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Chemical Homogeneity in the Hyades
We present an abundance analysis of the heavy elements Zr, Ba, La, Ce,and Nd for Hyades F-K dwarfs based on high-resolution, highsignal-to-noise ratio spectra from Keck HIRES. The derived abundancesshow the stellar members to be highly uniform, although some elementsshow a small residual trend with temperature. The rms scatter for eachelement for the cluster members is as follows: Zr=0.055, Ba=0.049,Ce=0.025, La=0.025, and Nd=0.032 dex. This is consistent with themeasurement errors and implies that there is little or no intrinsicscatter among the Hyades members. Several stars thought to be nonmembersof the cluster based on their kinematics are found to deviate from thecluster mean abundances by about 2 σ. Establishing chemicalhomogeneity in open clusters is the primary requirement for theviability of chemically tagging Galactic disk stars to common formationsites in order to unravel the dissipative history of early diskformation.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Searching for Planets in the Hyades. V. Limits on Planet Detection in the Presence of Stellar Activity
We present the results of a radial velocity survey of a sample of Hyadesstars and discuss the effects of stellar activity on radial velocitymeasurements. The level of radial velocity scatter due to rotationalmodulation of stellar surface features for the Hyades is in agreementwith the 1997 predictions of Saar & Donahue-the maximum radialvelocity rms of up to ~50 m s-1, with an average rms of ~16 ms-1. In this sample of 94 stars we find one new binary, twostars with linear trends indicative of binary companions, and noclose-in giant planets. We discuss the limits on extrasolar planetdetection in the Hyades and the constraints imposed on radial velocitysurveys of young stars.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration (NASA). TheObservatory was made possible by the generous financial support of theW. M. Keck Foundation. The authors wish to recognize and acknowledge thevery significant cultural role and reverence that the summit of MaunaKea has always had within the indigenous Hawaiian community. We are mostfortunate to have the opportunity to conduct observations from thismountain. Additional data were obtained with the Hobby-Eberly Telescope,which is operated by McDonald Observatory on behalf of the University ofTexas at Austin, the Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| 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
| Searching for Planets in the Hyades. IV. Differential Abundance Analysis of Hyades Dwarfs
We present a differential abundance analysis of Hyades F-K dwarfs insearch for evidence of stellar enrichment from accretedhydrogen-deficient disk material. Metallicities and relative abundanceratios of several species have been determined. We derive a cluster mean[Fe/H]=0.13+/-0.01. Two stars show abundances ~0.2 dex larger than thecluster mean. In addition, one star, which was added by a recent studyas a cluster member, shows significantly lower abundances than thecluster mean. These three stars have questionable membershipcharacteristics. The remaining stars in the survey have an rms of 0.04dex in the differential [Fe/H] values. The Hyades cluster members haveapparently not been significantly chemically enriched. The abundanceratios of Si, Ti, Na, Mg, Ca, and Zn with respect to Fe are in theirsolar proportions.Some data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Searching for Planets in the Hyades. II. Some Implications of Stellar Magnetic Activity
The Hyades constitute a homogeneous sample of stars ideal forinvestigating the dependence of planet formation on the mass of thecentral star. Because of their youth, Hyades members are much morechromospherically active than stars traditionally surveyed for planetsusing high-precision radial velocity techniques. Therefore, we haveconducted a detailed investigation of whether magnetic activity of ourHyades target stars will interfere with our ability to make preciseradial velocity (vrad) searches for substellar companions. Wemeasure chromospheric activity (which we take as a proxy for magneticactivity) by computing the equivalent of the R'HKactivity index (which is corrected for photospheric contributions) fromthe Ca II K line. The value of is notconstant in the Hyades: we confirm that it decreases with increasingtemperature in the F stars and also find it decreases for stars coolerthan mid K. We examine correlations between simultaneously measuredR'HK and radial velocities using both a classicalstatistical test and a Bayesian odds ratio test. We find that there is asignificant correlation between R'HK and theradial velocity in only five of the 82 stars in this sample. Thus,simple R'HK-vrad correlations willgenerally not be effective in correcting the measured vradvalues for the effects of magnetic activity in the Hyades. We argue thatthis implies long-timescale activity variations (of order a few years;i.e., magnetic cycles or growth and decay of plage regions) will notsignificantly hinder our search for planets in the Hyades if the starsare closely monitored for chromospheric activity. The trends in theradial velocity scatter (σ'v) with, vsini, and Prot for ourstars is generally consistent with those found in field stars in theLick planet search data, with the notable exception of a shallowerdependence of σ'v on for F stars. Data presented hereinwere obtained at the W.M. Keck Observatory, which is operated as ascientific partnership among the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration. The Observatory was made possible by the generousfinancial support of the W.M. Keck Foundation.
| 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.
| 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.
| The Hyades: distance, structure, dynamics, and age
{We use absolute trigonometric parallaxes from the Hipparcos Catalogueto determine individual distances to members of the Hyades cluster, fromwhich the 3-dimensional structure of the cluster can be derived.Inertially-referenced proper motions are used to rediscuss distancedeterminations based on convergent-point analyses. A combination ofparallaxes and proper motions from Hipparcos, and radial velocities fromground-based observations, are used to determine the position andvelocity components of candidate members with respect to the clustercentre, providing new information on cluster membership: 13 newcandidate members within 20 pc of the cluster centre have beenidentified. Farther from the cluster centre there is a gradual mergingbetween certain cluster members and field stars, both spatially andkinematically. Within the cluster, the kinematical structure is fullyconsistent with parallel space motion of the component stars with aninternal velocity dispersion of about 0.3 km s(-1) . The spatialstructure and mass segregation are consistent with N-body simulationresults, without the need to invoke expansion, contraction, rotation, orother significant perturbations of the cluster. The quality of theindividual distance determinations permits the cluster zero-age mainsequence to be accurately modelled. The helium abundance for the clusteris determined to be Y =3D 0.26+/-0.02 which, combined with isochronemodelling including convective overshooting, yields a cluster age of625+/-50 Myr. The distance to the observed centre of mass (a conceptmeaningful only in the restricted context of the cluster memberscontained in the Hipparcos Catalogue) is 46.34+/-0.27 pc, correspondingto a distance modulus m-M=3D3.33+/-0.01 mag for the objects within 10 pcof the cluster centre (roughly corresponding to the tidal radius). Thisdistance modulus is close to, but significantly better determined than,that derived from recent high-precision radial velocity studies,somewhat larger than that indicated by recent ground-based trigonometricparallax determinations, and smaller than those found from recentstudies of the cluster convergent point. These discrepancies areinvestigated and explained. } Based on observations made with the ESAHipparcos astrometry satellite. Table~2 is also available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/Abstract.html
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| The Kreinin-Murri Method for Absolute Determination of Equatorial-Star Declinations.
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