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Computing the Parallax of the Pleiades from the Hipparcos Intermediate Astrometry Data: An Alternative Approach
The inconsistency between the mean parallax of the Pleidaes open clusterfrom the Hipparcos catalog and that obtained from the stellar evolutiontheory and photometric measurements is probed by recomputing theHipparcos data in a different way that reduces the propagation of thealong-scan attitude errors. This is achieved by coupling observations ofstars made nearly simultaneously in the two separate fields of view ofthe telescope. A direct calculation of astrometric quantities of 54Pleiades members by the new method, based on the Intermediate AstrometryData, provides a correction of -0.71+/-0.14 mas to the weighted meanparallax of the cluster. The mean corrected parallax of the Pleiades is7.75+/-0.20 mas.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Search for gamma Doradus variable stars in the Pleiades cluster
Photometric observations in the uvbybeta system of A-F type stars in thePleiades cluster have been performed in order to detect pulsatingvariable stars of gamma Doradus type in the lower part of the Cepheidinstability strip. In order to obtain more information about thebehaviour of the studied objects and to be able to distinguish betweenlong period variable and non-variable stars, two statistical methodshave been developed. Several of these stars show some type ofvariability but only two of the observed objects, H1284 and S29, can besafely classified as gamma Dor stars. Furthermore, these observationshave provided us with Strömgren and Hβ photometry,non-existing up to now for some of them, which permitted us to perform aphotometric study of the Pleiades cluster.
| Statistics of binaries in the Pleiades cluster
We present a statistical analysis of binaries in galactic clusters,based on photometric properties. Synthetic clusters are used to solvethe deconvolution problem. If the colour-magnitude relation for singlestars is given and if triple stars are treated as binaries, thedistribution of systems in a colour-magnitude diagram (CMD) can be usedto determine cluster properties. They include the maximum number ofsingle stars N1max which is compatible with the distributionof systems, a standard deviation sigma (describing the scatter of starson the main sequence), a critical mass ratio q_c, and the mass ratiodistribution in the case of N_1=N1max. In the general case(N_1<= N1max) the mass ratio distribution can bedetermined for q>q_c, and in favourable cases (if N_1 is sufficientlylarge) for all mass ratios. A first application concerns thePleiades cluster in the colour range 0.2<= B-V<0.98. The concentration of systems near the main sequence in the CMDis used to derive an approximation for the colour-magnitude relation. Asmall positive number c is involved as a parameter. The mass ratiodistribution depends sensitively on c and increases towards small massratios, at least up to q =~ 0.5 and probably up to q =~ 0.3. Thedistribution is bimodal, with a peak at q=1. Photometric arguments showthat c<~ 0.02. A binary frequency of 60-70% as expected from clustersimulations (Kroupa \cite{krou}) requires c<~ 0.03. An adjustment oftwo parameters (c=0.02 and a 70% binary frequency) is sufficient toreproduce the mass ratio distribution for binaries in the galacticfield. This suggests that the mass ratio distribution in the Pleiades issimilar to the distribution in the field, in accordance with aconjecture of Bouvier et al. (\cite{brn}, BRN).
| 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
| The rotational velocity of low-mass stars in the Pleiades cluster
We present new {vsin i} measurements for 235 low-mass stars in thePleiades. The differential rotational broadening has been resolved forall the stars in our sample. These results, combined with previouslypublished measurements, provide a complete and unbiased rotation dataset for stars in the mass range from 0.6 to 1.2{Msun}.Applying a numerical inversion technique on the {vsin i} distributions,we derive the distributions of equatorial velocities for low-massPleiades members. We find that half of the Pleiades dwarfs with a massbetween 0.6 to 1 {Msun} have rotation rates lower than 10{ kms(-1) }. Comparison of the rotational distributions of low-mass membersbetween IC 2602/2391 (~ 35 Myr) and the Pleiades (~ 100 Myr) suggeststhat G dwarfs behave like solid-bodies and follow Skumanich's law duringthis time span. However, comparison between Pleiades and older clusters-M34 (~ 200 Myr) and Hyades (~ 600 Myr)- indicates that the braking ofslow rotators on the early main sequence is weaker than predicted by anasymptotical Skumanich's law. This strongly supports the view thatangular momentum tapped in the radiative core of slow rotators on thezero age main sequence (ZAMS) resurfaces into the convective envelopebetween Pleiades and Hyades age. For the G-dwarfs, we derive acharacteristic coupling time scale between the core and the envelope ofabout 100-200 Myr, which accounts for the observed evolution of surfacerotation from the ZAMS to the Hyades. The relationship between rotationand coronal activity in the Pleiades is in agreement with previousobservations in other clusters and field stars. We show that the Rossbydiagram provides an excellent description of the X-ray activity for allstars in the mass domain studied. The Pleiades data for slow andmoderate rotators fills the gap between the X-ray-rotation correlationfound for slow rotators and the X-ray ``saturation plateau'' observedfor young fast rotators. The transition between increasing X-ray fluxwith rotation and X-ray saturation is observed at log (P/tau)=0.8+/-0.1. These results strengthen the hypothesis that the``saturation'' of the angular momentum loss process depends on thestellar mass. Based on observations collected at the Observatoire deHaute-Provence with ELODIE at the 193cm telescope and with CORAVEL atthe 1m-swiss telescope
| Rotation and chromospheric emission among F, G, and K dwarfs of the Pleiades
High-resolution echelle spectra of more than 100 F, G, and K dwarfs inthe Pleiades are reported. Chromospheric activity in these stars ismeasured via comparisons of the profiles of H-alpha and the Ca II IRtriplet to chromospherically inactive field stars. Consistent dereddenedcolors are determined from the available photometry and temperatures arederived. Most G and K dwarfs in the Pleiades rotate slowly, but about 20percent of the stars are ultrafast rotators (UFRs). That fraction ofUFRs is independent of color, and the highest rotation rates are foundamong the K dwarfs. The Pleiades exhibit a broad range in the strengthof chromospheric emission at any one color. Most G and K dwarfs in thePleiades show H-alpha and the IR triple in absorption, with filling inof the line cores.
| Investigation of the Pleiades cluster. I - Radial velocities of corona stars
Coravel radial velocities of 83 stars ranging in spectral types from F5to K0 selected frozen van Leeuwen's et al. (1986) survey of the coronaof the Pleiades show that only 56 are actually members. Sixspectroscopic binaries among the members and two among the nonmembershave been found. The large extent of the cluster over the sky alsoinduces a radial-velocity gradient from which a convergent point hasbeen determined. Since the cluster's depth is so large, the width of themain sequence is increased. The present results lead to a star densityof 1.1 stars per sq deg for r between 2.5 and 3.5 deg. However, westress that this is only a preliminary result, since the sample of starsin the corona is incomplete.
| A VBLUW photometric survey of the Pleiades cluster
Photometric data are presented for 390 known or suspected members of thePleiades cluster. The data were obtained in 1979 at ESO using theWalraven VBLUW photometer and the Dutch 91-cm telescope. A comparisonwas made with subsets obtained with the same telescope andinstrumentation at the former Leiden Southern Station at the SAAO annexduring 1976 and 1977, and with data obtained at ESO during 1980 and1981. The much improved performance of the telescope and the photometerat their new site is obvious from these comparisons. The stars measuredcomprise the selection of possible members by Hertzsprung (1947)brighter than m(pg) = 14.5 and a selection of possible members in theouter region of the cluster by Pels et al. (1975). Of the starsselected, 66 were found not to be members. A few of these are possiblyescaping members. The present data set provides a well determined mainsequence over the range K2V to B9V, as well as data on some of theprobably premain-sequence K2-to-K5 and postmain-sequence B8-to-B6 stars.Finding charts for the stars selected by Pels are presented in anappendix.
| The physical members of the Pleiades group
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