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Search for X-ray flares in the Pleiades using SoHO LASCO C3 images.
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| 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).
| The Pleiades and alpha Persei Clusters
The upper-main-sequence members of the Pleiades and alpha Perseiclusters, considered as members of the Local Association, yield meanparallaxes that are only 4% larger than the mean values from Hipparcosobservations. The (log T_eff, M_V) diagram reveals that in thetemperature range from 6000 to 8000 K, the Hyades and alpha Perseimain-sequence members are nearly identical and several tenths of amagnitude brighter than similar stars on the Pleiades main sequence. Thedeparture of the Pleiades main sequence cannot be traced to either ageor heavy-element abundance differences in the range thought to apply tothese clusters. A 50% increase in the helium abundance of Pleiades overHyades stars could account for the luminosity difference. Alternativeexplanations are that the Pleiades cluster is rejected from superclustermembership and/or that the Hipparcos parallax results for the Pleiadesare in error by some 10%.
| 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
| Investigation of the Pleiades cluster. III. Additional corona members.
The analysis of CORAVEL radial velocities of 93 stars selected on thebasis of their proper motion and Geneva CCD photometric observations for57 stars have permitted to identify 25 new members in the outer part ofthe Pleiades. Several spectroscopic binaries have been discovered, buttheir membership is not clear. Two orbits with short periods have beendetermined, but both stars are probably non-members. The total number ofmember stars in the outer part of the Pleiades in the spectral rangeF5-K0 (0.45 < B-V < 0.90) is now 81 which is comparable to thenumber of stars known in Hertzsprung's central area (88 stars) in thesame spectral domain. Therefore at least 48 % of the F5-K0 main-sequencestars are located in the outer part of the cluster. And the census isprobably still incomplete.
| Membership probabilities in the Pleiades field.
A catalogue of proper motions and photographic B, V magnitudes for starsup to B=19mag within a region centered near Alcyone is presented. Thecatalogue is based on MAMA measurements of 8 plates taken with theTautenburg Schmidt telescope. The survey includes ca. 14500 stars andcovers a total field of about 9 square degrees. For stars brighter thanB=18mag an internal accuracy of 0.05mag and 2.5mas/year has beenestimated for photometric data and proper motions, respectively.Membership probabilities, proper motions and B, V magnitudes are listedfor 442 stars up to B=19mag in the Pleiades field.
| 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.
| The distance of the Pleiades cluster and the calibration of photometric luminosities for early-type stars
An examination of the lower main-sequence (mode-A) stars in the Pleiadescluster suggests an Fe/H abundance ratio between 0.0 and 0.1 dex with aresulting modulus of 5.65 + or - 0.1 mag, and fundamental defects in thecalculation of Balona and Shobbrook (1984), with an adopted modulus of5.50 mag, are discussed. It is suggested that the ZAMS of Balona andShobbrook, and of Mermilliod (1981), are too bright due to theirassumption that the color-luminosity arrays of such clusters as thePleiades represent isochrones, leading to uncertainties in the ZAMS,particularly with respect to slope. Several recently publishedphotometric luminosity calibrations for early-type stars may beincorrect due to their failing to recognize the probable presence of atleast two evolutionary modes and the apparent absence of ZAMS stars nearthe sun.
| Four-color and H-beta photometry for open clusters. XI - The Pleiades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..419C&db_key=AST
| The physical members of the Pleiades group
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