof cluster members. A striking blue object was detected in the clusterfield that could be a white dwarf candidate. Based on observationscollected at the European Southern Observatory ESO at La Silla, Chile.

Radial velocities, binarity, and kinematic membership in open clusters with blue straggler candidates
Not Available

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].

Periodic Pattern in the Residual-Velocity Field of OB Associations
Not Available

NGC 7419: An open cluster rich in Be stars
The results of our CCD photometric Hα observations of NGC7419 are presented. The observations resulted in a discoveryof 17 new Be stars and two other emission-line objects. In consequence,the number of known Be stars in this young cluster increased more thantwofold and equals now to 31. This is at present the largest number ofBe stars known in any galactic open cluster. Moreover, we estimate thatthese 31 Be stars constitute 36 +/- 7% of all cluster B-type starsbrighter than R_C = 16.1 mag. This locates NGC 7419 among these openclusters which are richest in Be stars such as galactic NGC663, NGC 330 in SMC, and NGC1818A in LMC.

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 ( or viahttp://cdsweb.u-strasbg.fr/Abstract.html

CCD UVBY Hβ Photometry in Clusters. I. The Open Cluster Standard, IC 4651
CCD photometry of the intermediate-age open cluster IC 4651 on theuvbyHβ system is presented and analyzed. By using a combination ofthe information from the color-magnitude diagram (CMD) and thecolor-color diagrams, a sample of 98 highly probable main-sequencecluster members with high photometric accuracy is isolated. From thissample, adopting the intrinsic color relation of Olsen,E(b-y)=0.062+/-0.003 and [Fe/H]=+0.077+/-0.012, where the errors quotedare the standard errors of the mean and refer to the internal errorsalone. Use of the Nissen intrinsic color relation produces E(b-y)=0.071and [Fe/H]=+0.115. Adopting the lower reddening, a direct main-sequencefit to the Hyades with (m-M)=3.33 leads to (m-M)=10.15, while isochroneswith convective overshoot and zeroed to the Hyades produce an age of1.7+/-0.1 Gyr, with an excellent match to the morphology of the turnoff.The higher reddening produces (m-M)=10.3 and an age lower by 0.1 Gyr.Comparison with the CMD of NGC 3680 shows that the two clusters havevirtually identical morphology, which in combination with their similarcompositions produces identical ages. Coincidentally, the shifts in theCMD necessary to superpose the two clusters require that the apparentmoduli of IC 4651 and NGC 3680 be the same, whileE(b-y)4651=E(b-y)3680+0.04.

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.

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.

Differences in the fractions of Be stars in galaxies
The number ratios Be/(B+Be) of Be to B-type stars in young, well studiedclusters of the Galaxy, the LMC and SMC are examined. In order todisentangle age and metallicity effects we choose clusters in the sameage interval and for which reliable photometric and spectroscopic dataare available. Number counts are made for various magnitude intervals,and the results are found to be stable with respect to this choice. Inthe magnitude interval MV = -5 to -1.4 (i.e. O9 to B3) weobtained a ratio Be/(B+Be) = 0.11, 0.19, 0.23, 0.39 for 21 clusterslocated in the interior of the Galaxy, the exterior of the Galaxy, theLMC and the SMC, respectively. Various hypotheses for these differencesare examined. An interesting possibility is that the average rotation isfaster at low metallicities as a result of star formation processes. Themuch higher relative N-enrichment found by Venn et al. (\cite{vencar})in A-type supergiants of the SMC, compared to galactic supergiants, alsostrongly supports the presence of more rotational mixing at lowmetallicities. We discuss whether high rotational mixing may be thesource of primary nitrogen in the early chemical evolution of galaxies.

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

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Observation and Astrometry data

Right ascension:11h36m13.30s
Apparent magnitude:5.3

Catalogs and designations:
Proper NamesPearl Cluster
NGC 2000.0NGC 3766

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