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Ýlgili Makaleler
Temperature Behavior of Elemental Abundances in the Atmospheres of Magnetic Peculiar Stars
We analyze the temperature dependence of the abundances of the chemicalelements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line(Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp)stars in the range 6000 15000 K. The Cr and Fe abundances in theatmospheres of Ap stars increase rapidly as the temperature rises from6000 to 9000 10000 K. Subsequently, the Cr abundance decreases to valuesthat exceed the solar abundance by an order of magnitude, while the Feabundance remains enhanced by approximately +1.0 dex compared to thesolar value. The temperature dependence of the abundances of theseelements in the atmospheres of normal and Am stars is similar in shape,but its maximum is several orders of magnitude lower than that observedfor Ap stars. In the range 6000 9500 K, the observed temperaturedependences for Ap stars are satisfactorily described in terms ofelement diffusion under the combined action of gravitational settlingand radiative acceleration. It may well be that diffusion also takesplace in the atmospheres of normal stars, but its efficiency is very lowdue to the presence of microturbulence. We show that the magnetic fieldhas virtually no effect on the Cr and Fe diffusion in Ap stars in therange of effective temperatures 6000 9500 K. The Ca abundance and itsvariation in the atmospheres of Ap stars can also be explained in termsof the diffusion model if we assume the existence of a stellar wind witha variable moderate rate of (2 4) × 10- 15 M ȯ yr-1.
| A Catalog of Temperatures and Red Cousins Photometry for the Hyades
Using Hyades photometry published by Mendoza and other authors,Pinsonneault et al. have recently concluded that Cousins V-I photometrypublished by Taylor & Joner is not on the Cousins system. Extensivetests of the Taylor-Joner photometry and other pertinent results aretherefore performed in this paper. It is found that in part, thePinsonneault et al. conclusion rests on (1) a systematic error inMendoza's (R-I)J photometry and (2) a small error in anapproximate Johnson-to-Cousins transformation published by Bessell. Forthe Taylor-Joner values of (V-R)C, it is found that there arepossible (though not definite) differences of several mmag with otherresults. However, the Taylor-Joner values of (R-I)C data aresupported at the 1 mmag level. Using the (R-I)C data andother published results, an (R-I)C catalog is assembled for146 Hyades stars with spectral types earlier than about K5. For singlestars with multiple contributing data, the rms errors of the catalogentries are less than 4.4 mmag. Temperatures on the Di Benedettoangular-diameter scale are also given in the catalog and are used tohelp update published analyses of high-dispersion values of [Fe/H] forthe Hyades. The best current mean Hyades value of [Fe/H] is found to be+0.103+/-0.008 dex and is essentially unchanged from its previous value.In addition to these numerical results, recommendations are made aboutimproving attitudes and practices that are pertinent to issues likethose raised by Pinsonneault et al.
| The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the HipparcosCatalogue and the ACT Reference Catalogue. The ROSAT selected X-raysources are found to be a mixed population. A few of them areassociated with the Tau-Aur or Orion Star Forming Regions (SFR). Some,with distances similar to that of Tau-Aur but with discrepant propermotions, are probable or sure Pleiades super-cluster members or otherlate type young active stars with unresolved nature, more likely tooriginate in rapidly moving cloudlets, or else having originated fromdifferent sites other than Tau-Aur and moved to the present locations. Agood many of the non-TTS X-ray sources are considered as Hyades clustermembers. Some TTS candidates could be foreground pre-main sequencestars or actually young dwarfs not yet depleted of their Lithium. Underthe hypothesis that the sources we studied are representative of theROSAT selected TTS candidates discovered in the outskirts of the Tau-Aurregion, we conclude that only up to one third of the weak-line TTScandidates could be expected to be physically associated with theTau-Aur association. Along with the parallax and proper motion analysisof the non-TTS X-ray sources around the Tau-Aur SFR, our result suggeststhat the vast majority of the young active X-ray sources within anangular diameter of about 30 ° of the Tau-Aur SFR, belong to fourmain subgroups that are spatially separate.
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| The O VI and C III Lines at 1032 and 977 Å in Hyades F Stars
We continue our investigations into the mechanisms heating the outerlayers of cool dwarf stars. In this study we specifically seek todetermine whether in the layers with temperatures around 250,000-300,000K, in which the O VI lines are emitted, the temperatures are determinedby heat conduction from the coronae or by the same processes that heatthe lower temperature regions. To study this we discuss here 22 spectraof Hyades F stars taken by the Far Ultraviolet Spectroscopic Explorer(FUSE) satellite to study the O VI lines at 1032 Å and the C IIIlines at 977 Å and compare them with other lower transition layerlines, observed with HST and IUE, and with existing X-ray data. For ourtargets with B-V>0.4, the X-ray fluxes of single F stars increase, onaverage, slowly with increasing B-V, while the O VI line fluxes show thesame steep decrease around B-V=0.43 as previously found for the lowertemperature transition layer lines. For single stars the X-ray fluxesdecrease with increasing vsini, except for the stars with B-V between0.418 and 0.455, while for the O VI lines, as for the other transitionlayer lines, fluxes increase with increasing vsini, if vsini is largerthan 30 km s-1. For smaller vsini, line fluxes areindependent of vsini. The B-V and vsini dependences of the O VI linefluxes are then very different from those of the X-ray fluxes. We thusconclude that for electron temperature Te below 300,000 K,the transition layers for Hyades F stars are not mainly heated by heatconduction from their coronae.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE) telescope, which is operated for NASA byJohns Hopkins University, under contract NAS5-32985.
| 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
| Speckle Observations of Binary Stars with the WIYN Telescope. IV. Differential Photometry
Five hundred seventy-six magnitude difference measures are presented for260 binary stars. These measures are derived from CCD-based speckleobservations taken at the WIYN 3.5 m telescope at Kitt Peak NationalObservatory during the period 1997-2000. Separations of the systemsrange from over 1" down to near the diffraction limit of the telescope.A study of multiple measures of the same targets indicates that themeasures have a typical uncertainty of better than 0.13 mag per 2 minuteobservation, and that multiple observations can be averaged to arrive atsmaller uncertainties. Results presented here are also compared, insofaras it is possible, with measures in the Hipparcos Catalogue and toprevious studies using adaptive optics. No major systematic errors wereidentified.The WIYN Observatory is a joint facility of the University ofWisconsin-Madison, Indiana University, Yale University, and the NationalOptical Astronomy Observatory.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| An Archive of IUE Low-Dispersion Spectra of the White Dwarf Stars
We have produced an archive of the ultraviolet low-dispersion spectrafor the full set of white dwarf stars observed with the InternationalUltraviolet Explorer (IUE) satellite over the course of its 18 yrmission. This archive contains the spectra of 322 individual degeneratestars which have been processed to optimize the signal-to-noise for eachstar. In particular, all spectra have been corrected for residualtemporal and thermal effects and placed on the Hubble Space TelescopeFaint Object Spectrograph absolute flux scale using procedures describedby Massa & Fitzpatrick. Wherever possible, multiple observations ofindividual stars have been co-added to further enhance signal-to-noiseand have been combined into a single spectrum including the full 1150 to3150 Å wavelength region observed by IUE. The contents of thisspectral archive are described and the details of data reductionprocedures are provided, along with the url for access to the electronicfiles of the processed spectra.
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/721
| The Rate of Visual Binaries among the Brightest X-Ray Stars
Using the Tycho Double Star Catalogue and the Hipparcos Catalogue, Icompare the rates of occurrence of resolved visual doubles among thebrightest X-ray stars from the ROSAT All-Sky Survey Bright SourceCatalog (RASS-BSC) and generally in these two catalogs. A strongcorrelation between visual binarity and brightness in X-rays is found.The rate of binarity, quantified as the fraction of resolved components,is 7 times higher for the RASS-BSC stars than generally for all Tycho-2stars. In order to eliminate the distance bias in the sample, I comparethe rates in a distance-limited sample of Hipparcos stars within 50 pc.The comparison has shown that the fraction of visual and unresolvedorbiting binaries among stars with counterparts in the RASS-BSC, hencebright in X-rays, is 2.4 times higher than among fainter X-ray stars,i.e., the ones whose X-ray brightness is below the limit of theRASS-BSC. This difference is not caused by the enhancement of theobserved flux due to the combined component's radiation. Previouslyfound in the nearby open clusters, the correlation between binarity andX-ray luminosity exists for nearby field stars as well. Possibleimplications of the discrepancy are discussed, including thebinarity-age and binarity-rotation velocity relations. The H-R diagramof components of nearby visual binaries indicates that the red clumpgiants luminous in X-rays may be unresolved binaries.
| What Is Happening at Spectral Type F5 in Hyades F Stars?
Aiming at a better understanding of the mechanisms heating thechromospheres, transition regions, and coronae of cool stars, we studyultraviolet, low-resolution Hubble Space Telescope/Space TelescopeImaging Spectrograph spectra of Hyades main-sequence F stars. We studythe B-V dependence(s) of the chromospheric and transition layer emissionline fluxes and their dependences on rotational velocities. We find thatthe transition layer emission line fluxes and also those of strongchromospheric lines decrease steeply between B-V=0.42 and 0.45, i.e., atspectral type F5, for which the rotational velocities also decreasesteeply. The magnitude of the line-flux decrease increases for lines ofions with increasing degree of ionization. This shows that the line-fluxdecrease is not due to a change in the surface filling factor but ratherdue to a change of the relative importance of different heatingmechanisms. For early F stars with B-V<0.42 we find for thetransition layer emission lines increasing fluxes for increasing vsini,indicating magnetohydrodynamic heating. The vsini dependence isstrongest for the high-ionization lines. On the other hand, the lowchromospheric lines show no dependence on vsini, indicating acousticshock heating for these layers. This also contributes to the heating ofthe transition layers. The Mg II and Ca II lines show decreasing fluxesfor increasing vsini, as long as vsini is less than ~40 kms-1. The coronal X-ray emission also decreases for increasingvsini, except for vsini larger than ~100 km s-1. We have atpresent no explanation for this behavior. For late F stars thechromospheric lines show vsini dependences similar to those observed forearly F stars, again indicating acoustic heating for these layers. Wewere unable to determine the vsini dependence of the transition layerlines because of too few single star targets. The decrease of emissionline fluxes at the spectral type F5, with steeply decreasing vsini,indicates, however, a decreasing contribution of magnetohydrodynamicheating for the late F stars. The X-ray emission for the late F starsincreases for increasing vsini, indicating magnetohydrodynamic heatingfor the coronae of the late F stars, different from the early F stars.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Incorporated,under NASA contract NAS5-26555.
| Beryllium in the Hyades F and G Dwarfs from Keck HIRES Spectra
Although there are extensive observations of Li in field stars of alltypes and in both open and (recently) globular cluster stars, there arerelatively few observations of Be. Because Be is not destroyed as easilyas Li, the abundances of Li and Be together can tell us more about theinternal physical processes in stars than either element can alone. Wehave obtained high-resolution (45,000) and high signal-to-noise ratio(typically 90 per pixel) spectra of the Be II resonance lines in 34Hyades F and G dwarfs with the Keck I telescope and HIRES. In additionwe took a spectrum of the daytime sky to use as a surrogate for thesolar spectrum so we could determine the value for Be in the Sun,analyzed in the same manner as that for the stars. We have adoptedstellar temperatures and some Li abundances for these stars from theliterature. For most of the F dwarfs we have rederived Li abundances.The Be abundances have been derived with the spectrum synthesis method.We find that Be is depleted, but detected, in the Li gap in the F starsreaching down to values of A(Be)=0.60 dex, or a factor of nearly 7 belowthe meteoritic Be abundance (a factor of 3.5 below the solar value ofChmielewski et al.). There is little or no depletion of Be in starscooler than 6000 K, in spite of the large depletions (0.5-2.5 dex) inLi. The mean value of A(Be) for the 10 coolest stars is 1.33+/-0.06, notfar from the meteoritic value of 1.42. The pattern in the Beabundances-a Be dip and undepleted Be in the cool stars-is well matchedby the predictions of slow mixing due to stellar rotation. We haveinterpolated the calculations of Deliyannis and Pinsonneault for Bedepletion due to rotational mixing to the age of the Hyades; we findexcellent agreement of the predictions with the observed Be abundancesbut less good agreement with the observed Li abundances. Some of ourHyades stars have photometrically determined rotation periods, but thereis no relation between Be and rotation period. (Generally, the lowermass stars have less Li and longer periods, which may indicate greaterspin-down and thus more Li depletion relative to Be.) The Li and Beabundances are correlated for stars in the temperature range of5850-6680 K, similar to results from earlier work on Li and Be in F andG field stars. This indicates that the depletions are not justcorrelated-the only claim that can be made for the field stars-but areprobably occurring together during main-sequence evolution. The Hyades Gdwarfs have more Be than the Sun; their initial Be may have been largeror they may not be old enough to have depleted much Be. For those Hyadesstars that seem to have little or no depletion of Li or Be, the Li/Beratio is found to be 75+/-30 the meteoritic ratio Li/Be is 78. TheHyades ratio is a representative value for the initial ratio in thematerial out of which the Hyades cluster was formed.
| Speckle Observations of Binary Stars with the WIYN Telescope. II. Relative Astrometry Measures during 1998-2000
Five hundred twelve relative astrometry measures are presented for 253double stars, including 53 double stars discovered by Hipparcos. In 15cases, relative astrometry is reported for the first time for newlyconfirmed pairs. In addition, 20 high-quality nondetections ofcompanions are reported for stars suspected of being nonsingle byHipparcos. Observations were taken using a fast-readout CCD camerasystem at the WIYN 3.5 m telescope at Kitt Peak, Arizona. In comparingthese measures with ephemeris predictions for binary stars with verywell known orbits, we find that the measurement precision is better than3 mas in separation and 1° in position angle per individualobservation. Measurement precision and detection capabilities are fullydiscussed, and confirmed orbital motion is reported in four cases of theHipparcos double star discoveries. The WIYN Observatory is a jointfacility of the University of Wisconsin-Madison, Indiana University,Yale University, and the National Optical Astronomy Observatory.
| 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
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
| CCD Speckle Observations of Binary Stars from the Southern Hemisphere. III. Differential Photometry
Two hundred seventy-two magnitude difference measures of 135 double starsystems are presented. The results are derived from speckle observationsusing the Bessel V and R passbands and a fast readout CCD camera.Observations were taken at two 60 cm telescopes, namely the Helen SawyerHogg Telescope, formerly at Las Campanas, Chile, and the Lowell-TololoTelescope at the Cerro Tololo Inter-American Observatory, Chile. Thedata analysis method is presented and, in comparing the results to thoseof Hipparcos as well as to recent results using adaptive optics, we findvery good agreement. Overall, the measurement precision appears to bedependent on seeing and other factors but is generally in the range of0.10-0.15 mag for single observations under favorable observingconditions. In four cases, multiple observations in both V and R allowedfor the derivation of component V-R colors with uncertainties of 0.11mag or less. Spectral types are assigned and preliminary effectivetemperatures are estimated in these cases.
| 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
| A Hipparcos study of the Hyades open cluster. Improved colour-absolute magnitude and Hertzsprung-Russell diagrams
Hipparcos parallaxes fix distances to individual stars in the Hyadescluster with an accuracy of ~ 6 percent. We use the Hipparcos propermotions, which have a larger relative precision than the trigonometricparallaxes, to derive ~ 3 times more precise distance estimates, byassuming that all members share the same space motion. An investigationof the available kinematic data confirms that the Hyades velocity fielddoes not contain significant structure in the form of rotation and/orshear, but is fully consistent with a common space motion plus a(one-dimensional) internal velocity dispersion of ~ 0.30 kms-1. The improved parallaxes as a set are statisticallyconsistent with the Hipparcos parallaxes. The maximum expectedsystematic error in the proper motion-based parallaxes for stars in theouter regions of the cluster (i.e., beyond ~ 2 tidal radii ~ 20 pc) isla 0.30 mas. The new parallaxes confirm that the Hipparcos measurementsare correlated on small angular scales, consistent with the limitsspecified in the Hipparcos Catalogue, though with significantly smaller``amplitudes'' than claimed by Narayanan & Gould. We use the Tycho-2long time-baseline astrometric catalogue to derive a set of independentproper motion-based parallaxes for the Hipparcos members. The newparallaxes provide a uniquely sharp view of the three-dimensionalstructure of the Hyades. The colour-absolute magnitude diagram of thecluster based on the new parallaxes shows a well-defined main sequencewith two ``gaps''/``turn-offs''. These features provide the first directobservational support of Böhm-Vitense's prediction that (the onsetof) surface convection in stars significantly affects their (B-V)colours. We present and discuss the theoretical Hertzsprung-Russelldiagram (log L versus log T_eff) for an objectively defined set of 88high-fidelity members of the cluster as well as the delta Scuti startheta 2 Tau, the giants delta 1, theta1, epsilon , and gamma Tau, and the white dwarfs V471 Tau andHD 27483 (all of which are also members). The precision with which thenew parallaxes place individual Hyades in the Hertzsprung-Russelldiagram is limited by (systematic) uncertainties related to thetransformations from observed colours and absolute magnitudes toeffective temperatures and luminosities. The new parallaxes providestringent constraints on the calibration of such transformations whencombined with detailed theoretical stellar evolutionary modelling,tailored to the chemical composition and age of the Hyades, over thelarge stellar mass range of the cluster probed by Hipparcos.
| CCD Speckle Observations of Binary Stars from the Southern Hemisphere. II. Measures from the Lowell-Tololo Telescope during 1999
Speckle observations of 145 double stars and suspected double stars arepresented and discussed. On the basis of multiple observations, a totalof 280 position angle and separation measures are determined, as well as23 high-quality nondetections. All observations were taken with the(unintensified) Rochester Institute of Technology fast-readout CCDcamera mounted on the Lowell-Tololo 61 cm telescope at the Cerro TololoInter-American Observatory during 1999 October. We find that themeasures, when judged as a whole against ephemeris positions of binarieswith very well-known orbits, have root mean square deviations of1.8d+/-0.3d in position angle and 13+/-2 mas in separation. Elevendouble stars discovered by Hipparcos were also successfully observed,and the change in position angle and/or separation since the Hipparcosobservations was substantial in three cases.
| ICCD Speckle Observations of Binary Stars. XXIII. Measurements during 1982-1997 from Six Telescopes, with 14 New Orbits
We present 2017 observations of 1286 binary stars, observed by means ofspeckle interferometry using six telescopes over a 15 year period from1982 April to 1997 June. These measurements constitute the 23dinstallment in CHARA's speckle program at 2 to 4 m class telescopes andinclude the second major collection of measurements from the MountWilson 100 inch (2.5 m) Hooker Telescope. Orbital elements are alsopresented for 14 systems, seven of which have had no previouslypublished orbital analyses.
| Kinematics versus X-ray luminosity segregation in the Hyades
State of the art Tycho-2 proper motions of bona fide Hyades members areused in combination with the ROSAT X-ray survey of this open cluster inStern et al. (1995) to study the internal velocity dispersion versusX-ray luminosity by the classical convergent point method. It is shownin this paper for the Hyades members with B-V below 0.7 mag that starsbrighter than 1.3x 1029 erg s-1 (0.1-1.8 keV) havemuch more coherent motions than those with smaller X-ray luminositiesand those not detected by ROSAT at all. In fact, the proper motions ofthe X-ray luminous members are in agreement with a null internaldispersion of velocities for this subpopulation, whereas theintermediate bright X-ray stars exhibit a dispersion at 320 ms-1. The non-emitters, despite their being more massive andoptically brighter, have a velocity dispersion of about 440 ms-1. Probably related to this fact, the strong X-ray emittersare more concentrated within the tidal radius (10 pc) of the clustercentre than the non-emitters. Nearly half of strong X-ray emitters withcoherent motions are visual or orbiting pairs with typical separations 5to 50 AU. This find implies that a group of intermediate mass (1.0 to1.5 solar masses) stars in the Hyades is significantly younger than therest and their system is not yet dynamically relaxed.
| Internal velocity dispersion in the Hyades as a test for Tycho-2 proper motions
The Hyades stars have highly coherent motions in space which reflect inthe convergence of their astrometric proper motions in or about a commonconvergent point. The internal dispersion of the Hyades velocities is aslow as about 0.3 km s-1. This is comparable with the expecteduncertainty of the Tycho-2 proper motion at the distance of the Hyades.The classical approach of using the proper motion components normal tothe convergent point direction is employed for some 200 bona fidecluster members located in an area of some 2 000 deg2 andranging in V magnitude 3.4 to 12.1. It is demonstrated that with themost precise Tycho-2 proper motions for some 50 stars, the intrinsicdispersion is measurable at the level of 0.32 km s-1, inagreement with previous estimations obtained from the Hipparcos data andradial velocities. When only 30 stars out of the 50 are considered,which are not known to be spectroscopic binaries, the observed scattersuggests a dispersion of only 0.22 km s-1. The approach inuse is model independent, and sets an upper limit on the intrinsicdispersion of velocities. It is shown for some 180 stars in common, thatthe Tycho-2 proper motions have a very similar precision to that ofHipparcos, but reveal fewer extreme deviants from the common convergentpoint direction. Tycho-2 proper motions are based on long series ofobservations, up to a 100 years, and are therefore less subject to theorbital motion effects in binaries. The ratio of the external error tothe formal error of the Tycho-2 proper motions may be up to 1.3 forstars fainter than V_T=8.5 mag. An alternative explanation for the extrascatter in low-precision subsets is discussed, that less massive starsmay indeed have larger observed velocity dispersion due to the binarityeffect. Based on observations made with the ESA Hipparcos satellite.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Speckle Observations of Binary Stars with the WIYN Telescope. I. Measures During 1997
Two hundred seventy-seven position angle and separation measures of 154double stars are presented. Three of the systems were previously unknownto be double, and 16 other systems were discovered earlier this decadeby the Hipparcos satellite. Measures are derived from speckleobservations taken with the Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5 mtelescope located at Kitt Peak, Arizona. Speckle images were obtainedusing two different imaging detectors, namely, a multianode microchannelarray (MAMA) detector and a fast-readout CCD. A measurement precisionstudy was performed on a sample of binaries with extremely well knownorbits by comparing the measures obtained here to the ephemerispredictions. For the CCD, the root mean square (rms) deviation ofresiduals was found to be 3.5 milliarcseconds (mas) in separation and1.2d in position angle, while the residuals of the MAMA data varieddepending on the magnification used and seeing conditions but can becomparable or superior to the CCD values. In addition, the two cameraswere compared in terms of the detection limit in total magnitude andmagnitude difference of the systems under study. The MAMA system has theability to detect some systems with magnitude differences larger than3.5, although reliable astrometry could not be obtained on theseobjects. Reliable astrometry was obtained on a system of magnitudedifference of 5.3 with the CCD system.
| Chemical abundances of A and F-type stars: the Hyades open cluster
Abundances of 11 chemical elements have been determined for 29 F and 19A dwarfs (``normal'' and chemically peculiar with apparent rotationalvelocities ranging from 11 to 237 km.s^{-1}) members of the Hyadescluster using Takeda's (1995) iterative procedure well suited for fastrotators. High quality spectra of high to moderate resolution have beensynthesized to derive the apparent rotational velocities, microturbulentvelocities and abundances of C, O, Na, Mg, Si, Ca, Sc, Fe, Ni, Y and Ba.For the F stars, the found abundances are compared to the predictions ofTurcotte et al.'s (1998) evolutionary models for the age of the Hyades.We fail to find the expected underabundances for light elements nor theoverabundances for the iron-peak elements for stars having effectivetemperatures greater than 6500 K. This suggests that diffusion alonecannot account for the derived abundances: other transport processeswhich counteract diffusion are probably at play. For the A stars,abundances of Na, Mg, Si, Ca, Fe, Ni and Ba are anticorrelated(correlated for O) with the apparent equatorial velocities forve\sin i higher than ~ 100 km.s^{-1}. Below this value, wefind no correlation in agreement with the predictions of Charbonneau& Michaud (1991) except possibly for the abundance of Ni which mightbe anticorrelated with ve\sin i. The fastest rotators(ve\sin i >= 150 km.s^{-1}) are strongly depletted in Mg,Si, Ca, Fe, Ni and Ba (between -0.3 dex and -0.94 dex) while their Oabundance is normal. Based on observations collected at Observatoire deHaute Provence (France)
| Visual binary orbits and masses POST HIPPARCOS
The parallaxes from Hipparcos are an important ingredient to derive moreaccurate masses for known orbital binaries, but in order to exploit theparallaxes fully, the orbital elements have to be known to similarprecision. The present work gives improved orbital elements for some 205systems by combining the Hipparcos astrometry with existing ground-basedobservations. The new solutions avoid the linearity constraints andomissions in the Hipparcos Catalog by using the intermediate TransitData which can be combined with ground-based observations in arbitarilycomplex orbital models. The new orbital elements and parallaxes give newmass-sum values together with realistic total error-estimates. To getindividual masses at least for main-sequence systems, the mass-ratioshave been generally estimated from theoretical isochrones and observedmagnitude-differences. For some 25 short-period systems, however, trueastrometric mass-ratios have been determined through the observedorbital curvature in the 3-year Hipparcos observation interval. Thefinal result is an observed `mass-luminosity relation' which falls closeto theoretical expectation, but with `outliers' due to undetectedmultiplicity or to composition- and age-effects in the nonuniformnear-star sample. Based in part on observations collected with the ESAHipparcos astrometry satellite. Tables~ 1, 3, 4 and 6 are also availablein electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr~(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Color indices of the Sun and Hyades stars in the WBVR system
Using an original setup at high altitudes, we measured the color indicesof the Sun in the WBVR photometric system relative to standard stars: (W- B)_solar = -0.05, (B - V)_solar = +0.67, and (V - R)_solar = +0.53. Wepresent the WBVR photometry for the Hyades members selected by vanBueren by their space velocities. The solar position is shown intwo-color diagrams relative to Hyades stars and bright G2 V stars. Acomparison of our results with Kurucz's models reveals a discrepancybetween the metallicities of the models and the Hyades members.
| Observations of Double Stars. XVIII.
Micrometer observations of 1350 pairs in 1995-1997 are listed.
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| Takýmyýldýz: | Avci |
| Sað Açýklýk: | 04h51m49.98s |
| Yükselim: | +13°39'18.5" |
| Görünürdeki Parlaklýk: | 6.287 |
| Uzaklýk: | 41.824 parsek |
| özdevim Sað Açýklýk: | 89.5 |
| özdevim Yükselim: | -16.8 |
| B-T magnitude: | 6.865 |
| V-T magnitude: | 6.335 |
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