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מאמרים קשורים
The SPITZER c2d Survey of Weak-Line T Tauri Stars. I. Initial Results
Using the Spitzer Space Telescope, we have observed 90 weak-line andclassical T Tauri stars in the vicinity of the Ophiuchus, Lupus,Chamaeleon, and Taurus star-forming regions as part of the Cores toDisks (c2d) Spitzer Legacy project. In addition to the Spitzer data, wehave obtained contemporaneous optical photometry to assist inconstructing spectral energy distributions. These objects werespecifically chosen as solar-type young stars with low levels ofHα emission, strong X-ray emission, and lithium absorption, i.e.,weak-line T Tauri stars, most of which were undetected in the mid- tofar-IR by the IRAS survey. Weak-line T Tauri stars are potentiallyextremely important objects in determining the timescale over which diskevolution may take place. Our objective is to determine whether theseyoung stars are diskless or have remnant disks that are below thedetection threshold of previous infrared missions. We find that only5/83 weak-line T Tauri stars have detectable excess emission between 3.6and 70 μm, which would indicate the presence of dust from the innerfew tenths of an AU out to the planet-forming regions a few tens of AUfrom the star. Of these sources, two have small excesses at 24 μmconsistent with optically thin disks; the others have optically thickdisks already detected by previous IR surveys. All of the sevenclassical T Tauri stars show excess emission at 24 and 70 μm althoughtheir properties vary at shorter wavelengths. Our initial results showthat disks are rare among young stars selected for their weak Hαemission.
| 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.
| Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Aims.We report a deep X-ray survey of the young (~140 Myr), rich opencluster NGC 2516 obtained with the EPIC camera on board the XMM-Newtonsatellite. Methods: .By combining data from six observations, ahigh sensitivity, greater than a factor of 5 with respect to recentChandra observations, has been achieved. Kaplan-Meier estimators of thecumulative X-ray luminosity distribution are built, statisticallycorrected for non members contaminants and compared to those of thenearly coeval Pleiades. The EPIC spectra of the X-ray brightest starsare fitted using optically thin model plasma with one or two thermalcomponents. Results: .We detected 431 X-ray sources and 234 ofthem have as optical counterparts cluster stars spanning the entire NGC2516 Main Sequence. On the basis of X-ray emission and opticalphotometry, we indicate 20 new candidate members of the cluster; at thesame time we find 49 X-ray sources without known optical or infraredcounterpart. The X-ray luminosities of cluster stars span the range logLX (erg s-1) = 28.4-30.8. The representativetemperatures span the 0.3-0.6 keV (3.5-8 MK) range for the coolcomponent and 1.0-2.0 keV (12-23 MK) for the hot one; similar values arefound in other young open clusters like the Pleiades, IC 2391, andBlanco 1. While no significant differences are found in X-ray spectra,NGC 2516 solar type stars are definitely less luminous in X-rays thanthe nearly coeval Pleiades. The comparison with a previous ROSAT surveyreveals the lack of variability amplitudes larger than a factor of 2 insolar type stars in a 11 yr time scale of the cluster and thusactivity cycles like in the Sun are probably absent or have a differentperiod and amplitude in young stars.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| 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.
| Caroline Herschel as observer
Not Available
| A Spitzer Study of Dusty Disks around Nearby, Young Stars
We have obtained Spitzer Space Telescope MIPS (Multiband ImagingPhotometer for Spitzer) observations of 39 A- through M-type dwarfs,with estimated ages between 12 and 600 Myr; IRAC observations for asubset of 11 stars; and follow-up CSO SHARC II 350 μm observationsfor a subset of two stars. None of the objects observed with IRACpossess infrared excesses at 3.6-8.0 μm however, seven objectsobserved with MIPS possess 24 and/or 70 μm excesses. Four objects(κ Phe, HD 92945, HD 119124, and AU Mic), with estimated ages12-200 Myr, possess strong 70 μm excesses, >=100% larger thantheir predicted photospheres, and no 24 μm excesses, suggesting thatthe dust grains in these systems are cold. One object (HD 112429)possesses moderate 24 and 70 μm excesses with a color temperature,Tgr=100 K. Two objects (α1 Lib and HD177724) possess such strong 24 μm excesses that their 12, 24, and 70μm fluxes cannot be self-consistently modeled using a modifiedblackbody despite a 70 μm excess >2 times greater than thephotosphere around α1 Lib. The strong 24 μm excessesmay be the result of emission in spectral features, as observed towardthe Hale-Bopp star HD 69830.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
| 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.
| New Debris-Disk Candidates: 24 Micron Stellar Excesses at 100 Million years
Sixty-three members of the 100 Myr old open cluster M47 (NGC 2422) havebeen detected at 24 μm with the Spitzer Space Telescope. The Be starV 378 Pup shows an excess both in the near-infrared and at 24 μm(K-[24]=2.4 mag), probably due to free-free emission from the gaseousenvelope. Seven other early-type stars show smaller excesses,K-[24]=0.6-0.9. Among late-type stars, two show large excesses: P922, aK1 V star with K-[24]=1.08+/-0.11, and P1121, an F9 V star withK-[24]=3.72+/-0.02. P1121 is the first known main-sequence star showingan excess comparable to that of β Pic, which may indicate thepresence of an exceptionally massive debris disk. It is possible that amajor planetesimal collision has occurred in this system, consistentwith the few hundred Myr timescales estimated for the clearing of thesolar system.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Recent developments on studies of open clusters
Studies on open clusters are of great significance for astronomy andastrophysics, for which distinct developments have been made recently inboth observational and theoretical respects. In this paper, currentresearch of open cluster is reviewed, and membership determination,basic parameters determination, evolution of cluster and clustermembers, structure and dynamics of open cluster are also described indetail.
| 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.
| Luminosity and Mass Function of the Galactic open cluster NGC 2422
We present UBVRI photometry of the open cluster NGC 2422 (age ~108 yr) down to a limiting magnitude V =~ 19. These data areused to derive the Luminosity and Mass Functions and to study thecluster spatial distribution. By considering the color-magnitude diagramdata and adopting a representative cluster main sequence, we obtained alist of candidate cluster members based on a photometric criterion.Using a reference field region and an iterative procedure, a correctionfor contaminating field stars has been derived in order to obtain theLuminosity and the Mass Functions in the M=0.4-3.5 Msunrange. By fitting the spatial distribution, we infer that anon-negligible number of cluster stars lies outside our investigatedregion. We have estimated a correction to the Mass Function of thecluster in order to take into account the ``missing'' cluster stars. ThePresent Day Mass Function of NGC 2422 can be represented by a power-lawof index alpha = 3.07 +/-0.08 (rms) - the Salpeter Mass Function inthis notation has index alpha = 2.35 - in the mass range 0.9 <=M/Msun<= 2.5 . The index alpha and the total mass of thecluster are very similar to those of the Pleiades.Based on observations made at the Cerro Tololo Inter-AmericanObservatory, Chile.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| ROSAT PSPC/HRI observations of the open cluster NGC 2422
We present the results of a ROSAT study of NGC 2422, a southern opencluster at a distance of about 470 pc, with an age close to thePleiades. Source detection was performed on two observations, a 10-ksPSPC and a 40-ks HRI pointing, with a detection algorithm based onwavelet transforms, particularly suited to detecting faint sources incrowded fields. We have detected 78 sources, 13 of which were detectedonly with the HRI, and 37 detected only with the PSPC. For each source,we have computed the 0.2-2.0 keV X-ray flux. Using optical data from theliterature and our own low-dispersion spectroscopic observations, wefind candidate optical counterparts for 62 X-ray sources, with more than80% of these counterparts being late type stars. We have assigned to theoptical sources an astrometric and/or a photometric membership flag,depending on which data are available to us. The number of sources (38of 62) with high membership probability counterparts is consistent withthat expected for Galactic plane observations at our sensitivity. Wehave computed maximum likelihood X-ray luminosity functions (XLF) for Fand early-G type stars with high membership probability. Heavy datacensoring due to our limited sensitivity permits determination of onlythe high-luminosity tails of the XLFs; the distributions areindistinguishable from those of the nearly coeval Pleiades cluster.
| NGC 2571: An intermediate-age open cluster with a White Dwarf candidate
CCD UBVI imaging photometry was carried out in the field of the opencluster NGC 2571. From the analysis of our data we state the cluster isat a distance of 1380 +/- 130 pc and its age is 50 +/- 10 x106 yr. The cluster mass function has a slope larger than atypical Salpeter's law. There are two notorious features in NGC 2571:the cluster contains a high proportion of stars located below thereference line that are serious candidates to be metallic line stars(probably Am-Fm), and shows also a sharp gap along its main sequencethat cannot be explained by a random process nor by a biased rejectionof cluster members. A striking blue object was detected in the clusterfield that could be a white dwarf candidate. Based on observationscollected at the European Southern Observatory ESO at La Silla, Chile.
| The rise and rise of the deep sky image
Presidential Address to the British Astronomical Association, 2000October 25
| Hipparcos Trigonometric Parallaxes and the Distance Scale for Open Star Clusters
Hipparcos trigonometric parallaxes are used to estimate the distances tothe maximum possible number of open star clusters (OSC); distance moduliare estimated for 45 clusters with maximum heliocentric distances ofabout 1000 pc. The latter value can serve as an estimate of the limit towhich it still makes sense to use Hipparcos trigonometric parallaxes todetermine the distances to small groups composed of 6-10 sufficientlybright stars. A systematic correction to the distance moduli of clustersfrom the homogeneous catalog of OSC parameters (Loktin et al. 1997,2000) is estimated, which turns out to be independent of the clusterage.
| The CFHT Open Star Cluster Survey. III. The White Dwarf Cooling Age of the Rich Open Star Cluster NGC 2099 (M37)
We present deep CCD photometry of the very rich, intermediate-age(similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-Vcolor-magnitude diagram (CMD) for the cluster shows an extremelywell-populated and very tightly constrained main sequence extending over12 mag from the turnoff. Both a well-defined main-sequence turnoff and ared giant population are also clearly evident. The CFH12K photometry forthis cluster is faint enough (V~24.5) to detect the remnants of the mostmassive progenitor cluster stars under the Type I SNe limit. Therefore,the CMD of the cluster also exhibits a well defined white dwarf``clump'' caused by the decreased rate of cooling of these stars as theyage, and a subsequent gap with very few objects. By using sourceclassification to eliminate faint blue galaxies and a statisticalsubtraction technique to eliminate foreground/background objects, wehave determined the age of the star cluster from the termination point(MV=11.95+/-0.30) in the white dwarf luminosity function. Thewhite dwarf cooling age is found to be566+/-154176 Myr from comparisons with white dwarfcooling models and is in excellent agreement with the main-sequenceturnoff isochrone age (520 Myr). After carefully accounting for theuncertainties in the white dwarf limiting magnitude, we show that thecooling age confirms that models including convective core overshootingare preferred for young-intermediate-aged clusters. This is particularlyimportant in the case of NGC 2099 as the age is similar to that of theHyades cluster, for which current models have difficulties inreproducing the details of the main-sequence turnoff. We also derive thereddening [E(B-V)=0.21+/-0.03] and distance[(m-M)V=11.55+/-0.13] to NGC 2099 by matching main-sequencefeatures in the cluster to a new fiducial main sequence for the Hyades,after correcting for small metallicity differences. As a continuing partof the goals of the CFHT Open Star Cluster Survey to better understanddynamical processes of open clusters, we also fit a King model to thecluster density distribution and investigate the cluster main-sequenceluminosity and mass functions in increasing concentric annuli. We findsome evidence for mass segregation within the boundary of NGC 2099 asexpected given the cluster's age relative to the dynamical age. Thepresent global mass function for the cluster is found to be shallowerthan a Salpeter IMF.
| 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 Praesepe Open Cluster and the Galactic Distance Scale
Updated proper motions for 328 probable members of the Praesepe are usedto determine the distance to this open cluster by Hertzsprung'sgeometric method. The cluster distance was found to be r = 171 +/-15 pc,which corresponds to the distance modulus V_0-M_V = 6.16 +/-0.19 mag.The distance scale for open clusters is discussed.
| 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
| Le satellite Hipparcos. Une moisson de resultats astrophysiques.
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.
| 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
| Search for star clustering: methodology and application to the Two Micron Galactic Survey
A new approach to the study of the large-scale stellar clusterdistribution in the Galaxy based on two-point correlation techniques ispresented. The basic formalism for this method is outlined, and itsapplications are then investigated by the use of a simple model ofcluster distribution in the Galaxy. This provides an estimate of thepotentials of the two-point correlation function for indicatingclustering in the measured star positions, which can be related to thepresence of star clusters in the observed field. This technique is thenapplied to several areas of the Two Micron Galactic Survey catalogue,from which information is obtained on the distribution of clustersaccording to position in the Galaxy, as well as about age, density ofstars, etc.
| Corridors in the regions of young open stellar clusters.
Not Available
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