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Magnesium Isotope Ratios in Hyades Stars
Using classical model atmospheres and an LTE analysis, Mg isotope ratios24Mg:25Mg:26Mg are measured in 32Hyades dwarfs covering effective temperatures4000K<=Teff<=5000K. We find no significant trend in anyisotope ratio versus Teff, and the mean isotope ratio is inexcellent agreement with the solar value. We determine stellarparameters and Fe abundances for 56 Hyades dwarfs covering4000K<=Teff<=6200K. For stars warmer than 4700 K, wederive a cluster mean value of [Fe/H]=0.16+/-0.02 (σ=0.1), in goodagreement with previous studies. For stars cooler than 4700 K, we findthat the abundance of Fe from ionized lines exceeds the abundance of Fefrom neutral lines. At 4700 K,[Fe/H]II-[Fe/H]I~=0.3dex, while at 4000 K[Fe/H]II-[Fe/H]I~=1.2dex. This discrepancy betweenthe Fe abundance from neutral and ionized lines likely reflectsinadequacies in the model atmospheres and the presence of non-LTE orother effects. Despite the inability of the models to reproduce theionization equilibrium for Fe, the Mg isotope ratios appear immune tothese problems and remain a powerful tool for studying Galactic chemicalevolution.Data presented here were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof 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.
| The Distances to Open Clusters as Derived from Main-Sequence Fitting. II. Construction of Empirically Calibrated Isochrones
We continue our series of papers on open cluster distances by comparingmulticolor photometry of single stars in the Hyades with theoreticalisochrones constructed with various color-temperature relations. Afterverifying that the isochrone effective temperatures agree well withspectroscopically determined values, we argue that mismatches betweenthe photometry and the theoretical colors likely arise from systematicerrors in the color-temperature relations. We then describe a method forempirically correcting the isochrones to match the photometry anddiscuss the dependence of the isochrone luminosity on metallicity.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| 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.
| A New Procedure for the Photometric Parallax Estimation
We present a new procedure for photometric parallax estimation. The datafor 1236 stars provide calibrations between the absolute magnitudeoffset from the Hyades main-sequence and the ultraviolet-excess foreight different (B-V)0 colour-index intervals, (0.3 0.4),(0.4 0.5), (0.5 0.6), (0.6 0.7), (0.7 0.8), (0.8 0.9), (0.9 1.0) and(1.0 1.1). The mean difference between the original and estimatedabsolute magnitudes and the corresponding standard deviation are rathersmall, +0.0002 and +/-0.0613 mag. The procedure has been adapted to theSloan photometry by means of colour equations and applied to a set ofartificial stars with different metallicities. The comparison of theabsolute magnitudes estimated by the new procedure and the canonical oneindicates that a single colour-magnitude diagram does not supplyreliable absolute magnitudes for stars with large range of metallicity.
| 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.
| 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
| 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.
| 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
| ROSAT All-Sky Survey Observations of the Hyades Cluster
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...448..683S&db_key=AST
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| Seven-Color Photoelectric Photometry of Stars in the Hyades Clusters
Not Available
| Low-Mass Stars in the Hyades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.265..785R&db_key=AST
| The low mass Hyades and the evaporation of clusters
The 135 single stars and 85 binary systems, redder than R-I = +0.34 magand brighter than V = 17 mag, between alpha = 3.75 h and 5.0 h and delta= +5 deg and + 25 deg show a luminosity function that differsconsiderably from that of the general field stars within 20 pc of theSun. The ratio of double star components to single cluster membersincreases markedly with decreasing luminosity. Forty-three single starsand 16 binary systems that are members of the Hyades supercluster within20 pc of the Sun show the same luminosity function as the field stars inthat region. Fifty percent of the cluster members and 40 percent of thesupercluster members are components of binary stars. The equivalentwidths of H-alpha appear to support a range of ages (approximately 8 to16 x 108 yr) for the cluster stars and demonstrate that theoldest objects are in the supercluster. A list of cluster members, whichmay include the end of the stable main sequence, but for which accurate(R-I) photometry is not available, is included. The half-dozen knownparallax stars of the faintest luminosity contain at least onesupercluster member, TVLM 868-110639, which is probably beyond thestable, nuclear burning main sequence as a 'transitional' or 'brown'dwarf.
| The chromospheric activity of low-mass stars in the Hyades
High-resolution spectra or narrow-band H-alpha photometry of 106 dwarf Kand M stars in the Hyades cluster have been used to determine H-alphaequivalent widths. These data reveal a sequence of Hyades members withH-alpha in absorption for photospheric temperatures hotter than about3500 K. Within this sequence, the dispersion about the mean equivalentwidth-color relation is only slightly larger than the measurementerrors. A second sequence of Hyades members, characterized by H-alphaemission and significant scatter at a given color, appears at effectivetemperatures cooler than about 4000 K. This bifurcation in H-alphaproperties at 4000 K coincides with a bifurcation in the rotationalcharacteristics of Hyades stars: the dispersion in the rotationalvelocities of the hotter Hyades members at a given effective temperatureis small while the cooler stars exhibit significant scatter in theirrotational velocities. On the basis of these data, it is suggested thatlow-mass stars spin down to rotational velocities dependent upon massand age, but independent of premain-sequence angular momentum.
| UBVRI photometry of G, K, M HIPPARCOS stars. II
As indicated by Grenon (1989), the data of the present series of reportson the UBVRI photometry of late-type stars in the Hipparcos InputCatalog are to be employed in computations of Hipparcos observing time,as well as in evaluating the observability of faint stars by thesatellite. Attention is here given to late type stars in the V = 8-12range, including distant red giants in the Galactic plane (Hipparcosproposal 189), as well as high proper motion stars included in the G,LTT, LP, and MCC catalogs.
| A radial-velocity survey of the Hyades
Photoelectric radial-velocity measurements, obtained with external erroras small as 0.1 km/s using the 200-inch Hale telescope at PalomarObservatory during the period 1971-1986, are reported for over 400candidate members (with V magnitude between 6 and 14) of the Hyadescluster. The history of Hyades observations is recalled; the Palomarinstrumentation and observing program are described; the data-reductionand standardization procedures are discussed in detail; and the data arepresented in extensive tables and graphs. About 200 of the stars areclassified as cluster members, including 60 spectroscopic binaries.
| Rotational velocities of low-mass stars in the Hyades
High-resolution echelle spectra have been used to estimate rotationalvelocities for K and M dwarfs in the Hyades. All of the K dwarfs haverotational velocities less than the instrumental limit of 10 km/s. Mostof the M dwarfs with (R - I)K larger than 1.0 have detectable rotationalvelocities, with v sin i between 10 and 20 km/s. Combining these datawith results from the younger Pleiades and Alpha Persei clusters, it isfound that G dwarfs spin down from about 100 km/s to about 10 km/s inless than 40 million yr, whereas M dwarfs take an order of magnitudelonger to spin down.
| Positions of 127 Hyads and 6-cm observations of 320 Hyads
Accurate positions are given for 127 Hyades cluster stars whichpreviously had only approximate positions known. The fields of 320Hyads, including 55 X-ray stars, were observed with the VLA to a 6-cmdetection level of approximately 0.5 mJy. There were no detections.
| Photometry of stars in the uvgr system
Photoelectric photometry is presented for over 400 stars using the uvgrsystem of Thuan and Gunn. Stars were selected to cover a wide range ofspectral type, luminosity class, and metallicity. A mean main sequenceis derived along with reddening curves and approximate transformationsto the UBVR system. The calibration of the standard-star sequence issignificantly improved.
| Systematic search for members of the Hyades Supercluster. VI - The lower main sequence
Proper motion has served as the basis for identifying 50 members of theHyades Supercluster from a complete sample of stars with visualmagnitude higher than 15 and annual proper motion greater than 0.5arcsec. The resulting main sequence is represented by M(bol) 4.51 + 4.31(R-I), with 0.15 mag dispersion. The supercluster stars may represent 15percent of the star density near the sun. The reddest superclusterdwarfs are noted to be cool to a degree that is nearly comparable tosuch extreme stars to VB 10.
| Photometry of Possible Members of the Hyades Cluster - Part Four
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1982PASP...94..475W&db_key=AST
| Luminosity distribution and shape of the Hyades cluster
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979A&A....78..312O&db_key=AST
| Photometry of new possible members of the Hyades cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977AJ.....82..978U&db_key=AST
| New members of the Hyades cluster and a discussion of its structure.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975A&A....43..423P&db_key=AST
| Charts for Hyades Stars
Not Available
| The Color-Magnitude Diagram of the Hyades Cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1962ApJ...136...75J&db_key=AST
| Dwarf M stars found spectrophotometrically .
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1956AJ.....61..201V&db_key=AST
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Membro dei seguenti gruppi:
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Osservazione e dati astrometrici
| Costellazione: | Toro |
| Ascensione retta: | 04h33m37.19s |
| Declinazione: | +21°09'03.0" |
| Magnitudine apparente: | 10.585 |
| Distanza: | 40.032 parsec |
| Moto proprio RA: | 109.5 |
| Moto proprio Dec: | -47.6 |
| B-T magnitude: | 12.574 |
| V-T magnitude: | 10.75 |
Cataloghi e designazioni:
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