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Dainis Dravins. Profile photo.

Dainis Dravins

Professor emeritus

Dainis Dravins. Profile photo.

Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations

Author

  • Søren Madsen
  • Dainis Dravins
  • Lennart Lindegren

Summary, in English

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<SUP>-1</SUP> (standard error), and that of its individual stars towithin 0.6 km s<SUP>-1</SUP>. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms<SUP>-1</SUP>. 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

ef{tab1} and

ef{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

Department/s

  • Lund Observatory

Publishing year

2002

Language

English

Pages

446-463

Publication/Series

Astronomy & Astrophysics

Volume

381

Document type

Journal article

Publisher

EDP Sciences

Topic

  • Astronomy, Astrophysics and Cosmology

Keywords

  • METHODS: DATA ANALYSIS
  • TECHNIQUES: RADIAL VELOCITIES
  • ASTROMETRY
  • OPEN CLUSTERS AND ASSOCIATIONS: GENERAL
  • STARS: DISTANCES - STARS: KINEMATICS

Status

Published

ISBN/ISSN/Other

  • ISSN: 0004-6361