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Revisiting the region of the open cluster NGC 5606
We present polarimetric observations in the UBVRI bands corresponding to54 stars located in the direction of NGC 5606. Our intention is toanalyze the dust characteristics between the Sun and the cluster, aswell as to confirm doubtful memberships using polarimetric tools. Wealso want to determine if a group of 11 B and A stars identified in thepast by Vázquez & Feinstein (1991, A&AS, 87, 383) infront of NGC 5606 has any physical entity. From polarimetric data wehave found at least two dust layers along the line of sight to the opencluster. The observations show that both dust layers have their localmagnetic field with an orientation which is close to the direction ofthe Galactic Plane (θ = 70.0 °), but the direction of thepolarimetric vector for the members of the cluster seems to be lowerthan this value. NGC 5606 can be polarimetricallycharacterized with P_max = 2.97% and θv = 63.5 °.The internal dispersion of the polarization values for the members ofNGC 5606 seems to be compatible with intracluster dust. We were able toadd four possible new members to the list of stars in NGC 5606, and toreject others accepted as members in past investigations; and weidentified 17 (out of the 54 observed stars) with intrinsic polarizationin their light. The group of late B- and A-type stars could bephysically related, with characteristics of an open cluster ofintermediate age (between 0.79 and 1. × 108 yr). Forthis group we obtained representative values of P = 2.12% andθv = 70.0 °. The new cluster, which we haveprovisionally named Anon (La Plata 1), extends from NE to SW across theface of NGC 5606 covering about 10'. The fitting procedure ofSchmidt-Kaler's ZAMS (1982, in Landolt/Bornstein, Neue Series VI/2b) wasused to derive the distance to the group, obtaining a distance modulusof Vo - Mv = 9.90 (649 pc from the Sun). The meancolor excess associated with the small group is EB-V = 0.27± 0.08 mag and the dust distribution shows great variabilityacross its face. We think that the small open cluster has goneundetected due to the lack of bright member stars in it.Based on observations obtained at Complejo Astronómico ElLeoncito, operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the Universities of La Plata,Córdoba, and San Juan. Table 1 is only available in electronicform at http://www.aanda.org
| Pre-main sequence stars in open clusters. I. The DAY-I catalogue
Context: We present the basic ideas and first results from the projectwe are carrying out at present, the search for and characterisation ofpre-main sequence (PMS) stars among the members of Galactic youngclusters. The observations of 10 southern clusters, nine of them locatedin the Carina-Sagittarius spiral arm of the Milky Way are presented. Aims: We aim at listing candidate PMS member stars in young clusters.The catalogued stars will serve as a basis for future spectroscopicstudies of individual objects to determine the properties of stellarformation in the last phases before the main sequence stage. Propertiessuch as the presence of residual envelopes or disks, age spread amongPMS members, and the possible presence of several episodes of starformation in the clusters, are to be addressed. Methods: Multicolourphotometry in the UBVR_CIC system has been obtained for 10southern young clusters in the fourth Galactic quadrant, located betweenGalactic longitudes l = 238° and l = 310°. For six clusters inthe sample, the observations presented here provide the first publishedstudy based on CCD photometry. A quantitative comparison is performedwith post-MS isochrones, and PMS isochrones from three differentevolutionary models are used in the photometric membership analysis forpossible PMS stars. Results: The observations produce photometricindices in the Johnson-Cousins photometric systems for a total of 26 962stars. The matching of our pixel coordinates with corresponding fieldsin the 2MASS data base provides astrometric calibration for allcataloged stars and JHK 2MASS photometric indices for 60% of them.Post-MS cluster ages range from 4 to 60 Myr, whereas the photometricmembership analysis assigns PMS membership to a total of 842 stars,covering an age range between 1 and 10 Myr. This information on the PMScandidate members has been collected into a catalogue, named DAY-I,which contains 16 entries for 842 stars in the field of 10 southernclusters.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1397
| 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.
| The origin of massive O-type field stars: II. Field O stars as runaways
In two papers we try to confirm that all Galactic high-mass stars areformed in a cluster environment, by excluding that O-type stars found inthe Galactic field actually formed there. In de Wit et al. (2004) wepresented deep K-band imaging of 5 arcmin fields centred on 43 massiveO-type field stars that revealed that the large majority of theseobjects are single objects. In this contribution we explore thepossibility that the field O stars are dynamically ejected from youngclusters, by investigating their peculiar space velocity distribution,their distance from the Galactic plane, and their spatial vicinity toknown young stellar clusters. We (re-)identify 22 field O-type stars ascandidate runaway OB-stars. The statistics show that 4 ± 2% ofall O-type stars with V<8m can be considered as formedoutside a cluster environment. Most are spectroscopically singleobjects, some are visual binaries. The derived percentage for O-typestars that form isolated in the field based on our statistical analysesis in agreement with what is expected from calculations adopting auniversal cluster richness distribution with power index of β= 1.7,assuming that the cluster richness distribution is continuous down tothe smallest clusters containing one single star.
| The Vela-Puppis open clusters Pismis 8 and Pismis 13
CCD UBVI imaging photometry was carried out in the fields of the openclusters Pismis 8 and Pismis 13, located in the Vela-Puppis region inour Galaxy. MK spectral types have also been determined for a number ofstars located in the fields of these two clusters which were used tosecure membership among the brightest stars. Since our photometry goesto a fainter limit than previous studies we could provide betterreddening, distance and age determinations. Both clusters are locatedclose to the edge of the local arm in the third quadrant. Pismis 8 is acluster about 5-7 My old located at 2000 pc from the Sun, while Pismis13 was found at 2750 pc with a probable age of about 100 My. Theestimate of the slopes of the mass functions in both cases yielded x≈1.7 and x≈ 2.1 for Pismis 8 and 13 respectively.Based on observations collected at the University of Toronto SouthernObservatory, Las Campanas, Chile, and the Complejo Astronómico ElLeoncito, San Juan, Argentina. Table 3 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/432/491
| Revisiting the area of the open cluster Stock 16
Deep CCD UBV(RI)C photometry has been carried out in the areaof the open cluster Stock 16. The different photometric diagramsindicate the presence of three superimposed stellar groups, all composedof early type stars. The first of them, the cluster Stock 16, suffersfrom slightly variable reddening with a mean color excess = 0.51 . The second group, reddened by = 0.84 , includes the Wolf-Rayet star WR 50 (aWC7+OB), and the third one, with a mean < EB-V> = 1.18, also includes another Wolf-Rayet star, WR 51 (a WN4+OB?). Our resultsconfirm the distance and reddening already stated for Stock 16 butinclude new members. Concerning the other two highly reddened groups, weshow that the first one - located at a distance of 3600 pc - is probablyrelated to an anonymous OB association behind Cen OB1 also at 3600 pcalready mentioned in the literature; the other seems to be a distant OBassociation - at more than 9000 pc - which may belong to the Scutum-Cruxspiral arm. In these two groups we find evidence that the absorptionlaw, AV/EB-V = R amounts to 4.0 approximately. Theages of the different star populations in the zone range from 5 ×106 yr to 6.4 × 106 yr for Stock 16, 8× 106 for the second most reddened group and 5 ×106 for the probable association in Scutum-Crux. Adetermination of the IMF slope of Stock 16 was also carried out forstars in the mass range 1.8 < M < 14 Mȯ, givinga normal value x = 1.3.Based on observations collected at the University of Toronto SouthernObservatory (UTSO), Las Campanas, Chile.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/471
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| The origin of massive O-type field stars. I. A search for clusters
We present a study aimed at clarifying the birthplace for 43 massiveO-type field stars. In this first paper we present the observationalpart: a search for stellar clusters near the target stars. We derivestellar density maps at two different resolving scales, viz. 0.25pc and 1.0 pc from NTT and TNG imaging and the 2MASS catalogue.These scales are typical for cluster sizes. The main result is that thelarge majority of the O-type field population are isolated stars: only12% (5 out of 43) of the O-type field stars is found to harbour asmall-scale stellar cluster. We review the literature and aim atcharacterizing the stellar field of each O-type field star with theemphasis on star formation and the presence of known young stellarclusters. An analysis of the result of this paper and a discussion ofthe O-type field population as products of a dynamical ejection event ispresented in an accompanying paper.Based on observations collected at the European Southern Observatory,Chile, and at the Italian Telescopio Nazionale Galileo (TNG) operated onthe island of La Palma by the Centro Galileo Galilei of the CNAA(Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.Table 2 and Figs. 4 to 17 are available in electronic form athttp://www.edpsciences.org
| 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.
| Lyngå 1, A Small Open Cluster Containing a Red-Supergiant Member
We present CCD {UBV}RI} (Cousins system) photometric observationscomplemented with spectroscopic and polarimetric observations that werecarried out in the open cluster Lyngå 1. Our data indicate thatthe cluster reddening is E(B-V)= 0.45+/-0.03, the ratio A[V]/E(B-V)= Rsuggests that the extinction law may be slightly anomalous ( R ≈3.5) and that the cluster distance modulus is V0 - M[V] =11.40+/-0.2. The age of Lyngå 1 is between 100 and 125 Myraccording to a fitting of theoretical isochrones, and the slope of itsmass spectrum is x ≈ 1.7. The brightest red star in the field isa cluster member of spectral type K2 II-Ib.
| 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.
| Imaging study of NGC 3372, the Carina nebula - I. UBVRIJHK photometry of Tr 14, Tr 15, Tr 16 and Car I
We present the results of a large-scale imaging photometric study of thestellar population in the northern part of NGC 3372 in the UBVRIJHKbands with a wavelength coverage from 0.33 to 2.5 μm. Theobservations were made at Las Campanas Observatory. The optical CCDmosaics cover an area approximately of 32 × 22 arcmin2centred between the Tr 14 and Tr 16 clusters. The survey was extended tocover 12 × 12 arcmin2 at the location of Tr 15.Near-infrared NICMOS3 mosaics covering the areas occupied by theseclusters were obtained in the JHK photometric bands. By means of starcounts in V, the centres and sizes of each cluster were redeterminedyielding: Tr 14 (r= 264 arcsec), Tr 15 (r= 320 arcsec) and Tr 16 (r= 320arcsec). It was confirmed that Cr 232 is not a true cluster. Multicolouroptical photometry was obtained for 4152 stars. Two colour andcolour-magnitude diagrams are presented and analysed for each individualcluster and compared to those of the field. We confirm the widespreadvariations in the dust density and also in the dust size distributionleading to widely different values of AV and reddening lawstowards Tr 14 and Tr 16. No spatial patterns were found for thesevariations. Spectroscopic parallaxes were computed and the results areconsistent with all three clusters being at a similar distance from theSun (= 2.7 kpc) but the data have shown very large scatter inboth AV and d. Analyses of the extinction-correctedcolour-magnitude diagrams suggest ages between 3 and 60 million yearsfor the stars in Tr 15 and between less than 1 and 6 million years forTr 14 and Tr 16. A small number of infrared-excess stars were found inTr 16 and Tr 14 but not in Tr 15. The distribution of stars in Tr 14seen in the near-infrared suggests that this cluster is partiallyembedded in a molecular cloud. This molecular cloud extends towards thewest reaching its highest density, marked by a CO peak emission, somethree arcmin to the south-west of the nucleus of Tr 14. The richultraviolet field created by the Tr 14 stars ionizes most of the visibleHII region in its vicinity and most of the radio HII region Car I.Evidence is found of ionization fronts leading into the molecular cloud,which appears to be `wrapping' the Tr 14 cluster. Deep JHK images of theCar I region reveal the presence of an embedded stellar populationilluminating a large infrared reflection nebula. It includes at leastone O9-B0 star associated with an ultracompact HII region. Nebulous 2.2μm emission from three of the mid-infrared sources in the Tr 14region is also found.
| 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 spectral evolution of Galactic open clusters
We present a library of 47 open-cluster integrated spectra, mostlyobtained at CASLEO (San Juan, Argentina) in the range 3600 < λ< 7400 Å, which are made available at CDS. The data arecombined with previous spectra to obtain 10 high signal-to-noise ratiobasic templates in the young and intermediate-age domains, which arealso provided in the library. These Galactic disc templates representthe increased time resolution spectral evolution of a stellar populationunit around the Solar metallicity level. The improved signal-to-noiseratio of the present templates with respect to previous template lists,together with their increased time resolution, allowed us to improve thefundamental parameters of some open clusters. The present spectrallibrary will be useful for several astrophysical applications,particularly for population syntheses of star-forming giant galaxies.
| 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.
| Integrated Spectral Properties of 13 Open Galactic Clusters
Not Available
| Reddening and age for 13 southern Galactic open clusters determined from integrated spectra
In this study we present flux-calibrated integrated spectra in the range3800-6800 Å for 13 concentrated open clusters with Galacticlongitudes between 219deg and 316deg, nine ofwhich have not been previously studied. Using the equivalent widths ofthe Balmer lines and comparing the cluster spectra with template spectraof Magellanic Clouds and Galactic star clusters with known parameters,we derive both foreground interstellar reddening values and age. Fornine clusters these two parameters have been determined for the firsttime, while for the rest of the sample the results show good agreementwith previous studies. The present analysis indicates four very young(Hogg 11, NGC 5606, vdB-RN 80 and Pismis 17), seven moderately young(ESO 429-SC13, Hogg 3, Hogg 12, Haffner 7, BH 87, NGC 2368 and Bochum12) and two intermediate-age (Berkeley 75 and NGC 2635) open clusters.The derived foreground interstellar reddening values are in the range0.00 <= E(B-V) <= 0.38. The age and reddening distributions of thepresent sample of relatively faint open clusters match those of openclusters with known parameters in a 90deg sector centered atl = 270deg. Based on observations made at ComplejoAstronómico El Leoncito, which is operated under agreementbetween the Consejo Nacional de Investigaciones Científicas yTécnicas de la República Argentina and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina.
| 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
| The open cluster Havlen-Moffat No. 1 revisited
A deep CCD UBVRI photometric survey combined with UBVRI polarimetricobservations of 21 bright stars was carried out in the region of theopen cluster Havlen-Moffat No. 1. Our data reveal that the extinctionlaw in this cluster is variable and that six cluster stars show veryhigh polarisation values (>4%), probably because of the presence of anearby small dust cloud. The cluster is at a distance of d = 3300 pc, itis 2-4 Myr old and the initial mass function of its most massive stars(M > 3 Msun ) has a flat slope of x~ 0.7. As an additionalresult, it was possible to reconcile the absolute magnitudes of the twoWN7-type members using the R-values valid in the regions where they arelocated. Based on observations collected at the University of TorontoSouthern Observatory, Las Campanas, Chile, and the ComplejoAstronómico El Leoncito, (CASLEO), Argentina, operated underagreement between the Consejo Nacional de InvestigacionesCientíficas y Técnicas de la República Argentinaand the National Universities of La Plata, Córdoba and San Juan.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/371/908.
| CCD UBVI Photometry and Polarimetry in the Open Cluster Trumpler 21
CCD UBVI photometry was carried out in the field of the open clusterTrumpler 21. Our data locate the cluster at a distance of 1380 pc in alow absorption zone with E(B-V) = 0.25. The age of the cluster isestimated to be 25-30 × 106 yr based on thesuperposition of isochrones computed with mass loss and overshooting.The cluster mass spectrum has a slope of 1.44 similar to the typicalvalue 1.35 for field stars. UBVRI polarimetry of its brightest membersled us to the finding that diffuse interstellar material is responsiblefor both polarization percentage and its direction.
| 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
| 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.
| UBVI imaging photometry of NGC 6231
CCD UBVI photometry in the field of the open cluster NGC 6231 wasobtained for 1060 stars down to V~ 19 mag. Memberships, reddening,distance and age of this cluster were investigated. Its lower sequencedisplays a notorious bend at V~ 13.5 mag followed by a high number offaint stars showing a large magnitude spread at constant colour. Thisdistribution of stars does not seem to be produced by a mereaccumulation of field stars seen in the direction of the cluster but bya real star excess in the zone. The evidence suggests these stars areserious candidates to be cluster members caught in their way towards theZAMS. Fitting them with pre main sequence isochrones an age spread ofabout 10 Myr is obtained. The luminosity and mass functions were foundto be flat for -7.5 < MV < 1.5 and from 3 to 80 {calMsun} respectively. Based on observations collected at theUniversity of Toronto Southern Observatory, Las Campanas, Chile. Table 1is available only in electronic form at the CDS via anonymous ftp(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Absolute proper motions of 181 young open clusters.
Not Available
| Investigation on the region of the open cluster TR 14.
We present UBVRI CCD imaging photometry down to approximately V=19mag inthe area of the very young open cluster Tr 14. Our results confirm theexistence of an anomalous extinction law in the area if it isinterpreted in terms of the UBVRI photometry. According to our analysisthe cluster distance is d=3100 (V_0_-M_V_=12.50mag) and its age is1.5+/-.5x10^6^y. We want to draw the attention on stars which, from theobserved magnitude-spread at constant color, could be stars in an earlystage of evolution. In that case, the observed spread leads to assumethat the process of star formation in Tr 14 is continuous whereas, frompre-main sequence evolutionary tracks, a duration of about 5x10^6^y isdeduced for it. The computation of the cluster initial mass functionreveals a flat slope even though it still compares to a Salpeter law. Atthe same time, we found a spatial variation of the cluster IMF, whichcould be explained by mass segregation in the star formation process.The cluster youth and its compact appearance seem to confirm that it isfar from virial equilibrium. The observed total cluster mass is around2000Msun_ and its core contains about 40% of the total.
| A detailed study of the open cluster PISMIS 20.
The very young open cluster Pis 20 has been investigated using CCD UBVRphotometry. New blue faint members were found down to V=19mag. With themwe obtain a better definition of the lower cluster main sequence. Usingthe fitting method, we derive a cluster distance of d=3600pc(V_0_-M_V_=12.80). The mean color excess is =1.24mag. Fromthe observed characteristics and spatial location of the bluest clusterstars we conclude that the star WR 67 has a good chance to be a clustermember. In that case, the mass of its progenitor should be greater than50Msun_ according to evolutionary models. We obtain for Pis20 an age of 5(+/-1)x10^6^yr from isochrone fitting. We discuss thecluster's mass spectrum whose slope, within the observationaluncertainties, appears to be similar to the one in Salpeter's law. Wenotice that high WR/OB and WC/WN number ratios are present not onlywithin the cluster itself but also in the association where Pis 20 issituated. Although the cluster distance derived here places Pis 20nearer than the supernova remnant MSH 15-52, we cannot/rule out thepossibility that they are spatially related. The presently observedcluster mass is around 600Msun_.
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