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Публикации по объекту
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.
| Beryllium Abundances in F and G Dwarfs in Praesepe and Other Young Clusters from Keck HIRES Observations
The study of both Be and Li gives useful clues about stellar internalstructure. Of particular interest is the study of these light elementsin open clusters, which have a known age and metallicity. In this paperwe present a study of Be abundances in 10 F-type stars in Praesepe and acomprehensive discussion about Be abundances in other open clusters:Hyades, Pleiades, α Per, Coma, and UMa. We have made observationsof the doublet of Be II around 3130 Å in Praesepe stars, using theKeck I telescope and the High Resolution Echelle Spectrometer (HIRES).Beryllium abundances were derived from the spectra using the spectrumsynthesis method. We find four stars with definite Be depletion in thetemperature range of the Li dip like we found in our previous clusterstudies, notably for the Hyades and Coma clusters. Putting all theclusters together, we confirm the existence of a Be dip in a narrowtemperature range for F stars. Beryllium depletion in this dip is lesspronounced than Li depletion. For the cooler stars there is little or noBe depletion, even though there are large depletions of Li. For starsthat have little or no Li depletion, A(Li)>=3.0, the ratio Li/Be is75+/-4.6, compared to the meteoritic ratio of 77.6. For stars coolerthan ~5900 K there appears to be little or no Be depletion, and the meanA(Be) is 1.30+/-0.02. For these cooler stars within a given clusterthere is no evidence for intrinsic star-to-star differences in A(Be),with the possible exception of the cool Pleiades stars. In thetemperature range of the Li-Be dip, a strong correlation exists betweenLi and Be, consistent with the theory of rotationally induced mixing.Moreover, the slopes of the Li versus Be correlations are differentdepending on the temperature range. For the full sample of 42 starsbetween 5900 and 6650 K the slope is 0.43+/-0.05 [where A(Li) is theabscissa]. The slope is 0.48+/-0.08 for 6300K
| 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)
| 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.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| The Lithium Dip in M67: Comparison with the Hyades, Praesepe, and NGC 752 Clusters
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...446..203B&db_key=AST
| 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.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Chemical composition of open clusters. III - Iron and carbon in F dwarfs in Coma, Praesepe, and M67
Iron and carbon abundances are determined for F dwarfs in several openclusters and the field in order to investigate the homogeneity ofchemical mixing in the Galactic disk. Abundances are based onmeasurements of eight Fe I lines and six high-excitation C I lines inthe spectral region 7065-7155 A analyzed with model atmospheres. Clustermean values of Fe/H and C/H show cluster-to-cluster variations well inexcess of observational errors, indicating both that the overall metalcontent of the gas from which these clusters formed preserved small butsignificant differences over periods of several billion years, and thatthe production of carbon closely followed that of iron so that theseregions of star formation had the same ratio of C/Fe.
| 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.
| 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.
| 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.
| The nature of the F STR 4077-A stars. II - Frequency, kinematics, metallicity, binarity and rotational velocities
Fundamental characteristics of the F str 4077-A stars are investigated.Their frequency in the solar neighborhood is estimated at about 1.4percent of all main-sequence F stars. The space velocities areconsistent with Population I membership, with the exception of oneobject (in a sample of 17) whose velocity is above 100 km/s. The binaryfrequency is higher than normal, and the rotational velocities arenormal for the cooler stars but rather low for the hotter ones. Overallmetallicity, deduced from photometric and CORAVEL data, suggests adivision of these stars into two groups: metal-rich objects (generallythe hotter ones), which may resemble the Am or Ap stars, and slightlymetal-weak objects (generally the cooler ones), which may be linked withthe Ba II giants or the CH subgiants.
| New calibrations of blanketing parameters Delta m2 and delta m1 in terms of Fe/H
New calibrations are derived for the blanketing parameters Delta m2 anddelta m1, of the Geneva and Stromgren photometric system, respectively,in terms of Fe/H. Based on a sample of 164 A-F main-sequence and giantstars, two quadratics relations in a metallicity range from -2.13 to 0.5are defined.
| Chemical abundances and ages of open clusters
The metal content of nine open clusters is discussed with the help ofresults from detailed spectral analyses of main-sequence and slightlyevolved stars, belonging to those clusters. This small sample ofclusters, out of about 1200 known, has, however, the advantage ofrepresenting the whole span of ages of open clusters with two young(Alpha Per, the Pleiades, with five intermediate age, including U Ma,Hyades, Coma, Praesepe, and NGC 752) and with two old (M 67 and NGC 188)clusters. It has also the advantage that the observations of almost allthe stars in these different clusters have been carried out on similar,when not identical telescopes, on similar high-resolution spectrographs,and similar detectors. The high signal/noise spectra of these clusterstars have been reduced with similar procedures, and interpreted withsimilar model atmosphere analyses. In taking into account the error barsattributed to the mean metal content parameter, Fe/H, of each cluster,one may conclude that only the Hyades cluster has a metal content whichdiffers significantly from that of the sun, being between 30 and 40percent higher than that of the sun.
| Metallicity in galactic clusters from high signal-to-noise spectroscopy
High-quality spectroscopic data on selected F dwarfs in six Galacticclusters are used to determine global (Fe/H) values for the clusters.For the two youngest clusters, Pleiades and Alpha Per, the (Fe/H) valuesare solar: 0.017 + or - 0.055. The Hyades and Praesepe are slightlymetal-enhanced at (Fe/H) = + 0.125 + or - 0.032, even though they are anorder of magnitude older than the Pleiades. Coma and the UMa Group atthe age of the Hyades are slightly metal-deficient with (Fe/H) = - 0.082+ or - 0.039. The lack of an age-metallicity relationship indicates thatthe enrichment and mixing in the Galactic disk have not been uniform ontime scales less than a billion years.
| Lithium in the Hyades, the Hyades moving group, and Praesepe
High-resolution spectra of 35 F main-sequence stars have been obtainedat high SNRs in the region of the Li I resonance line at 6708 A.Fourteen of the stars are members of the Hyades, six are members ofPraesepe, and the remaining 15 stars are members of the Hyades MovingGroup. Equivalent widths of the Li resonance line and of Fe lines in thesame spectral region were measured and Li and Fe abundances werecalculated from spectral synthesis using Kurucz model atmospheres. Amean metallicity of (Fe/H) = + 0.17 + or - 0.06 for the Hyades, + 0.13 +or - 0.07 for Praesepe, and + 0.11 + or - 0.09 for the Hyades MovingGroup is obtained. The Li abundance patterns are consistent with thosefound previously for the Hyades stars and for the Praesepe stars, butsome of the Hyades Moving Group members do not fit this Li-temperatureprofile well. The possibility exists that the Hyades Moving Group is notcoeval and that its members have little in common other than kinematics.
| A Proper Motion Membership Study of Praesepe
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1983AJ.....88..215J&db_key=AST
| Meridian observations made in Brorfelde (Copenhagen University Observatory) 1975-76.
This catalogue presents positions for selected faint stars mainly fromAGK3 observed with the 7" transit circle at Brorfelde. The stars aredistributed in 43 selected areas around radio sources and are suitableas reference stars for measuring the optical counterparts to the radiosources. The obscr 'itioi cre carried out from 1975.16 to 1976.04, andevery star was observed at least two times giving a m.s.e. of "13 forthe catalogue positions given in the FK4 system. These positions usedtogether with AGK3 proper motions will in the period 1975 to 1980 have am.s.e. at the epoch ranging from `:13 to `:15, which is less than halfof what is expected when using AGK3 positions. Key words: transit circle- catalogue of positions
| 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
| 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
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Наблюдательные данные и астрометрия
| Созвездие: | Рак |
| Прямое восхождение: | 08h39m58.07s |
| Склонение: | +19°12'05.9" |
| Видимая звёздная величина: | 9.231 |
| Собственное движение RA: | -36.5 |
| Собственное движение Dec: | -11.2 |
| B-T magnitude: | 9.778 |
| V-T magnitude: | 9.277 |
Каталоги и обозначения:
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