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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.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Coronal X-ray sources in the Hyades: A 40 kilosecond ROSAT pointing
We present results of a 40 ks ROSAT pointed observation of the Hyadescluster. The limiting Lx is approximately = 2 x1027 ergs/sec at field center, increasing to approximately =2 x 1028 ergs/sec at 40 min off-axis. This represents themost sensitive X-ray observation to date in the Hyades region. More than30 sources have been detected in the Position Sensitive ProportionalCounter (PSPC) standard processing, of which 15 are Hyades members, fiveare cluster candidates that are likely non-members, four are foregroundor background stars, and the remainder are unidentified. One Hyad, VB173 (= VA 276), lies in a confused region, but is detected as a distinctsource in the soft band only (E approximately less than 0.5 keV). Wereport upper limits for four other Hyades members in our field, all Mdwarfs. Of the 16 Hyades detections, two represent the opticallyfaintest members seen to date in X-rays; VA 260 (V = 16.68) and VA 368(V = 16.25). These are both M dwarfs of mass approximately = 0.15-0.2solar mass and are fully convective stars according to current theory.Analysis of X-ray light curves using 1 ks bins indicates somevariability in the strongest sources and a possible flare in VA 383. TwoHyades stars, VB 141 and VB 71, were also detected with the co-alignedWide Field Camera (WFC) EUV instrument. VB 141, the second brightestX-ray source in the Hyades, remains an enigma: a rapidly rotating FOstar with a fainter, long-period companion, this object has an X-rayspectrum indicative of strong coronal activity. X-ray pulse-heightanalysis demonstrates that coronal models with at least two temperaturesare required for most of the stronger X-ray sources. The ROSAT X-rayspectra generally require higher temperatures for the hotter componentin the M dwarfs compared to the F-G dwarfs.
| Parallaxes of stars in the Hyades Cluster region - A comparison of hand and automatic measures
Parallaxes and proper motions are presented for 30 members of the HyadesCluster and 223 field stars in 13 fields located near the center of thecluster. The series of photographic plates are extensive and they weremeasured twice; first, by hand, using a conventional two-screw measuringmachine, and also with a PDS microdensitometer. A comparison of the twosets of measures reveals that the internal and external errors inparallax average about + or - 0.005 arcsec for the central stars fromautomatic PDS measures and about twice this amount for the data measuredby hand. The parallaxes of the member stars yield a distance modulus of3.30, in excellent agreement with the average of many prior distancedeterminations for the cluster. The parallaxes of the field starsconfirm a small amount of coma or higher-order terms in the opticalsystem of the Van Vleck refractor. They also confirm that, within afield of one half degree radius centered on the optical axis and over arange in visual magnitude from 8.5 to 13, the errors in parallax varyonly slightly over the most optimum conditions.
| 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.
| The Einstein Observatory survey of stars in the Hyades cluster region
The paper reports the results of an extensive X-ray study of the Hyadesregion and improves on previous studies by using refined X-ray sourcedetection algorithms and the complete set of Einstein Observatory IPCexposures covering the Hyades cluster region (a total of 63 1 x 1 degimages). Maximum likelihood integral X-ray luminosity functions werecomputed for the Hyades stars for given color index (B-V) ranges. Thepresent results agree substantially with a previous survey about theubiquity of the stellar activity in the Hyades cluster, especially amongsolar-type stars. The result of Stern et al. (1981) that stellar X-rayemission is dependent on stellar age is confrimed.
| 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.
| Subluminous stars in the Hyades region
Proper-motion and photometric surveys of the Hyades cluster havediscovered some 50 stars sharing the Hyades motion but lying about twomagnitudes below the cluster main sequence. The long-standing questionof the nature of these 'subluminous stars in the Hyades' (SLH stars) hasnever been genuinely resolved. In this paper the observationalstatistics of the Hyades proper motions, photometry, and new radialvelocities are discussed. It is demonstrated that the SLH stars areneither subluminous nor Hyades members. Comparisons with known Hyadesfield stars and with a simple theoretical model show quantitatively thatthe SLH stars are ordinary background field stars.
| Infrared photometry of Hyades dwarfs
JHK infrared photometry is presented for 76 main-sequence proper motionmembers brighter than V = 12 in the Hyades. Known binaries are confirmedand new candidates proposed using color-magnitude and color-colordiagrams. The cluster binary frequency (about 40%) is discussed, as wellas its possible effects on the cluster distance modulus and stellarchromospheric/coronal activity.
| Erratum - Errors or Omissions in Star-Identifications in the General Catalogue of Trigonometric Stellar Parallaxes
Not Available
| 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
| 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.
| The Distance to the Hyades Cluster from R - i Photometry
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974ApJ...193..359U&db_key=AST
| 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
| 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
| The Hyades and Coma Berenices Star Clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1955ApJ...122..209J&db_key=AST
| Some new members of the Hyades cluster
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