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New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| 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.
| Observations of 14 young open star clusters with the HEGRA system of Cherenkov telescopes
Context: .A sample of 14 young open star clusters has been observed inthe TeV energy regime with the stereoscopic system of the HEGRA (HighEnergy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 to 2002,resulting in more than 300 h of observation time. Aims: .Youngopen star clusters may contribute to the acceleration of cosmic rays.The detection of γ-rays (from decaying π^0s produced inhadronic interactions) from these objects could be evidence for such acontribution. The results of our observations are compared to availableγ-ray data and to a simple hadronic model in the framework ofshock front acceleration of cosmic rays in the stellar winds of thecluster members to test the potential of the presently available data onyoung open star clusters to constrain this type of model. Methods:.The stereoscopic system of HEGRA Cherenkov telescopes makes use of theatmospheric imaging technique. Air showers initiated by primaryGamma-Rays are recorded as elliptical images in the telescope cameras.The images from the different telescopes are then superimposed toreconstruct the parameters of the primary particle. This technique(stereoscopy) was pioneered by the HEGRA experiment. Results: .Nosignificant excess has been found in the analysed data set of young openstar clusters. The derived upper limit on the TeV gamma-ray flux fromBerkeley 87 and the available EGRET data from the same direction do notallow us to fully constrain the simple hadronic model used here. Thecomparison of the upper limits derived for all 14 objects with the fluxdetected from TeV J2032+4130 (under the assumption of an association ofthe TeV-signal with the compact stellar association Cyg OB2) suggeststhat γ-ray emission from young open star clusters as an objectclass cannot be ruled out.
| 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.
| New galactic open cluster candidates from DSS and 2MASS imagery
An inspection of the DSS and 2MASS images of selected Milky Way regionshas led to the discovery of 66 stellar groupings whose morphologies,color-magnitude diagrams, and stellar density distributions suggest thatthese objects are possible open clusters that do not yet appear to belisted in any catalogue. For 24 of these groupings, which we consider tobe the most likely to be candidates, we provide extensive descriptionson the basis of 2MASS photometry and their visual impression on DSS and2MASS. Of these cluster candidates, 9 have fundamental parametersdetermined by fitting the color-magnitude diagrams with solarmetallicity Padova isochrones. An additional 10 cluster candidates havedistance moduli and reddenings derived from K magnitudes and (J-K) colorindices of helium-burning red clump stars. As an addendum, we alsoprovide a list of a number of apparently unknown galactic andextragalactic objects that were also discovered during the survey.
| Eclipsing binary stars in open clusters.
Not Available
| 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.
| Evolutionary constraints on the masses of the components of the HDE 226868/Cyg X-1 binary system
Calculations carried out to model the evolution of HDE 226868, underdifferent assumptions about the stellar wind mass-loss rate, providerobust limits on the present mass of the star. It has to be in the range40 +/- 5Msolar if the distance to the system is in the range1.95-2.35 kpc and the effective temperature of HDE 226868 in the range30000-31000 K. Extending the possible intervals of these parameters to1.8-2.35 kpc and 28000-32000 K, one gets for the mass of the star therange 40 +/- 10Msolar. Including into the analysisobservational properties such as the profiles of the emission lines,rotational broadening of the absorption lines and the ellipsoidal lightvariations, one can estimate also the mass of the compact component. Ithas to be in the ranges 20 +/- 5Msolar and13.5-29Msolar for the cases described above. The sameanalysis (using the evolutionary models and the observational propertieslisted above) yields a lower limit to the distance to the system of ~2.0kpc, if the effective temperature of HDE 226868 is higher than 30000 K.This limit to the distance does not depend on any photometric orastrometric considerations.
| Eclipsing binaries in open clusters - III. V621 Per in χ Persei
V621 Persei is a detached eclipsing binary in the open cluster χPersei, which is composed of an early B-type giant star and amain-sequence secondary component. From high-resolution spectroscopicobservations and radial velocities from the literature, we determine theorbital period to be 25.5 d and the primary velocity semi-amplitude tobe K= 64.5 +/- 0.4 km s-1. No trace of the secondary star hasbeen found in the spectrum. We solve the discovery light curves of thistotally eclipsing binary and find that the surface gravity of thesecondary star is loggB= 4.244 +/- 0.054. We compare theabsolute masses and radii of the two stars in the mass-radius diagram,for different possible values of the primary surface gravity, with thepredictions of stellar models. We find that loggA~ 3.55, inagreement with values found from fitting Balmer lines with syntheticprofiles. The expected masses of the two stars are 12 and 6Msolar and the expected radii are 10 and 3 Rsolar.The primary component is near the blue loop stage in its evolution.
| Eclipsing binaries in open clusters - II. V453 Cyg in NGC 6871
We derive absolute dimensions of the early B-type detached eclipsingbinary V453 Cygni (B0.4 IV + B0.7 IV, P= 3.89 d), a member of the opencluster NGC 6871. From the analysis of new, high-resolution spectroscopyand the UBV light curves of Cohen, we find the masses to be 14.36 +/-0.20 and 11.11 +/- 0.13 Msolar, the radii to be 8.55 +/- 0.06and 5.49 +/- 0.06 Rsolar, and the effective temperatures tobe 26600 +/- 500 and 25500 +/- 800 K for the primary and secondarystars, respectively. The surface gravity values of logg= 3.731 +/- 0.012and 4.005 +/- 0.015 indicate that V453 Cyg is reaching the end of itsmain-sequence lifetime. We have determined the apsidal motion period ofthe system to be 66.4 +/- 1.8 yr using the technique of Lacy extended toinclude spectroscopic data as well as times of minimum light, giving adensity concentration coefficient of logk2=-2.226 +/- 0.024.Contaminating (third) light has been detected for the first time in thelight curve of V453 Cyg; previous analyses without this effectsystematically underestimate the ratio of the radii of the two stars.The absolute dimensions of the system have been compared with thestellar evolution models of the Granada, Geneva, Padova and Cambridgegroups. All model sets fit the data on V453 Cyg for solar helium andmetal abundances and an age of 10.0 +/- 0.2 Myr. The Granada models alsoagree fully with the observed logk2 once general relativisticeffects have been accounted for. The Cambridge models with convectivecore overshooting fit V453 Cyg better than those without. Given thissuccess of the theoretical predictions, we briefly discuss whicheclipsing binaries should be studied in order to challenge the modelsfurther.
| 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.
| The Lack of Blue Supergiants in NGC 7419, a Red Supergiant-rich Galactic Open Cluster with Rapidly Rotating Stars
According to previous studies based on photometry alone, NGC 7419reveals a surprisingly low ratio of blue to red supergiants: only oneblue supergiant (BSG) along with a record number of five red supergiants(RSGs). However, for a cluster like NGC 7419 with solar metallicity, oneexpects twice as many BSGs as RSGs. To verify the small ratio of BSGs toRSGs, we have obtained spectroscopic observations of the seven mostluminous blue member stars using the 1.6 m telescope of the MountMégantic Observatory. (The RSGs have already been studiedspectroscopically.) To classify the stars, we have developed a systemespecially adapted for these heavily reddened stars in a spectral regionfrom 8400 to 8920 Å, near the hydrogen Paschen series limit. Thisclassification system is based on standard stars of known MK spectraltype extending over O9-B5 and all luminosity classes and is linkedthrough a grid of synthetic spectra to the atmospheric physicalparameters Teff and logg. We also include Be stars. Among theseven blue stars observed in NGC 7419, four have red spectra that aredominated by absorption lines and three by emission lines. The spectraltypes for the former are B2.5 II-III, B2.5 III, B0 III, and B4 III (e),while those for the latter are Be, B1 III-Ve, and Be, respectively. Theaverage heliocentric radial velocity of these stars is -66+/-6 kms-1, compatible with the value of -74+/-9 km s-1measured for the five RSG members. A distance of 1.7+/-0.4 kpc for thiscluster was estimated using the blue-star spectral types, in agreementwith the value of 2.3+/-0.3 kpc found by Beauchamp and coworkers, basedon isochrone fitting in the color-magnitude diagram. With no BSG starsdetected spectroscopically, we confirm the low number, in this caseabsence, of BSGs in this cluster. The high fraction of Be stars detectedby us and others among the bright blue member stars could be explainedby an average rotational velocity for the stars in NGC 7419, which issignificantly higher than in other clusters of similar age andmetallicity. Since higher stellar rotation rates shorten the BSG phase,we suggest that this explains why the evolved stars in NGC 7419 havebecome RSGs. Thus, NGC 7419 is an exceptional case, since high stellarrotation normally tends to occur at lower metallicity.
| Photoelectric Observations of V448 CYGNAE
The research literature on the star V448 Cyg is reviewed in more detail.An analysis of three color photoelectric observations made at Abastumaniis reported in detail with corresponding tables and graphs. Inparticular, the gaseous structure and evolutionary status of the systemare discussed. Newly published spectroscopic data suggest that our lightcurves might be interpreted by more modern nonclassical methods assuminga new model. Thus, comparing data obtained in this fashion with the newspectroscopic data may help improve the evolutionary status and modelfor the V448 Cyg system. It is also seems possible to obtain bettervalues of the physical parameters for circumstellar structures andobtain more precise information on the physical conditions in theatmospheres of the stars and on the structure of the circumstellarshells.
| 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.
| Spectroscopic Survey of the Galactic Open Cluster NGC 6871. I. New Emission-Line Stars
We analyze spectra of 44 emission-line stars detected in alow-resolution optical spectroscopic survey of the galactic open clusterNGC 6871. The survey of 1217 stars is complete to V=14.9 and includesstars with V<16.5 between the zero-age main sequence and the107 yr pre-main-sequence (PMS) isochrone. Of the 44emission-line stars in this survey, 28 show obvious emission in Hαand 16 have weak Hα absorption (compared with Hγ). We usethe reddening to separate foreground and background stars from thecluster members; the position in the HR diagram or the presence offorbidden emission lines ([N II] and [S II]) then yields theevolutionary status of the emission-line stars. A comparison of theHα spectral index distribution in NGC 6871 with the distributionof young stars in the Taurus-Auriga molecular cloud indicates that thelate-type PMS stars in our sample may be weak-line T Tauri stars. Manyof these stars show [S II] (36%) and [N II] (45%) emission.
| Characteristics of new star cluster candidates in the Cygnus area
The morphology and stellar content of 22 clusters in the Cygnus area hasbeen determined using 2MASS infrared observations in the J and K bands.7 of the clusters are well-known objects and our results are in goodagreement with previous work. 12 objects are recently-discovered clustercandidates (Dutra & Bica \cite{dutra01}) and 3 are new clustercandidates. Adopting distance estimates from the literature or byconsidering cluster counterparts, we derive a consistent set ofparameters suitable for analysis and comparison: center coordinates,radius, stellar population, IMF slope and mass range. We find a mean IMFslope of Gamma =-1.30 for the known clusters and Gamma =-1.40 for theothers. Infrared data having better completeness, we derive largerstellar contents and cluster masses than previous works. An importantfraction of the cluster stellar population in Cygnus is hidden in veryobscured areas. A clear tendency towards mass segregation appears formost of the clusters. Some of the new clusters seem to be quite massive,probably hosting very massive stars at the center. They are privilegedtargets for more detailed investigations using infrared spectroscopy.
| 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
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/168
| 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).
| A search for optical counterparts to X-ray sources in the WGA catalogue.
Not Available
| Multiperiodicity and physical nature of the δ Sct star GSC 2683-3076
We present the results of a CCD Johnson V and photoelectricStrömgren uvbyβ photometric study of the recently discoveredmultiperiodic δ Sct star GSC 2683-3076. Our data set mainlyconsists of 2874 differential measurements in V together with a few datacollected into the uvbyβ system. Additional unfiltered CCDmeasurements were also carried out. A set of seven best-fittingpulsation frequencies representing the light variations of the variablehas been detected. The spectral type of the variable is found to be A9Vor F0V. Using the uvbyβ indices the following main physicalparameters for the variable have been derived: Teff=7230K,MV=1.95mag, logg=3.90, M=1.85Msolar,R=2.30Rsolar, ρ=0.16ρsolar, age=1.0Gyr,metal abundance [Me/H]=0.16 and distance modulus=8.4mag. GSC 2683-3076is found to be a Population I δ Sct star, slightly metal-enriched,evolving on its main-sequence stage. A mixture of radial and non-radialmodes seems to be present in the pulsation of this variable. This staris also known as H133 in the young open cluster NGC 6871, howeverarguments are given that address this star as a non-member of thecluster. The uvbyβ photometry available in the literature for NGC6871 is also discussed.
| 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].
| Fundamental parameters and new variables of the galactic open cluster NGC 7128
CCD photometry in Johnson UBV and Strömgren uvby systems andmedium-resolution spectroscopy of the galactic open cluster NGC 7128 arepresented. Spectral types of the brightest 12 stars in the cluster fieldwere determined based on equivalent widths of the Hα and the Hei6678-Å line. The spectroscopic observations also revealed twoobvious and one probable Be-type stars showing Hα emission. Theanalysis of the photometric diagrams gave a colour excess ofE(B-V)=1.03+/-0.06mag, a distance modulus DM=13.0+/-0.2mag and an ageabove 10Myr. Time-resolved photometric observations obtained on onenight resulted in the detection of short time-scale light variations ofseven new and three already known variable stars in the cluster field.
| Bi-modal pulsation of the delta Scuti star V1821 Cygni
The results of Johnson V time-series CCD photometry of the delta Scutistar V1821 Cygni are presented. Our data set consists of 2431differential magnitudes and spans the period of July 1999 to September2000. We detect two best-fit pulsation frequencies representing thelight variations of the variable. V1821 Cyg is most likely pulsating ina mixture of radial and nonradial modes. The uvbybeta data in theexisting catalogues have been used to derive the main physicalparameters for the variable. We obtain: Teff=7490+/- 200 K,MV=1fm02 +/-0.2, log g=3.75+/- 0.1 dex, M=2.3+/-0.2Msun, R=3.3+/-0.3 Rsun, bar ρ=0.06+/-0.01 barρsun, age=1.0+/- 0.1 Gyr and a distance modulus of ~ 9fm12. V1821 Cyg is suggested to be a Population I delta Scuti star withmetal-enriched abundances ([M/H]=0.366) evolving on its late mainsequence stage on the basis of the colour indices and derivedproperties.
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations
In a previous paper on the Magellanic Clouds, we demonstrated thatcoeval clusters provide a powerful tool for probing the progenitormasses of Wolf-Rayet (W-R) stars and luminous blue variables (LBVs).Here we extend this work to the higher metallicity regions of the MilkyWay, studying 12 Galactic clusters. We present new spectral types forthe unevolved stars and use these, plus data from the literature, toconstruct H-R diagrams. We find that all but two of the clusters arehighly coeval, with the highest mass stars having formed over a periodof less than 1 Myr. The turnoff masses show that at Milky Waymetallicities some W-R stars (of early WN type) come from stars withmasses as low as 20-25 Msolar. Other early-type WN starsappears to have evolved from high masses, suggesting that a large rangeof masses evolve through an early WN stage. On the other hand, WN7 starsare found only in clusters with very high turnoff masses, over 120Msolar. Similarly, the LBVs are only found in clusters withthe highest turnoff masses, as we found in the Magellanic Clouds,providing very strong evidence that LBVs are a normal stage in theevolution of the most massive stars. Although clusters containing WN7stars and LBVs can be as young as 1 Myr, we argue that these objects areevolved, and that the young age simply reflects the very high massesthat characterize the progenitors of such stars. In particular, we showthat the LBV η Car appears to be coeval with the rest of theTrumpler 14/16 complex. Although the WC stars in the Magellanic Cloudswere found in clusters with turnoff masses as low as 45Msolar, the three Galactic WC stars in our sample are allfound in clusters with high turnoff masses (>70 Msolar)whether this difference is significant or due to small number statisticsremains to be seen. The bolometric corrections of Galactic W-R stars arehard to establish using the cluster turnoff method but are consistentwith the ``standard model'' of Hillier.
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