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Multicolour CCD measurements of visual double and multiple stars. III
Context: Recent CCD observations were performed in the period 1998-2004for a large sample of visual double and multiple stars selected from theHipparcos Catalogue and/or from the Gliese Catalogue of Nearby Stars. Aims: Accurate astrometric and photometric data allowing us tocharacterise the individual components are provided. These data arecompared to Hipparcos data or to data from an older epoch to assess thenature of the observed systems. Methods: We simultaneously apply aMoffat-Lorentz profile with a similar shape to all detected componentsand adjust the profile parameters from which we obtain the relativeastrometric position (epoch, position angle, angular separation) as wellas differential multi-colour photometry (filters (B)VRI). Results: Wethus acquired recent data for 71 visual systems of which 6 are orbitalbinaries, 27 are nearby, and 30 are multiple systems. In three of thesecases, the systems remained unresolved. 23 new components were detectedand measured. Two new visual double stars of intermediate separationwere also found. The estimated accuracies in relative position are0.04° and 0.01 arcsec respectively, while those in differentialphotometry are of the order of 0.01-0.02 mag in general. Conclusions:.The nature of the association of 55 systems is evaluated. New basicbinary properties are derived for 20 bound systems. Component coloursand masses are provided for two orbital binaries.Based on observations collected at the National AstronomicalObservatory, Rozhen, and the Astronomical Observatory, Belogradchik,both operated by the Institute of Astronomy, Bulgarian Academy ofSciences. Also based on data obtained by the Hipparcos astrometrysatellite. Appendix A is only available in electronic form athttp://www.aanda.org Tables 4-6 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/641
| The Monitor project: searching for occultations in young open clusters
The Monitor project is a photometric monitoring survey of nine young(1-200Myr) clusters in the solar neighbourhood to search for eclipses byvery low mass stars and brown dwarfs and for planetary transits in thelight curves of cluster members. It began in the autumn of 2004 and usesseveral 2- to 4-m telescopes worldwide. We aim to calibrate the relationbetween age, mass, radius and where possible luminosity, from the Kdwarf to the planet regime, in an age range where constraints onevolutionary models are currently very scarce. Any detection of anexoplanet in one of our youngest targets (<~10Myr) would also provideimportant constraints on planet formation and migration time-scales andtheir relation to protoplanetary disc lifetimes. Finally, we will usethe light curves of cluster members to study rotation and flaring inlow-mass pre-main-sequence stars.The present paper details the motivation, science goals and observingstrategy of the survey. We present a method to estimate the sensitivityand number of detections expected in each cluster, using a simplesemi-analytic approach which takes into account the characteristics ofthe cluster and photometric observations, using (tunable) best-guessassumptions for the incidence and parameter distribution of putativecompanions, and we incorporate the limits imposed by radial velocityfollow-up from medium and large telescopes. We use these calculations toshow that the survey as a whole can be expected to detect over 100 younglow and very low mass eclipsing binaries, and ~3 transiting planets withradial velocity signatures detectable with currently availablefacilities.
| 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.
| Is the Cepheid V1726 Cygni an overtone pulsator?
Not Available
| 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.
| Photometric Monitoring of Open Clusters. II. A New M Dwarf Eclipsing Binary System in the Open Cluster NGC 1647
We have discovered a new M dwarf eclipsing binary system in the opencluster NGC 1647. Unlike field binaries, accurate age and metallicityestimates for the system are potentially available through an analysisof the brighter cluster members. This information will ultimately allowa detailed and robust comparison with the appropriate stellar evolutionmodels. Analysis of our spectroscopic radial velocity data andphotometric light-curve data gives an orbital period of0.618790+/-0.000005 days and the following properties for the individualstellar components: M1=0.47+/-0.05 Msolar,M2=0.19+/-0.02 Msolar, R1=0.57+/-0.02Rsolar, R2=0.21+/-0.01 Rsolar,Teff1=3320+/-150 K, and Teff2=2910+/-150 K. Thesmall mass ratio and low secondary mass provide an unprecedentedopportunity to test stellar models. Adopting an age of 150 Myr,consistent with the cluster turn-off, and testing a range ofmetallicities, covering plausible values for local open clusters, wefind that none of the models are consistent with all the properties ofboth M dwarf stars in the eclipsing binary.
| 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.
| Seven-Color Photometry of the Open Cluster NGC 1647 Area
The area of the open cluster NGC 1647 in Taurus is investigated by CCDphotometry in the Vilnius seven-color system. Magnitudes and colorindices are determined for 433 stars down to V = 15.0 mag in the 45arcmin diameter area. For 252 of them photometric spectral andluminosity classes, interstellar reddenings, extinctions and distancesare obtained. According to the CDS WEBDA database, 89 of them have ahigh cluster membership probability. Their mean distance from the Sunis 555±74 pc, excluding four stars which seem to be field stars.The main sequence starts at spectral class B7 V which corresponds to acluster age of about 150 million years. Cluster members show adifferential interstellar extinction ranging from 0.8 to 1.8 mag. Themean extinction of the cluster stars is 1.12±0.25 mag.Interstellar extinction in the area is dominated by the Taurus darkcloud complex at 160 pc. Color excesses of individual stars correlatewell with the 100 μm dust thermal emission intensity. The clustershape is investigated by counting stars down to K=15.6 mag and is foundto be elongated in the direction roughly perpendicular to the Milky Way,with the flattening 0.4.
| Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas
We present a catalogue (CSOCA ) of stars residing in 520 Galactic opencluster sky areas which is the result of the kinematic (proper motion)and photometric member selection of stars listed in the homogeneousAll-sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). We describethe structure and contents of the catalogue, the selection procedureapplied, and the proper motion and photometric membership constraintsadopted. In every cluster area the CSOCA contains the complete list ofthe ASCC-2.5 stars regardless of their membership probability. Forevery star the CSOCA includes accurate J2000 equatorial coordinates,proper motions in the Hipparcos system, BV photometric data in theJohnson system, proper motion and photometric membership probabilities,as well as angular distances from the cluster centers for about 166 000ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types,multiplicity and variability flags from the ASCC-2.5, and radialvelocities with their errors from the Catalogue of Radial Velocities ofGalactic Stars with high precision Astrometric Data (CRVAD) are alsogiven.
| Photometric Monitoring of Open Clusters. I. The Survey
Open clusters, which have age, abundance, and extinction informationfrom studies of main-sequence turnoff stars, are the ideal location inwhich to determine the mass-luminosity-radius relation for low-massstars. We have undertaken a photometric monitoring survey of openclusters in the Galaxy designed to detect low-mass eclipsing binarysystems through variations in their relative light curves. Our aim is toprovide an improved calibration of the mass-luminosity-radius relationfor low-mass stars and brown dwarfs, to test stellar structure andevolution models, and to help quantify the contribution of low-massstars to the global mass census in the Galaxy. In this paper we presentour survey, describing the data and outlining the analysis techniques.We study six nearby open clusters, with a range of ages from ~0.2 to 4Gyr and metallicities from approximately solar to -0.2 dex. We monitor afield of view of greater than 1 deg2 per target cluster, wellbeyond the characteristic cluster radius, over timescales of hours,days, and months with a sampling rate optimized for the detection ofeclipsing binaries with periods of hours to days. Our survey depth isdesigned to detect eclipse events in a binary with a primary star of<~0.3 Msolar. Our data have a photometric precision of ~3mmag at I~16.
| 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.
| The Distance Scale for Classical Cepheid Variables
New radii, derived from a modified version of the Baade-Wesselink (BW)method that is tied to published KHG narrowband spectrophotometry, arepresented for 13 bright Cepheids. The data yield a best-fittingperiod-radius relation given bylog=1.071(+/-0.025)+0.747(+/-0.028)logP0. In combination with other high-quality radiusestimates recently published by Laney & Stobie, the new data yield aperiod-radius relation described bylog=1.064(+/-0.0006)+0.750(+/-0.006)logP0, which simplifies to ~P3/4.The relationship is used to test the scale of Cepheid luminositiesinferred from cluster zero-age main-sequence (ZAMS) fitting, for whichwe present an updated list of calibrating Cepheids located in stellargroups. The cluster ZAMS-fitting distance scale tied to a Pleiadesdistance modulus of 5.56 is found to agree closely with the distancescale defined by Hipparcos parallaxes of cluster Cepheids and alsoyields Cepheid luminosities that are a good match to those inferred fromthe period-radius relation. The mean difference between absolute visualmagnitudes based on cluster ZAMS fitting,C, and those inferred for 23 clusterCepheids from radius and effective temperature estimates,BW, in the sense of C-BW is+0.019+/-0.029 s.e. There is no evidence to indicate the need for amajor revision to the Cepheid cluster distance scale. The absolutemagnitude differences are examined using available [Fe/H] data for thecluster Cepheid sample to test the metallicity dependence of theperiod-luminosity relation. Large scatter and a small range ofmetallicities hinder a reliable estimate of the exact relationship,although the data are fairly consistent with predictions from stellarevolutionary models. The derived dependence isΔMV(C-BW)=+0.06(+/-0.03)-0.43(+/-0.54)[ Fe/H].
| Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations. II. Open cluster Platais 1 (C2128+488) and small-amplitude Cepheid V1726 Cygni
The small-amplitude Cepheid V1726 Cyg and two membersof open cluster Platais 1 (Platais 1 star No. 1(1921) and Platais 1 star No. 111 (1600))were investigated, using high-resolution CCD spectra. The followingresults were obtained: 1) All objects have the same metallicities, closeto that of the Sun (for V1726 Cyg weighted average [Fe/H]=+0.05, forPlatais 1 star No. 1 (1921) [Fe/H]=+0.13); 2) values of Teffand log g for the B-stars are in excellent agreement with thosedetermined from (B-V) colour indices using a (B-V) ~ (Teff,log g) calibration; 3) the elemental abundances indicate that V1726 Cygis in the post first dredge-up stage with an age near 1.5 x108, and is crossing the Cepheid instability strip for thethird time. Mean values of Teff=6100 K and log g=2.35 +/-0.05 permit us to refine its colour excess to EB-V=0fm 33,which for a distance of d = 1568+/- 13 pc corresponds toMV=-2fm 99. The Cepheid could therefore be pulsating in thefundamental tone, although pulsation in the first overtone is notexcluded; 4) Platais 1 Star No. 1 (1921) is a slowly rotating HgMn-starwith a high helium content, while Platais 1 star No. 111 (1600) is arapidly rotating main-sequence star with a helium content comparable tothat of the Sun; 5) the age of the open cluster is estimated to be about2.5 x 108 yr. Based on the spectra collected with the 6-mtelescope SAO RAS.
| Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
| The CFHT Open Star Cluster Survey. I. Cluster Selection and Data Reduction
We present this paper in conjunction with a companion paper as the firstresults in the Canada-France-Hawaii Telescope Open Star Cluster Survey.This survey is a large BVR imaging data set of 19 open star clusters inour Galaxy. This data set was taken with the CFH12K mosaic CCD(42'×28'), and the majority of the clusters were imaged underexcellent photometric, subarcsecond seeing, conditions. The combinationof multiple exposures extending to deep (V~25) magnitudes with short(<=10 s) frames allows for studies ranging from faint white dwarfstars to bright turnoff, variable, and red giant stars. The primary aimof this survey is to catalog the white dwarf stars in these clusters andestablish observational constraints on the initial-final massrelationship for these stars and the upper mass limit to white dwarfproduction. Additionally, we hope to better determine the properties ofthe clusters, such as age and distance, and also test evolution anddynamical theories by analyzing luminosity and mass functions. In orderto more easily incorporate these data in further studies, we haveproduced a catalog of positions, magnitudes, colors, and stellarityconfidence for all stars in each cluster of the survey. This reduction,along with the computed calibration parameters for all three nights ofthe observing run will encourage others to use these data in differentastrophysical studies outside of our goals. Additionally, the data setis reduced using the new TERAPIX photometric reduction package, PSFex,which is found to compare well with other packages. This paper isintended both as a source for the astronomical community to obtaininformation on the clusters in the survey and as a detailed reference ofreduction procedures for further publications of individual clusters. Wediscuss the methods employed to reduce the data and compute thephotometric catalog. We reserve both the scientific results for eachindividual cluster and global results from the study of the entiresurvey for future publications. The first of these further publicationsis devoted to the old rich open star cluster, NGC 6819, and appears as acompanion paper in the same issue of the Journal.
| The Pulsation Mode of the Cluster Cepheid V1726 Cygni
CCD V-band observations and archival O-C estimates are presented for the4.237 day Cepheid V1726 Cyg, which is a member of the open clusterC2128+488 (Anon. Platais), and the data are analyzed in conjunction withother available photometric data in order to study the star's periodvariations. An O-C analysis of the data yields the following improvedephemeris for the variable,HJDmax=2,444,020.5892+4.2369334E+2.044×10-8E2, which implies that the period of V1726 Cyg isincreasing at a rate of +0.304+/-0.026 s yr-1, consistentwith a star in the third crossing of the instability strip provided thatit is pulsating in the first-overtone, rather than fundamental, mode.Overtone pulsation for V1726 Cyg is also indicated by its low amplitude,sinusoidal light curve, and derived Fourier parameters, despitereservations about the significance of the latter. No evidence is foundfor random cycle-to-cycle variations in period for V1726 Cyg.
| Luminosity and mass function of galactic open clusters I. NGC 4815
We present deep V and I photometry for the open cluster NGC 4815 andfour surrounding Galactic fields down to a limiting magnitude V ~ 25.These data are used to study cluster spatial extension by means of starcounts, and to derive the luminosity (LF) and mass function (MF). Theradius turns out to be 3.6+/-0.3 arcmin at V=19.0, whereas the mass is880+/-230 msun down to V=20.8. From the color-magnitudediagram, we obtain the LFs in the V and I bands, using both the standardhistogram and an adaptive kernel. After correction for incompletenessand field star contamination, the LFs were transformed into the presentday mass functions (PDMF). The PDMFs from the V and I photometry can berepresented as a power-law with a slope alpha = 3.1+/-0.3 and alpha =2.9+/-0.3 (the (Salpeter \cite{salp55}) MF in this notation has a slopealpha = 2.35) respectively, in the mass range 2.5 <=(m)/(msun) <= 0.8. Below this mass, the MF cannot beconsidered as representative of the cluster IMF, as it is the result ofthe combined effect of strong irregularities in the stellar background,probable internal dynamical evolution of the cluster and/or interactionof the cluster with the dense Galactic field. Unresolved binaries andmass segregation can only flatten the apparent derived IMF, so we expectthat the real IMF must be steeper than the quoted slope by an unknownamount. Based on observations made at the European Southern Observatory,La Silla, Chile.
| Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations. I. Association Cas OB2 and the small-amplitude Cepheid SU Cassiopeae
The small-amplitude Cepheid SU Cas and four membersof the association Cas OB2 (HD 16893, HD17327a and b, HD 17443) were investigated,using high-resolution CCD spectra. The following results were obtained:1) All these objects have the same metallicity values, close to that ofthe Sun; 2) Elemental abundance indicates that SU Cas is a post firstdredge-up star with an age from 1 108 to 1.45 108yr, and it is not crossing the Cepheid instability strip for the firsttime. The mean value of log g = 2.35 corresponds to pulsations in thefundamental tone, although errors in gravity estimations provideovertone pulsations. The questions about its pulsational mode andmembership in Cas OB2 remained open; 3) HD17327a is a slowly rotating HgMn-star with the highest heliumcontent among such objects, while HD 16893 also has a manganeseoverabundance and might be classified as an Am-star; 4) HD17327b and HD 17443 are rapidly rotating main-sequence stars,while HD 17443 has a helium content comparable with that of the Sun.
| 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
| Dense Star Clusters as the Sources of Gamma-Ray-Burst Progenitors
Two independent sets of arguments lead us to conclude that theprogenitors of superintense bursts (with an energy yield larger thanthat for ordinary supernovae by one or two orders of magnitude) are bornin massive dense star clusters, but generally flare up only after theyhave left the cluster; these are the same objects that are theprogenitors of gamma-ray bursts (GRBs). Each of the giant stellar arcswhich are grouped into multiple systems of stellar complexes in the LMCand NGC6946 could only be produced by a single powerful energy releasenear its center. The progenitors of these systems of arc-shaped stellarcomplexes must have had a common source nearby, and it could only be amassive star cluster. Such clusters are actually known near bothsystems. On the other hand, calculations of the dynamical evolution ofstar clusters show that close binary systems of compact objects areformed in the dense central parts of the clusters and are then ejectedfrom them during triple encounters. Mergers of the components of suchsystems are believed to be responsible for GRBs. Since their progenitorsare ejected from the cluster before merging, the arc-shaped stellarcomplexes produced by GRBs are observed near (but not around) the parentclusters. If a considerable fraction of the GRB progenitors are formedas a result star encounters in massive star clusters, and if the GRBsthemselves trigger star formation near the parent clusters, thenobservations of GRBs in star-forming regions are consistent with theirorigin during mergers of pairs of compact objects.
| 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.
| 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
| A photometric and spectroscopic study of the brightest northern Cepheids - II. Fundamental physical parameters
We present a new Cepheid reddening and effective temperature scale basedon the uvby photometry published in the first paper of this series.Using all available information about the companion stars in Cepheidswith bright blue secondaries, we remove their light from the observedlight and colour curves. The resulting corrections are as large as0.05-0.15 mag in several cases for different colour indices. A newphotometric approach based on the (b -y) versus (B-V) two-colour diagramis tested with three other previous calibrations taken from theliterature. Two uvby relations in earlier studies turn out to be themost reliable and consistent, and so they are used in deriving colourexcesses. We determine systematically higher reddenings for Cepheidswith a significant secondary light correction. The dereddened Stromgrencolours are calibrated in terms of T_eff and logg using the most recentsynthetic colour grids. Our temperature scale is very close to that ofKraft, which is supported by other recent temperature determinationsusing the infrared flux method or Geneva photometry. The photometricgravities fit some of the earlier theoretical and observational (mainlyspectroscopic) results very well.
| Very Accurate Distances and Radii of Open Cluster Cephids from a Near-Infrared Surface Brightness Technique
We have obtained the radii and distances of 16 galactic Cepheidssupposed to be members in open clusters or associations using a newoptical and two near-infrared calibrations of the surface brightness(Barnes-Evans) method. We find excellent agreement of the radii anddistances produced by both infrared techniques, which use the V, V - K(K on the Carter system) and the K, J - K magnitude-color combinations,respectively, with typical random errors that are as little as ~2%. Wediscuss possible systematic errors in our infrared solutions in detailand conclude that the typical total uncertainty of the infrared distanceand radius of a Cepheid is about 3% in both infrared solutions, providedthat the data are of excellent quality and that the amplitude of thecolor curve used in the solution is larger than ~0.3 mag. The optical V,V - R distance and radius of a given Cepheid can deviate by as much as~30% from the infrared value because of large systematic and randomerrors caused by microturbulence and gravity variations: these affectthe optical but not the infrared colors. We find excellent agreement ofour infrared radii with the infrared radii derived previously for thesevariables by Laney & Stobie from an application of the maximumlikelihood technique, which further increases our confidence that thetotal errors in our infrared solutions are not larger than ~3%. In anAppendix we discuss the relative advantages and disadvantages of ourinfrared surface brightness technique and the maximum likelihoodtechnique. We compare the adopted infrared distances of the Cepheidvariables to the zero-age main-sequence--fitting (ZAMS-fitting)distances of their supposed host clusters and associations (assuming aPleiades distances modulus of 5.57) and find an unweighted mean value ofthe distance ratio of 1.02 +/- 0.04. A detailed discussion of theindividual Cepheids shows that the uncertainty of the ZAMS-fittingdistances varies considerably from cluster to cluster. We find clearevidence that four Cepheids are not cluster members (SZ Tau, T Mon, UCar, and SV Vul), while we confirm cluster membership for V Cen and BBSgr, for which former evidence for cluster membership was only weak.After rejection of nonmembers, we find a weighted mean distance ratio of0.969 +/- 0.014, with a standard deviation of 0.05, which demonstratesthat both distance indicators are accurate to better than 5%, includingsystematic errors, and that there is excellent agreement between bothdistance scales.
| Cluster membership determinations from proper motion surveys
Not Available
| The astrometric accuracy of "Carte DU Ciel" plates and proper motions in the field of the open cluster NGC 1647.
The astrometric accuracy of triple image Carte du Ciel plates has beenanalysed using plates of the open cluster NGC 1647. A new multithresholdtechnique was used for the treatment of the triple image plates.Accuracies ranging from 100 mas to 200 mas were found. We determinedproper motions with a median accuracy of 1.6 mas/a for 2220 stars in thefield of NGC 1647. The membership probabilities of stars in the clusterregion agree very well with the data of Francic (1989). Due to asignificant difference of the proper motions of SZ Tau and the clustermembers, SZ Tau seems not to be a member of NGC 1647.
| The Progenitors of Classical Cepheid Variables
Properties are deduced for the main-sequence O and B-type stars thatwere the immediate progenitors to classical Cepheids belonging to thelocal sample of galactic calibrating clusters. The sample of suchclusters and stellar groups is presently large enough to allow thederivation of statistically-significant properties related to theirCepheid members. In particular, the turnoff point colour for a clusteris demonstrated to be linearly related to the pulsational period (Po) ofits associated Cepheid via the empirical relationship: (B=96V)o(turnoff) =3D =960.057 =96 0.113= log Po; the masses of stars at the redturnoff point for the cluster follow a semi-empirical relationshipdescribed by: log M(RTO)/Ms =3D +0.36 + 0.50 log Po. The adoptedrelationship for the Cepheid progenitors is: log M/Ms =3D +0.41 + 0.50log Po. The properties deduced for Cepheid progenitors as a consequenceof these relationships reveal intriguing information about classicalCepheids themselves.
| BVRIJHK period-luminosity relations for Galactic classical Cepheids.
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