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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.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| 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.
| Chromospheric Activity in the Late A- and Early F-Type Stars of Open Clusters. I. Hyades, Praesepe, and Coma
We report observations of the He i lambda5876 (D3) line in the late A-and early F-type stars in the Hyades, Praesepe, and Coma star clustersused to determine chromospheric activity levels. The average activitylevel in the early F-type stars is statistically identical in the threeclusters and in field dwarfs, and the dispersion in activity levels ineach sample is similar. The earliest star detected in each clusterranges from B-V = 0.26 in the Hyades to B-V = 0.31 in Praesepe. However,four stars in the Hyades in the range 0.28-0.32 are not detected. Thedetection of Hyades star VB 84 at B-V = 0.26 represents the bluest starobserved to date with a statistically nonzero chromospheric D3equivalent width. In nine early F-type stars we compare D3 strengtheither during two different observing runs or in several individualintegrations. In two stars we find evidence for short-term variabilityof a factor of 2 on timescales of 1 day or less. The timescale of thevariability is consistent with rotational modulation. Finally, thesimilarity in chromospheric activity in the Hyades and Praesepe starscontrasts with the significant weakness of coronal X-ray emission ofearly F-type stars in Praesepe compared to Hyades. We have noexplanation for this difference; however, this result suggests that theobserved disappearance of D3 in the late A-type stars is not directlycaused by the weak coronal emission of these stars.
| 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
| Spectrophotometry of 237 Stars in 7 Open Clusters
Spectrophotometry is presented for 237 stars in 7 nearby open clusters:Hyades, Pleiades, Alpha Persei, Praesepe, Coma Berenices, IC 4665, andM39. The observations were taken by Lee McDonald and David Bursteinusing the Wampler single-channel scanner on the Crossley 0.9m telescopeat Lick Observatory from July 1973 through December 1974. Sixteenbandpasses spanning the spectral range 3500 Angstroms to 7780 Angstromswere observed for each star, with bandwidths 32Angstroms, 48 Angstromsor 64 Angstroms. Data are standardized to the Hayes-Latham system tomutual accuracy of 0.016 mag per passband. The accuracy of thespectrophotometry is assessed in three ways on a star-by-star basis.First, comparisons are made with previously published spectrophotometryfor 19 stars observed in common. Second, (B-V) colors and uvby colorsare compared for 236 stars and 221 stars, respectively. Finally,comparsions are made for 200 main sequence stars to the spectralsynthesis models of Kurucz, fixing log g = 4.0 and [Fe/H] = 0.0, andonly varying effective temperature. The accuracy of tests using uvbycolors and the Kurucz models are shown to track each other closely,yielding an accuracy estimate (1 sigma ) of 0.01 mag for the 13 colorsformed from bandpasses longward of the Balmer jump, and 0.02 mag for the3 colors formed from the three bandpasses below the Balmer jump. Incontrast, larger scatter is found relative to the previously publishedspectrophotometry of Bohm-Vitense & Johnson (16 stars in common) andGunn & Stryker (3 stars). We also show that the scatter in the fitsof the spectrophotometric colors and the uvby filter colors is areasonable way to identify the observations of which specific stars areaccurate to 1 sigma , 2 sigma , .... As such, the residuals from boththe filter color fits and the Kurucz model fits are tabulated for eachstar where it was possible to make a comparison, so users of these datacan choose stars according to the accuracy of the data that isappropriate to their needs. The very good agreement between the modelsand these data verifies the accuracy of these data, and also verifiesthe usefulness of the Kurucz models to define spectrophotometry forstars in this temperature range (>5000 K). These data define accuratespectrophotometry of bright, open cluster stars that can be used as asecondary flux calibration for CCD-based spectrophotometric surveys.
| 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.
| Photometry of astrometric reference stars
UBVRI, DDO, and uvby, H-beta photometry of astrometric reference starsis presented. Spectral types and luminosity classifications made fromthe colors are used to determine their spectroscopic parallaxes. In thispaper, colors for 309 stars in 25 regions are given, and classificationsfor 210 stars have been made. These stars form reference frames in theAllegheny Observatory Multichannel Astrometric Photometer astrometricprogram, and in the Praesepe cluster reduced by Russell (1976). It isfound that the present photometric spectral types are reliable to within2.5 spectral subclasses.
| 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.
| 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.
| Lunar occultations of Praesepe. I - Mauna Kea
Results of lunar occultations of the Praesepe cluster during theDecember 1981 and April 30, 1982 passages as observed from Mauna Keawith the IR telescope facility are reported. Timings of 40 events aregiven. Four Praesepe binaries, previously unknown, were resolved, and anadditional measurement of the occultation-speckle system, 63 Gem, isreported. Two of the Praesepe systems have separations small enough togive hope for spectroscopic resolution.
| Internal Motions in Praesepe
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..470J&db_key=AST
| Faint members of the Praesepe cluster.
Not Available
| Effects of rotation in the colour-magnitude diagrams of Praesepe and the Hyades.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971MNRAS.151..463S&db_key=AST
| 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
| Effect of rotation on the colors and magnitudes of stars in Praesepe.
Not Available
| Catalogue des étoiles mesurées dans le système photométrique de l'Observatoire de Genève
Not Available
| La mesure des vitesses radiales au prisme objectif. XVIII-Classifications spectrales et mesure de vitesses radiales de 112 étoiles situées dans Praesepe
Not Available
| Infrarot-Photometrie der Praesepe. Mit 10 Textabbildungen
Not Available
| 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
| Spectral Classification of the Brighter Stars of the Praesepe Cluster
Not Available
| Praesepe: Magnitudes and Colors.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1952ApJ...116..640J&db_key=AST
| Visuelle und photographische Helligkeiten von 218 Präsepe Sternen
Not Available
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Observation and Astrometry data
| Constellation: | Cancer |
| Right ascension: | 08h39m05.23s |
| Declination: | +20°07'01.8" |
| Apparent magnitude: | 9.38 |
| Proper motion RA: | -34.6 |
| Proper motion Dec: | -12.3 |
| B-T magnitude: | 9.854 |
| V-T magnitude: | 9.42 |
Catalogs and designations:
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