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The blue stragglers formed via mass transfer in old open clusters
Aims.In this paper, we present the simulations for the primordial bluestragglers in the old open cluster M 67 based on detailed modelling ofthe evolutionary processes. The principal aim is to discuss thecontribution of mass transfer between the components of close binariesto the blue straggler population in M 67. Methods: .First, wefollowed the evolution of a binary of 1.4 M_ȯ+0.9 M_ȯ. Thesynthetic evolutionary track of the binary system revealed that aprimordial blue straggler had a long lifetime in the observed bluestraggler region of color-magnitude diagram. Second, a grid of modelsfor close binary systems experiencing mass exchange were computed from 1Gyr to 6 Gyr in order to account for primordial blue-straggler formationin a time sequence. Based on such a grid, Monte-Carlo simulations wereapplied for the old open cluster M 67. Results: .Adoptingappropriate orbital parameters, 4 primordial blue stragglers werepredicted by our simulations. This was consistent with the observationalfact that only a few blue stragglers in M 67 were binaries with shortorbital periods. An upper boundary of the primordial blue stragglers inthe color-magnitude diagram (CMD) was defined and could be used todistinguish blue stragglers that were not formed via mass exchange.Using the grid of binary models, the orbital periods of the primordialBSs could be predicted. Conclusions: .Compared with theobservations, it is clear that the mechanism discussed in this workalone cannot fully predict the blue straggler population in M 67. Theremust be several other processes also involved in the formation of theobserved blue stragglers in M 67.
| The Integrated Spectrum of M67 and the Spectroscopic Age of M32
We construct an integrated spectrum of the intermediate-age,solar-metallicity Galactic cluster M67, from individual spectroscopicobservations of bona fide cluster members. The spectrum so obtained isused as a template to test our stellar population synthesis models, inan age and metallicity regime where such models remain largely untested.As a result, we demonstrate that our models predict a spectroscopic ageof 3.5+/-0.5 Gyr for M67, which is the same age we obtain from fittingisochrones to the color-magnitude diagram of the cluster. Fullconsistency is reached when using either Hβ, Hγ, or Hδas the age indicator. We also check if the models, when applied to thecluster integrated spectrum, predict elemental abundances in agreementwith the known detailed abundance pattern of the cluster. The modelsalso pass the latter test, by predicting the abundances of iron,magnesium, carbon, and nitrogen in agreement with detailed abundanceanalyses of cluster stars to within 0.1 dex. Encouraged by the highdegree of consistency of our models, we apply them to the study of theintegrated spectrum of the central 3" of the compact elliptical galaxyM32. The resulting luminosity-weighted age of the galaxy ranges between2 and 3.5 Gyr, depending on the age indicator adopted. According to ourmodels, the center of M32 seems to have a supersolar iron abundance,ranging between [Fe/H]~+0.1 and +0.3, depending on the spectral indexadopted. The light element magnesium seems to be underabundant in thecenter of M32 relative to iron by about ~0.1-0.2 dex, whereas the dataare consistent with nearly solar carbon and nitrogen abundances relativeto iron. We find that single-age, single-metallicity stellar populationmodels with solar-scaled abundance patterns cannot fit all the Balmerand metal lines in the integrated spectrum of M32. In particular, thereis a systematic trend in the sense that bluer absorption lines indicatea younger age and a higher metallicity. This slight inconsistency can bedue either to (unaccounted for) abundance ratio effects on blue iron andBalmer line indices or to a spread of the ages of the stellarpopulations in M32. Current stellar population models cannot break thisdegeneracy at the level of accuracy required to address this problem.
| CCD Photometry of the M 67 Cluster in the Vilnius System. II. New Photometry of High Accuracy
Seven color CCD photometry in the Vilnius system, supplemented by theCousins I passband, has been obtained for 412 stars down to V = 16 magin the M 67 open cluster area. A special method of flat-fielding, givinghigh accuracy photometry, was used. Photometric spectral types andinterstellar reddenings of all stars were determined. The mean reddeningof the cluster stars is EB-V = 0.04 mag and the distance is770 pc. Considerable systematic errors were found in some otherphotometric investigations of the same cluster.
| L' and M' standard stars for the Mauna Kea Observatories Near-Infrared system
We present L' and M' photometry, obtained at the United Kingdom InfraredTelescope (UKIRT) using the Mauna Kea Observatories Near-Infrared(MKO-NIR) filter set, for 46 and 31 standard stars, respectively. The L'standards include 25 from the in-house `UKIRT Bright Standards' withmagnitudes deriving from Elias et al. and observations at the InfraredTelescope Facility in the early 1980s, and 21 fainter stars. The M'magnitudes derive from the results of Sinton and Tittemore. We estimatethe average external error to be 0.015 mag for the bright L' standardsand 0.025 mag for the fainter L' standards, and 0.026 mag for the M'standards. The new results provide a network of homogeneously observedstandards, and establish reference stars for the MKO system, in thesebands. They also extend the available standards to magnitudes whichshould be faint enough to be accessible for observations with moderndetectors on large and very large telescopes.
| Time Series Photometry of M67: W Ursae Majoris Systems, Blue Stragglers, and Related Systems
We present an analysis of over 2200 V images taken on 14 nights at theMount Laguna 1 m telescope of the open cluster M67. We show variabilityin the light curves of all four known W UMa variables on timescalesranging from a day to weeks. We have modeled the light curve of AH Cnc,and the total eclipses allow us to determineq=0.16+0.03-0.02 andi=86+4-8 deg. The position of this system near theturnoff of M67 makes it useful for constraining the turnoff mass for thecluster. We have also detected two unusual features in the light curveof AH Cnc that may be caused by prominences. We have also monitoredcluster blue stragglers for variability. We present evidence hinting atlow-level variations in the stragglers S752, S968, and S1263, and weplace limits on the variability of a number of other cluster bluestragglers. Finally, we provide photometry of the subsubgiant branchstar S1063 showing variability on timescales similar to the orbitalperiod, while the ``red straggler'' S1040 shows evidence of anunexplained drop in brightness at phases corresponding to the passage ofthe white dwarf in front of the giant.
| CCD photometry of RV-Tau type stars. Part 1. TT Ophiuchi.
Not Available
| JHK standard stars for large telescopes: the UKIRT Fundamental and Extended lists
We present high-precision JHK photometry with the 3.8-m UK InfraredTelescope (UKIRT) of 82 standard stars, 28 from the widely usedpreliminary list known as the `UKIRT Faint Standards', referred to hereas the Fundamental List, and 54 additional stars referred to as theExtended List. The stars have 9.4
| Direct N-body modelling of stellar populations: blue stragglers in M67
We present a state-of-the-art N-body code which includes a detailedtreatment of stellar and binary evolution as well as the clusterdynamics. This code is ideal for investigating all aspects relating tothe evolution of star clusters and their stellar populations. It isapplicable to open and globular clusters of any age. We use the N-bodycode to model the blue straggler population of the old open cluster M67.Preliminary calculations with our binary population synthesis code showthat binary evolution alone cannot explain the observed numbers orproperties of the blue stragglers. On the other hand, our N-body modelof M67 generates the required number of blue stragglers and providesformation paths for all the various types found in M67. Thisdemonstrates the effectiveness of the cluster environment in modifyingthe nature of the stars it contains, and highlights the importance ofcombining dynamics with stellar evolution. We also perform a series ofN=10000 simulations in order to quantify the rate of escape of starsfrom a cluster subject to the Galactic tidal field.
| 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
| The Blue Stragglers in M67 and Single-Population Synthesis
Based on the photometry of the classical open cluster M67 and thethorough membership survey, we made a color-magnitude diagram (CMD) ofhigh-membership stars for the cluster. Apart from the stars fitted by aconventional isochrone (single-star evolutionary model) scheme, wenoticed a number of features on the CMD that are usually ignored whenconstructing the corresponding single stellar population (SSP),especially the large number of blue stragglers. As for the study of theintegrated properties of SSPs, we argue that the contributions from allingredients in a single population of stars, such as the luminous bluestragglers, should be included. These bright blue objects may modifyremarkably the spectral energy distribution (SED) in the blue andultraviolet. In this paper we investigate their effects on theintegrated light of single stellar populations. We will show in thiswork that, by using the observation of M67 in a multicolorintermediate-band system, we can assign adequate theoretical spectra toindividual stars. The integrated spectrum of this cluster isdramatically different from the SSP built using isochrones only, whichshows increasing enhancement toward shorter wavelengths (starting fromabout 5000 Å) of the spectrum. Taking this as a general case forSSPs, we can expect a considerable amount of modification to theintegrated SEDs, which might cast new light on the analysis of thestellar content in the complex stellar systems.
| The Hot Stars of Old Open Clusters: M67, NGC 188, and NGC 6791
We analyze ultraviolet (~1500 Å) images of the old open clustersM67, NGC 188, and NGC 6791 obtained with the Ultraviolet ImagingTelescope (UIT) during the second flight of the Astro observatory in1995 March. Twenty stars are detected on the UIT image of M67, including11 blue stragglers, seven white dwarf candidates, and the yellowgiant-white dwarf binary S1040. The ultraviolet photometry of the bluestragglers F90 (S975) and F131 (S1082) suggests that these stars havehot subluminous companions. We present a semiempirical integratedultraviolet spectrum of M67 and show that the blue stragglers dominatethe integrated spectrum at wavelengths shorter than 2600 Å. Thenumber of white dwarfs in M67 is roughly consistent with the numberexpected from white dwarf cooling models. Eight candidate sdB/sdO starsare detected in NGC 6791, and two are detected in NGC 188. Theluminosity range 1.10 < log L/L_ȯ < 1.27, derived from theultraviolet photometry of the six sdB candidates, is consistent withtheoretical models of metal-rich hot horizontal branch (HB) stars. Thefraction of hot HB stars in both NGC 6791 and NGC 188 is about 30%,implying that the integrated spectra of both clusters should show a UVturnup at least as strong as that observed in any elliptical galaxy.
| UBVRI photometry using CCD cameras
Methods for determining the magnitudes of astronomical objects using CCDcameras are discussed. A filter set for measurement in the Kron-CousinsU, B, V, R, and I passbands is recommended. Practical techniques andhardware are discussed. A CCD calibration procedure based on observationof the open cluster M67 is described with worked examples.
| CCD Photometry of the M 67 Cluster in the Vilnius Photometric System
Seven-color CCD photometry in the Vilnius photometric system of 279stars down to V = 15 mag in the open cluster M 67 area is obtained. 13standard stars in the cluster are measured photoelectrically.Photometric spectral types have been determined for all the stars. Thereddening of the cluster is found to be E_{B-V} = 0.045, the truedistance modulus is 9.38 mag and the age is 4times 10(9) years.
| The Mount Wilson Halo Mapping Project 1975-1985 I: The UBV(RI)_M_W Photometric System Compared with Other Standard Systems: The Adopted Trigonometric HR Diagram in (R-I)_M_W and (V-I)_M_W
Photometry of stars is described that defines a UBV(RI)_M_W photometricsystem, established between 1975 and 1980 using the Mount Wilson 60-inchand 100-inch reflectors. The MW natural system has been reduced to theUBV(RI)_C Cape-Cousins system by linear equations for stars hotter thanM0. Strong non-linear color and magnitude equations exit in R and Ibetween the two systems for stars cooler than M0. The non-linearity isdue to the change of the effective wavelengths of the red passbandscaused by effect of the TiO b ands combines with the substantialdifference between red response of the extended S20 cathodes used hereand the GaAs cells used by Cousins (1976, 1980a,b), Bessell (1979,1990), Landolt (1983, 1992), and others for the Cape system. We report(R-I)_M_W and (V-I)_M_W colors based on a linear extension of the _M_Wnatural system to the red for cool stars with (V-R)_Cape greater than0.8, (V-I)_Cape greater than 1.8 (spectral types later than M0), aszero-pointed to the Cape system for bluer stars. Color equations arederived between the MW, the Cape, the Kron-Eggen, and Johnsonphotometric systems, and also the Hubble Space Telescope (V-I) colors asreduced to the (V-I)_C Cape system by Holtzman et al. (1995). TheKron-Eggen and the Cape-Cousins R and I systems differ in their zeropoints of color and magnitudes, showing that the Kron-Eggen and theCousins systems are not the same. The appellation of a "Kron-Cousins"photometric system used in much of the current literature is incorrect.Preparatory to determining photometric parallaxes of local higher propermotion M dwarfs in Paper IV, trigonometric HR diagrams are determined inV, (V-R)_M_W and (V-I)_M_W on the Mount Wilson system using 160available parallax stars with parallaxes larger than 0.099 arcsec. Afirst appendix illustrates the difference in the effective wavelengthsof the Cape-Cousins, Kron-Eggen, and Johnson systems for cool stars,based on the observed slope coefficients of the color equations thatlink them. A second appendix contains explicit color equations based onstars in common between Bessell's (1990) definition of the Cape systemfor the reddest stars and the Kron-Eggen and the MW photometric systems.(SECTION: Stars)
| The First Direct Measurement of the Mass of a Blue Straggler in the Core of a Globular Cluster: BSS 19 in 47 Tucanae
Hot, young blue straggler stars are observed by the Hubble SpaceTelescope to exist in the cores of very old globular clusters. Theorymaintains that these stars must be more massive than hydrogen-burningcluster members that are as old as the clusters themselves. We have, forthe first time, verified this theoretical claim by directly measuringthe mass of the blue straggler BSS 19 in the core of 47 Tucanae. Thederived mass, M=1.7+/-0.4 Msolar , is nearly twice that ofthe oldest primordial hydrogen-burning cluster star. BSS 19 is found tobe rotating rapidly, with a derived projected rotational velocity ofv{sin}i=155+/-55 km s ^{-1} . This large value for the rotationalvelocity tends to support a model for the formation of BSS 19 in whichthe blue straggler was formed from a binary system via a noncollisionalmechanism (e.g., the coalescence of a contact binary).
| The Consistency of Stromgren-Beta Photometry for Northern Galactic Clusters. III. M67
We have measured M67 stars in a Stromgren-beta system based ultimatelyon data for the Hyades and Coma clusters. By comparing our results topreviously published data, we have derived formal corrections requiredto put the other data on our system. Corrections which are statisticallysignificant by our criteria are found for Stromgren measurements made byStrom et al. (1971, PASP, 83, 768). Those corrections range between 36and 61 mmag. In addition, corrections of 17 and 8 mmag, respectively,are found for beta measurements made by Eggen (1981, ApJ, 247, 503) andNissen et al. (1987, AJ, 93, 634). In addition to considering M67, wehave updated earlier work of ours which was done on Praesepe and NGC752. After revising our criterion for statistical significance, we havechanged our judgment about two corrections which we had derived forpublished data for those clusters. Further work will be required toverify the reality of those corrections. We have also compared ourresults to closely similar work which was done by Nissen (1988, A&A,199, 146) and which had previously escaped our notice. Differencesbetween our results and those of Nissen appear to exist for beta inPraesepe and for c_1 in NGC 752. Independent measurements support ourPraesepe result, but the problem for NGC 752 remains unresolved.(SECTION: Stellar Clusters and Associations)
| The Implications of the Binary Properties of the M67 Blue Stragglers
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...470..521L
| Deep Wide-Field Spectrophotometry of the Open Cluster M67
We present nine color CCD intermediate-band spectrophotometry of a twosquare degree field centered on the old open cluster M67, from 3890$\rm\AA$ to nearly 1$\mu$. These observations are taken as a part of theBATC (Beijing-Arizona-Taipei-Connecticut) Color Survey of the Sky, forboth scientific and calibration reasons. With these data we show thatthe BATC survey can reach its goal of obtaining spectrophotometry to azero point accuracy of 0.01 mag, and down to V = 21 with 0.3 mag randomerror. We fit the color-magnitude diagrams (CMDs) with Worthey'stheoretical models. The net result is the excellent fit of the 4.0 Gyr,[Fe/H] = $-0.10$ model to our data, including a good fit to the mainsequence (MS) turn-off. Our data are consistent with a toy model with50\% of the stars in M67 being binaries and a random distribution ofbinary mass-ratios, although other models with different mass-ratiodistributions cannot be ruled out. The spatial distribution and massfunction (MF) of stars in M67 show marked effects of dynamical evolutionand evaporation of stars from the cluster. Blue stragglers and binarystars are the most condensed within the cluster, with degree ofcondensation depending on mass.We find M67 to have an elongated shape,oriented at an angle of $15^{\circ}$ relative to the galactic plane.Within its tidal radius, the observed MF of M67 between 1.2 $\rmM_\odot$ and $\rm 0.8 M_\odot$ has a Salpeter slope $\rm \eta = -1.93\pm 0.66$. For stars of mass below 0.8 $\rm M_\odot$, $\rm \eta \sim 0$.It is plausible that the leveling-off of the MF at lower masses is aresult of evaporation of lower mass stars in this mass range at a rateof one every $\sim 10^7$ years. If so, it is plausible that the IMF ofM67 has the canonical field value of $\rm \eta = -2.0$.
| Moderate-resolution spectral standards from lambda 5600 to lambda 9000
We present a grid of stellar classification spectra of moderateresolution (R approximately 1500) in the range lambda lambda 5600-9000A, compiled from high signal-to noise spectra of 275 stars, most in theopen clusters Praesepe and M67. The grid covers dwarfs from types B8through M5, giants from G8 through M7, and subgiants from F5 through K0.We catalog atomic and molecular absorption features useful for stellarclassification, and demonstrate the use of luminosity-sensitive featuresto distinguish between late-type dwarf and giant stars. The entiredatabase is made available in digital format on anonymous ftp andthrough the World Wide Web.
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| Radial velocities of blue stragglers. 1: A catalog of candidates in six open clusters
We discuss the observational definition of blue stragglers and present acatalog of 58 blue-straggler candidates in six open clusters (NGC 752,2360, 2420, 2682, 6939, and 7789) chosen to cover the range of ages from1.6 to 5 Gyr. Candidates which are nonmembers based on proper motionsare not included in the catalog. New radial-velocity observations areused to identify additional candidates which are nonmembers. We findthat the blue-straggler samples in 6939 and 7789 are significantlycontaminated by nonmembers.
| Secondary photometric standard stars for the Thuan-Gunn and Johnson-Kron-Cousins systems
CCD photometry is presented for 119 stars observed in the Thuan-Gunnphotometric system (the passbands vgr). The sample includes 82 stars forwhich magnitudes in this system have not previously been published.Photometry in the Johnson-Kron-Cousins photometric system (BV or UBV)has been obtained for a subsample of 85 of the stars. Of those starswhich are not primary standard stars for the Thuan-Gunn system 70observed in Gunn r, 48 observed in Gunn g, and 37 observed in Gunn vhave sufficient repeat measurements that they may be used as secondarystandard stars for CCD photometry. The typical uncertainties for themagnitudes of these stars are 0.006 mag.
| CCD photometry of the old open cluster M67
We present a CCD photometric survey of the central one-half degree ofthe old open cluster, M67, in U, B, V, and I colors to magnitude V = 20.Extensive comparison of our photometry with other published datasetsshows excellent agreement, indicating that CCD photometry is capable ofproducing a uniform set of measurements consistent with the photometricsystem defined primarily by the Landolt standard sequence. Thecolor-magnitude diagram of the cluster shows a well-defined mainsequence extending at least to the limit of the photometry at M(V) =10.55 and a substantial binary sequence. At least 38 percent of clusterstars are binaries. The current generation of theoretical isochronescannot be fit to the observed sequences within the observational errors.We find a tendency for more massive members of the cluster to be morecentrally concentrated, along with a turnover in the cluster luminosityfunction at low masses, which may be due to dynamical relaxation of thecluster. To the limit of our photometry, we find a mass of the clusterof 724 solar masses. In addition, we present a sample of stars ofwell-determined standard magnitudes that are suitable as photometricstandards for further studies of this cluster and for generalcalibration of UBVI photometry using CCDs.
| CCD surface photometry for E and S0 galaxies in the Coma cluster
The paper presents CCD surface photometry in Johnson B and Gunn r for 33galaxies in the Coma cluster and for 13 galaxies in the field or inother clusters, obtained with the Danish 1.54-m telescope at the ESO, LaSilla, Chile in the period April 3-10, 1989. The data were calibratedwith standard stars. Data for all galaxies include derivations ofeffective radius, mean surface brightness, and total magnitude. Goodagreement was found between the photometry and the global parameterspresented with data of Dressler, Faber, Burstein and theircollaborators, indicating that the quality of the CCD data is fullyadequate for studies of the large-scale motions in the universe.
| Can Evolution in Close Binaries Account for the Blue Stragglers in M67
Not Available
| Blue stragglers - A search for binaries in the infrared
Infrared photometry in the J, H and K bands, performed on a sample of 33blue stragglers (BS) belonging to 8 open clusters is presented. Thisphotometry has been combined with published Johnson UBV photometry tosearch for possible cold companions among the BS, as expected for Case Bclose binary evolution. Probable binaries were identified by comparisonwith standard mean color relations for single stars. The number ofbinary detections is discussed together with the results of a previousspectroscopic search for binaries among BS (Manteiga et al. 1989), andthe consequences for the hypothesis of binarity for the origin of BS areanalyzed.
| Cousins BVRI CCD photometry of stars in the M67 'dipper asterism'
The paper presents Cousins BVRI CCD photometric data for 29 stars in theM67 'dipper asterism' with a standard deviation per star not greaterthan 1 percent for stars brighter than V = 13 and 1-3 percent for V =14. Comparison is made with other data recently published.
| A Collisional Origin for the Blue Stragglers in the Old Open Clusters M67 and NGC 188
Not Available
| Cousins VRI standard stars in the M 67 dipper asterism
Cousins VRI data for 19 standard stars in the M 67 dipper asterism arepresented. With one exception, the values of sigma per mean for thesedata are lsss than 10 mmag. Because these stars are close together inthe sky, they can be used with economy of effort in standardizing CCDimages. New Cousins VRI photometry is reported for 23 M 67 stars. For 22additional cluster stars, new V magnitudes are reported. It is foundthat some published V data for the Hyades and M 67 must be corrected bysmall amounts to recover the Landolt (1983) zero point. In addition,faint-star M 67 measurements by Eggen and Sandage (1964) display scatterwhich is not easily interpreted. The same is true for the (largely)bright-star M 67 data of Johnson and Sandage (1955). The photomultipliercolors of Janes and Smith (1984) and the CCD data of Schild (1983, 1985)are also considered.
| Relative proper motions and the stellar velocity dispersion of the open cluster M67
Relative proper motions for 663 stars in the field of the old opencluster M67 have been determined using 44 Yerkes 40-in refractor plates.The resulting proper-motion marginal distributions have been fit with atwo-component model representing the sum of a cluster distribution and amuch wider field distribution, both of which are assumed to beintrinsically Gaussian in form. The observed marginal distributions areactually quite non-Gaussian due to the effects of the proper-motionmeasurement errors, and thus a modeling procedure that realisticallyincludes the measurement errors has been devised. The procedure yieldsexcellent fits to the observed distributions, and thus allows reliablecluster-membership probabilities to be calculated. The cluster'sintrinsic velocity dispersion is estimated from the proper motions ofthe about 80 brightest cluster stars to be 0.81 + or - 0.10 km/s. Thisis marginally higher than a dispersion estimate based on publishedradial-velocity measurements of a similar sample of cluster members.
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