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Na, Al, and O Abundances of Open Clusters NGC 7142, NGC 6939, and IC 4756
We present an analysis of echelle spectra of stars in three openclusters obtained with the Hydra multiobject spectrograph on the WIYN3.5 m telescope. Abundances of Fe, O, Si, Ca, Na, Al, and Ni have beendetermined via equivalent width analysis and spectrum synthesis. Meanabundances for each cluster are compared to those of previous studiesand of other clusters in the literature, with emphasis on exploring theenhancements of Na and Al seen in many open clusters. All three clustersshow enhanced values of [Na/Fe] and [Al/Fe], while the abundances of Fe,O, Si, and Ca are consistent with their ages and locations in theGalactic disk.
| Young Stars in the Camelopardalis Dust and Molecular Clouds. I. The Cam OB1 Association
The distribution of dust and molecular clouds in the direction ofGalactic longitudes 132--158° and latitudes ± 12\degr\ isinvestigated. The maps of dust distribution in the area were plottedfrom the following surveys: the star counts in the DSS I database byDobashi et al. (2005), the survey of the average infrared color excessesby Froebrich et al. (2007) and the thermal dust emission survey at 100μ m by Schlegel et al. (1998). The distribution of molecular cloudswas taken from the whole sky CO survey by Dame et al. (2001). All thesesurveys show very similar cloud patterns in the area. Using the radialvelocities of CO, the distances to separate clouds are estimated. Arevised list of the Cam OB1 association members contains 43 stars andthe open cluster NGC 1502. 18 young irregular variable and Hαemission stars are identified in the area. All this proves that the starforming process in the Camelopardalis clouds is still in progress.
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| Accretion in young low intermediate mass stars
We present preliminary results of a study of the evolution of accretionrates in some young members of open clusters: NGC1502(1), NGC884(3),Trumpler37(2), Biurakan2(3), Berkeley87(1). Accretion rates werecomputed using UVI photometry and the Gullbring relation. In addition wefound that accretion rates of three members of the MBM12 youngassociation ( ˜ 2Myr) are in agreement with the expected valuesfor CTTS.
| 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.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| CCD photometric search for peculiar stars in open clusters. VI. NGC 1502, NGC 3105, Stock 16, NGC 6268, NGC 7235 and NGC 7510
In a sample of six young open clusters (NGC 1502, NGC 3105, Stock 16,NGC 6268, NGC 7235, and NGC 7510) we investigated 1753 objects using thenarrow band, three filter Δ a photometric system resulting in thedetection of eleven bona-fide magnetic chemically peculiar (CP) starsand five Be or metal-weak stars. The results for the distant cluster NGC3105 is most important because of the still unknown influence of theglobal metallicity gradient of the Milky Way. These findings confirmthat CP stars are present in open clusters of very young ages (log t≥ 6.90) at galactocentric distances up to 11.4 kpc. For all programmeclusters the age, reddening, and distance modulus were derived using thecorresponding isochrones. Some additional variable stars within Stock 16could be identified by comparing different photometric studies.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| SZ Camelopardalis - ein Bedeckungsveraenderlicher in einem gravitativen Vierfachsystem.
Not Available
| Bright OB stars in the Galaxy. I. Mass-loss and wind-momentum rates of O-type stars: A pure H\alpha analysis accounting for line-blanketing
We study mass-loss and wind momentum rates of 29 Galactic O-type starswith luminosity classes I, III and V by means of a pure H\alpha profileanalysis and investigate to what extent the results compare to thoseoriginating from a state-of-the-art, complete spectral analysis. Ourinvestigation relies on the approximate method developed by\citet{Puls96} which we have modified to account for the effects ofline-blanketing. Effective temperatures and gravities needed to obtainquantitative results from such a simplified approach have been derivedby means of calibrations based on most recent spectroscopic NLTEanalyses and models of Galactic stars by \citet{Repo03} and\citet{Martins02}. Comparing (i) the derived wind-densities to thosedetermined by \citet{Repo03} for eleven stars in common and (ii) theWind-momentum Luminosity Relationship (WLR) for our sample stars tothose derived by other investigations, we conclude that our approximateapproach is actually able to provide consistent results. Additionally,we studied the consequences of ``fine tuning'' some of the direct andindirect parameters entering the WLR, especially by accounting fordifferent possible values of stellar reddening and distances. Combiningour data set with the corresponding data provided by \citet{Herrero02}and \citet{Repo03} we finally study the WLR for the largest sample ofGalactic O-type stars gathered so far, including an elaborate errortreatment. The established disagreement between the theoreticalpredictions and the ``observed'' WLRs being a function of luminosityclass is suggested to be a result of wind clumping. Different strategiesto check this hypothesis are discussed, particularly by comparing theH\alpha mass-loss rates with the ones derived from radio observations.
| Studies of Young Stellar Objects
We present a detailed study of four young open star clusters and two OBassociations using deep CCD U B V R I and 2MASS J H Ks measurements.Physical parameters of the clusters have been estimated using optical aswell as near-IR data. Ages of objects under study range 10-100 Myr. Massfunction slope of the sample of clusters are in agreement with theSalpeter (1955) value. From a comparison of mass function slope ofclusters and OB associations in our Galaxy with those in externalgalaxies, we urgue that the star formation processes are such that theyyield almost similar stellar mass distribution in vastly different starforming environments. Mass segregation suggests that the clusters understudy are dynamically relaxed and hence mass segregation may be due todynamical evolution or imprint of star formation or both. The extinctionproperties of a sample of 18 young star clusters have been investigatedutilizing recent data available in literature. The study suggests thatthere is no uniformity in extinction properties amongst these clusters.A non-uniform extinction has been noticed for the first time in NGC 1502and Tr 37 along with the existence of circumstellar shell around someearly type stars in NGC 884, NGC 2264, Tr 14 and Tr 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.
| Interstellar extinction in the open clusters towards galactic longitude around 130deg
In this paper we present a detailed study of the intra-cluster reddeningmaterial in the young open clusters located around l ~ 130degusing colour-excess diagrams and two-colour diagrams. The study supportsthe universality of the extinction curves for lambda >= lambdaJ, whereas for shorter wavelengths the curve depends upon thevalue of the Rcluster (total-to-selective absorption in thecluster region). The value of Rcluster in the case of NGC654, NGC 869 and NGC 884 is found to be normal, whereas the value ofRcluster in the cluster regions NGC 1502 and IC 1805indicates an anomalous reddening law in these regions. In the case ofNGC 1502 the value of Rcluster is found to be lower(2.57+/-0.27) whereas in the case of IC 1805 it is higher (3.56+/-0.29)than the normal value of 3.1. Although the intra-cluster materialindicates a higher value of Rcluster in the NGC 663 region,the error in the estimation of Rcluster is too large toconclude anything. It is also found that the extinction process in the Uband in the case of NGC 663 seems to be less efficient, whereas in thecase of NGC 869 the process is more efficient.
| UBV and vbybeta stellar photometry in the eastern part of M 31
We present UBV and the first vbybeta Strömgren stellar photometryin a field centered on the stellar association OB31 in the Eastern partof the Andromeda galaxy. The ages of the stellar populations aredetermined from the color-magnitude diagrams using the latesttheoretical isochrones. HR diagram for all stars in the area isconstructed. The mass of the brightest blue star is about 60 {cal M}_sunand for the brightest red star is about 20 {cal M}_sun. The slope ofthe Stellar Initial Mass Function (IMF) for blue stars is -2.0 +/- 0.1which is similar to that of other nearby galaxies. We investigate thestellar content of three van den Bergh associations as well as that ofsome new OB associations determined by cluster analysis. Ages of the vanden Bergh associations are 4Myr for OB31 and 6Myr for OB32, OB34 andOB35. Eight blue supergiant candidates are isolated from the rest of thestars by using suitable color-color diagrams in Strömgren system.The results from UBV and Strömgren uvby photometry are compared.
| Confirmation of a Double Nature of the Third Body in SZ Cam
Not Available
| Spectral variability of luminous early type stars . II. Supergiant alpha Camelopardalis
Time-series of Hα spectra with relatively high resolution inwavelength (R = lambda /delta lambda of 15 000 to 22 000) and time(Delta t = 1d) of the late-type O supergiant alpha Cam areanalysed in terms of line-profile variability (lpv). The spectra coveran interval of one year, from February 1998 to February 1999. Theanalysis provides clear evidence of a continuous deep-seated windactivity traced by variations in the Hα emission and He I lambda6678.15 absorption lines. The observations indicate that the wind is notsmooth but perturbed, starting from its base up to velocities of ~ 500km s-1. The character of the medium-term (days) variationsfound in Hα changes between epochs, and appears to require anexplanation involving different kinds of wind perturbations. Inparticular, we found that in June and July 1998 as well as in February1999 the lpv of Hα was dominated by low-amplitude (<=+/-10%)variations in line flux which usually occupy the central part of theprofile symmetrically with respect to the line center while in December1998 and January 1999 the variations were organised in two wave-likemodulations that run from ``red'' to ``blue" and back to ``red" withinthe profile (between +/-300 km s-1), being most of the timein antiphase. The timescale of variation, revealed via Fourier analysis,is respectively ~ 7 and ~ 10 days. Significant variations in emissionequivalent width (up to 35%), closely linked to those in the lineprofile, are also noted. Short-term (3 to 4 days), low-amplitude(<=22%) variation in mass loss rate which recurs on a timescale of ~7 days giving rise to the formation of outward accelerating consecutiveshells or/and blobs was suggested to explain the lpv of Hα inJune-July 1998 and in February 1999. Whereas the variability patternobserved in December-January 1999 seems to be qualitatively consistentwith a model involving two rotationally-modulated wind perturbations,one of enhanced density and another of reduced density with respect tothe mean, which are not symmetric about the center of the star.Strange-mode oscillations or radial fundamental pulsation are discussedas possible mechanisms generating the established wind variability.
| Non-uniform extinction in young open star clusters
The extinction law and the variation of colour excess with position,luminosity and spectral class in young open star clusters NGC 663, NGC869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14,Tr 15, Tr 16, Coll 228, Tr 37 and Be 86 have been studied. Thedifference in the minimum and maximum values of E(B-V) of clustermembers has been considered as a measure of the presence of non-uniformgas and dust inside the clusters. Its value ranges from 0.22 to 1.03magin the clusters under study, which indicates that non-uniform extinctionis present in all the clusters. It has been noticed for the first timein NGC 1502 and Tr 37. It is also found that the differential colourexcess in open clusters, which may be caused by the presence of gas anddust, decreases systematically with the age of clusters, indicating thatmatter is either used in star formation or blown away by hot stars orboth. There is no uniformity in the variation of E(B-V) with position orspectral class or luminosity. Except in Tr 14, all clusters show arandom spatial distribution of E(B-V), indicating a random distributionof gas and dust inside the clusters. The E(B-V) value correlates withboth luminosity and spectral class only in the case of Coll 228, Tr 16and Be 86. The members of these clusters atλ>=λR show larger values of colour excessratios than the normal ones. The value of E(U-V)/E(B-V) for most of thecluster members is close to the normal interstellar value of 1.73.However, the colour excess ratios with E(B-V) atλ>=λJ are smaller than the normal value forNGC 663, NGC 869, NGC 884 and NGC 1502, while they are larger for NGC6611, Coll 228, Tr 16 and Tr 14. Thus there is no uniformity in therelationship of extinction properties amongst the clusters under study.
| The Stars in Camelopardalis OB1: Their Distance and Evolutionary History
An investigation is undertaken of the optically identified Population Iobjects in the Cam OB1 region. From the analysis of these objects it isdetermined that the distance to the Cam OB1 stars is 975+/-90 pc. Atheoretical H-R diagram for the Cam OB1 stars reveals that starformation has been going on in the region for1×106-1×108 yr. At present, the mostactive site of star formation is Cam R1, which shows an asymmetry in thedistribution of optically identified Population I objects. The majorityof the pre-main-sequence stars lie in the northern half of theassociation, while the main-sequence stars and evolved stars lie in thesouthern half.
| 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
| 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.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| UBVR-Photometry of the Eclipsing Binary SZ Cam
Not Available
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Interacting OB star binaries: LZ Cep, SZ Cam and IU AUR
New spectroscopic observations of three O+O binary stars (LZ Cep, SZ Camand IU Aur), obtained over a number of years at the DominionAstrophysical Observatory, are analysed using state-of-the-artcross-correlation techniques to produce radial velocity curves. Theseresults are combined with new (LZ Cep) and published (SZ Cam and IU Aur)photometry to yield masses, temperatures, radii and luminosities. Wefind that LZ Cep is a semi-detached system, in agreement with theresults of Howarth et al., with the secondary star filling its Rochelobe. We find a distance modulus of 9.6+/-0.2, which is slightly smallerthan the distance modulus to Cep OB2 (=9.9).The reddening of LZ Cep isalso at the lower end of the cluster range, suggesting that the binaryis located on the near side of the cluster. SZ Cam is a triple system,and we are able to measure the radial velocities of the three componentsthat are observed in the absorption lines. A light-curve solutionemploying our new mass ratio shows that the short-period binary isdetached. The third body emits about 40 per cent of the radiation fromthe system, and is probably a binary itself. The distance modulus for SZCam (=9.88) is in good agreement with that of its parent cluster NGC1502 (=9.71). IU Aur is another triple system, although we are unable tofind spectroscopic evidence for the tertiary. The close binary issemi-detached, with the secondary filling its Roche lobe. The distancemodulus of the system is 11.0, which is in approximate agreement withthat of the Aur OB1 association (=10.6+/-0.2). We note that the currentmodels for interacting massive binaries all assume case B mass transfer,in which the mass-loser becomes a Wolf-Rayet star. Case A mass-transfermodels (in which the mass-loser fills its Roche lobe while still on themain sequence) are required in order to see whether or not they canexplain the observed properties of these interacting systems.
| SZ Camelopardalis - an early-type eclipsing binary embedded in a multiple system
We present a spectroscopic and photometric analysis of a complexmultiple system: the early-type eclipsing binary SZ Cam (O9 IV + B0.5 V)is physically bound to a third body, which itself is a close binary;this quadruple system has a visual companion, which shows evidence forduplicity as well. A careful deconvolution of spectral lines in our CCDspectra yields rv curves of the SB2 system (K_1 = 180 km s(-1) , K_2 =261 km s(-1) ; P = 2fd 6984) and of its SB1 companion (K_{1{,T}} = 25 kms(-1) ; P 2fd 7966). UBV light curves of Kitamura & Yamasaki (1970)are reanalyzed and absolute dimensions of the eclipsing pair arederived. Both components of this close detached binary are overluminousfor their masses. The photometric solution suggests third light of ~20-30%, which confirms the spectroscopic and speckle interferometricdetection (Mason 1995) of the third component. The observed light timeeffect encompasses the long period range to ~ 50-60 years. Comparison ofcalculated orbits with the speckle interferometric result yields adistance of 1050 pc, which is ~ 10% larger than the value derived fromthe absolute bolometric magnitude, and ~ 20% above the photometricdistance of the open cluster NGC 1502. The minimum mass of the thirdcomponent is 20-30 M_ȯ, depending on the long orbital period andinclination assumed. This high mass supports the binary nature of thethird body. An analysis of the spectra of the visual companion ADS 2984A confirms its binarity. Based on observations collected at theGerman-Spanish Astronomical Centre, Calar Alto, operated by theMax-Planck-Institut für Astronomie, Heidelberg, jointly with theSpanish National Commission for Astronomy
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