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Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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.
| 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.
| A Precision Test of HIPPARCOS Systematics toward the Hyades
We propose a test that can in principle detect any systematic errors inthe Hipparcos parallaxes toward the Hyades cluster at the level of 0.3mas. We show that the statistical parallax algorithm subsumes theclassical moving cluster methods and provides more precise estimates ofthe distance and the first two moments of the velocity distribution ofthe Hyades cluster, namely, its bulk space velocity and the velocitydispersion tensor. To test the Hipparcos parallaxes, we first rescalethe bulk velocity determined from statistical parallax to forceagreement with the distance scale determined from Hipparcos parallaxes.We then predict the parallaxes of Hyades cluster members using thiscommon cluster space velocity and their Hipparcos proper motions. Weshow that the parallaxes determined in this manner (pi_pm) areconsistent at the 1 sigma level with the parallaxes (pi_orb) of threeHyades spectroscopic binary systems with orbital solutions. We find that=0.52+/-0.47 mas, where the error is dominated bythe errors in the orbital parallaxes. A reduction in these errors wouldallow a test of the systematic errors in the Hipparcos parallaxes at the0.3 mas level. If the Hyades distance scale is fixed by Hipparcosparallaxes, then its bulk velocity in equatorial coordinates is(V_x,V_y,V_z)=(-5.70+/-0.20, 45.62+/-0.11, 5.65+/-0.08) kms^-1, itsvelocity dispersion is 320+/-39 m s^-1, and the distance modulus to thecentroid of our sample of 43 cluster members is 3.34+/-0.02 mag.
| 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
| 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.
| 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.
| Photometry of possible members of the Hyades cluster. VI
Photoelectric observations in BVRI colors have been obtained for about200 stars suspected of being members of the Hyades cluster, mostly onthe basis of their proper motions. More than half of this total havebeen observed photometrically for the first time; the rest are mostlystars covered in the early papers of this series for which the V - Rcolors had not been determined and for which the accuracy of thephotometry could be improved by reobservation. Based on the photometry,about half of these stars appear to be probable cluster members. Most ofthe stars included here are faint and have appeared in proper-motionlists for which identification charts have not been available. Theiridentifications were confirmed from their motions measured on platestaken in the course of the Lick Proper Motion Survey. This list raisesto about 810 the total number of stars in the Hyades region withbroadband photometry. Of these, 365 appear to be probable or possiblemembers of the cluster.
| 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
| A study of the motion, membership, and distance of the Hyades cluster
Particular attention is given to the role of the convergent-point, ormoving cluster, method in the determination of the Hyades distance, andthe apparent discrepancy between its results and those of other Hyadesdistance indicators. The Hyades photographic plate material isconsidered along with the approaches used in the measurements, propermotion reductions, coordinate reductions, and aspects of photography.Problems of cluster membership determination are also discussed.
| Low-luminosity members of the Hyades cluster. II.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1969AJ.....74....2V&db_key=AST
| Low-luminosity members of the Hyades cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966AJ.....71..482V&db_key=AST
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