Zero-Age Main Sequence in the HR Diagram of the Vilnius Photometric System
The zero-age main sequence for solar metallicity stars in the absolutemagnitude vs. color diagram of the Vilnius seven-color photometricsystem is determined. The calibration is based on the results ofphotoelectric photometry of stars in the Hyades, Pleiades and Praesepeopen clusters and the Ori OB1 association. A theoretical Victoria-Reginaisochrone, corresponding to an age of 10 million years, coincides wellwith the lower envelope of the unevolved main sequence.
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The formation and evolution of binary systems. III. Low-mass binaries in the Praesepe cluster
With the aim of investigating the binary population of the 700 Myr oldPraesepe cluster, we have observed 149 G and K-type cluster membersusing adaptive optics. We detected 26 binary systems with an angularseparation ranging from less than 0.08 to 3.3 arcsec (15-600 AU). Aftercorrecting for detection biases, we derive a binary frequency (BF) inthe log P (days) range from 4.4 to 6.9 of 25.3 +/- 5.4%, which issimilar to that of field G-type dwarfs (23.8%, Duquennoy & Mayor\cite{r20}). This result, complemented by similar ones obtained for the2 Myr old star forming cluster IC 348 (Paper II) and the 120 Myr oldPleiades open cluster (Paper I), indicates that the fraction oflong-period binaries does not significantly evolve over the lifetime ofgalactic open clusters. We compare the distribution of cluster binariesto the binary populations of star forming regions, most notably Orionand Taurus, to critically review current ideas regarding the binaryformation process. We conclude that it is still unclear whether thelower binary fraction observed in young clusters compared to Tassociations is purely the result of the early dynamical disruption ofprimordial binaries in dense clusters or whether it reflectsintrinsically different modes of star formation in clusters andassociations. We also note that if Taurus binaries result from thedynamical decay of small-N protostellar aggregates, one would predictthe existence of a yet to be found dispersed population of mostly singlesubstellar objects in the Taurus cloud. Based on observations obtainedat the Canada-France-Hawaii Telescope.
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Investigation of the Praesepe cluster III. Radial velocity and binarity of the F5-K0 Klein-Wassink stars
Coravel observations of 103 F5-K0 stars in the Praesepe cluster yielded24 spectroscopic binaries (3 are non-members), and 20 orbits weredetermined, with periods from 4 to 7400 days. Based on a complete samplein the colour range 0.40 <= B-V >= 0.80 (80 stars, including KW244 = TX Cnc), the companion star fraction CSF = 0.45. The percentage ofspectroscopic binaries with P < 1000fd is 20% (16/80). The combinedphotometric and spectroscopic analysis showed that 12 among 18single-lined spectroscopic binaries are located within the `` single"star sequence in the (V,B-V) diagram and cannot be detected by thephotometric analysis in the UBV system. In addition, sevenphotometrically analysed binaries were not detected with the radialvelocity observations, but are confirmed members. The number ofsingle:binary:triple stars is 47:30:3. Based on observations collectedat the Haute-Provence Observatory (France)
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Stellar Activity in Coeval Open Clusters: Praesepe and the Hyades
Randich & Schmitt found that the coronal activity of solar-type andlow-mass stars in Praesepe is significantly lower than that of stars inthe Hyades cluster. This result is quite surprising, since the Hyadesand Praesepe have approximately the same age and metallicity and areoften thought to have originated in the same giant molecular cloudcomplex. We have carried out several tests in order to find a possibleexplanation for this result. We have measured radial velocities of twogroups of Praesepe stars (a dF-dK sample and a dM sample) and havemeasured Hα as a chromospheric activity index for the dM sample.Based on analyses of these data, we conclude that the Praesepe catalogused in the X-ray analysis does not contain a significant number ofnonmembers and, thus, that membership problems do not seem to be thecause of the Randich & Schmitt result. The comparison of theHα equivalent widths for the M dwarfs in Praesepe with those inthe Hyades indicates that, at least for stars in this mass range, thePraesepe stars are as active or more active than their Hyadescounterparts. The similarity of chromospheric emission allows us toreject differences in the rotational velocity distribution as the originof the dissimilar L_x luminosity functions. We have also analyzed a fewROSAT PSPC pointings of Praesepe in order to obtain a new andindependent estimate of the X-ray luminosities and upper limits for asmall sample of Praesepe members. This analysis suggests that theprevious ROSAT PSPC analysis produced slightly optimistic X-ray upperlimits; however, the differences between the old and new upper limitsare not large enough to explain the dichotomy in the X-ray properties ofPraesepe and the Hyades. Therefore, our examination of the availabledata does not provide a clear reason to explain why the X-ray luminosityfunctions of the two clusters are different. Part of the explanationcould be found in the binaries. Speculatively, these clusters could havedifferent orbital period distributions, with more short-period binariesamong the Hyades, which would show larger coronal activity.
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Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
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High-precision study of proper motions and membership of 924 stars in the central region of Praesepe.
High-precision proper motions for 257 and 296 stars in a 90'x90' regioncentered on BD+20 2170 were obtained from seven plates taken with the40cm refractor (f=6895mm) at Zo-Se station of Shanghai Observatory withsix AC plates and the stellar positions in Russell's (1976) catalogue,respectively. Combining with the data given by Klein-Wassink (1927),Jones (1971), Artyukhina (1971), and Jones & Cudworth (1983), theseproper motions give a sample of 924 stars in the region mentioned above.With membership probabilities estimated by an improved maximumlikelihood method, a very good sample of 198 members is obtained withproper motion accuracies ranging from +/-0.2 to +/-5.0mas/yr, of which60 per cent are better than +/-1.0mas/yr, and the completeness nearlydown to B=15.5.
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A ROSAT X-ray study of the Praesepe cluster.
We present the results of ROSAT PSPC observations of the Praesepecluster. 68 Praesepe candidates have been detected, above a threshold of=~2x10^28^erg/s, in the ~4x4deg area of the cluster covered by theobservations. 56 out of the 68 detected objects are cataloged as highprobability Praesepe members. Praesepe members of all spectral typeshave been detected with X-ray luminosities ranging from the sensitivitylimit to approximately 10^30^erg/s in the ROSAT broad band. The highestX-ray luminosity has been measured for a very short period W UMa typeSB2 binary. 2 out of the 4 Praesepe late-type giants have also beendetected. X-ray luminosity distribution functions have been derived forlate-type stars in the sample, taking into account both detections andupper limits. The main and most surprising finding are the low detectionrates derived for Praesepe low mass dwarfs. We detected about 30% of theF and G stars, and the detection rate among K and M dwarfs is evenlower. Correspondingly, the luminosity distribution functions for starsin selected color intervals are dominated by the contribution of upperlimits, with the medians below the sensitivity threshold. The comparisonwith the Hyades all-sky survey results shows an evident discrepancybetween the average X-ray properties of late-type dwarfs in the twoapparently coeval clusters; such a discrepancy must be an intrinsic one,since the observations are characterized by similar sensitivities.
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The evolution of the lithium abundances of solar-type stars. IV - Praesepe
Echelle observations are presented of lithium in 63 F and G dwarfs ofthe Praesepe cluster. For stars earlier than about G0V, Praesepe followsthe same trends seen in the Hyades, which has approximately the same ageand composition. Stars in Praesepe later than about G5V have more Lithan their Hyades counterparts, possibly because Praesepe is slightlyyounger than the Hyades or has slightly lower metallicity. Significantdifferences in the abundance of Li are seen among stars of the samecolor, and, as in the Hyades, there is a tendency for the deviant starsto be binaries to the extent that duplicity in Praesepe is known. Thereare also stars with much less Li than most cluster members yet whichappear to be true members of Praesepe. The close binary KW 181 has anormal Li abundance, despite the fact that similar close binaries in theHyades are Li rich.
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The luminosity function of Praesepe. I - A proper motion and photometric search for candidate members
Proper motions have been measured for a color selected sample of starsin a square 4 x 4 deg region centered on the Praesepe open cluster. Themagnitude limit of the photometry is about 19 and of the proper motionsabout 18. A list is presented of 765 probable and possible members from9 to 18 based on the proper motions, magnitudes, and colors. The resultsare compared with other proper motion surveys, and a preliminaryluminosity function of M(v) = 11 is calculated.
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Photometric binary stars in Praesepe and the search for globular cluster binaries
A radial velocity study of the stars which are located on a secondsequence above the single-star zero-age main sequence at a given colorin the color-magnitude diagram of the open cluster Praesepe, (NGC 2632)shows that 10, and possibly 11, of 17 are binary systems. Of the binarysystems, five have full amplitudes for their velocity variations thatare greater than 50 km/s. To the extent that they can be applied toglobular clusters, these results suggests that (1) observations of'second-sequence' stars in globular clusters would be an efficient wayof finding main-sequence binary systems in globulars, and (2) currentinstrumentation on large telescopes is sufficient for establishingunambiguously the existence of main-sequence binary systems in nearbyglobular clusters.
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J 2000.0 positions and proper motions of 257 stars in the central part of the Praesepe astrometric standard region
Data are presented on the J2000.0 positions and proper motions of 257stars in the central 1.5 deg x 1.5 deg area of the Praesepe astrometricstandard region. These data obtained with accuracies of 0.005-0.10arcsec for the positions in each direction and 0.0002-0.006 arcsec/yrfor the proper motions in each direction. The list includes stars forwhich proper motions were not given in the Russell (1976) catalog.
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Statistical identification of binaries in star clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PASP...87..707B&db_key=AST
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Four-color and H-bet photometry of open clusters. III. Praesepe.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1969AJ.....74..818C&db_key=AST
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Effect of rotation on the colors and magnitudes of stars in Praesepe.
Not Available
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La mesure des vitesses radiales au prisme objectif. XVIII-Classifications spectrales et mesure de vitesses radiales de 112 étoiles situées dans Praesepe
Not Available
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Infrarot-Photometrie der Praesepe. Mit 10 Textabbildungen
Not Available
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Stellar rotation in galactic open clusters
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1960MNRAS.121..503T&db_key=AST
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Spectral Classification of the Brighter Stars of the Praesepe Cluster
Not Available
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Praesepe: Magnitudes and Colors.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1952ApJ...116..640J&db_key=AST
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Visuelle und photographische Helligkeiten von 218 Präsepe Sternen
Not Available
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