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Astron. Astrophys. 331, 81-120 (1998)

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The Hyades: distance, structure, dynamics, and age *

M.A.C. , Perryman 1, 2, A.G.A. , Brown 1, Y. , Lebreton 3, A. , Gómez 3, C. , Turon 3, G. , Cayrel de Strobel 3, J.C. , Mermilliod 4, N. , Robichon 3, J. , Kovalevsky 5, F. and Crifo 3

1 Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
2 Astrophysics Division, European Space Agency, ESTEC, Noordwijk 2200 AG, The Netherlands
3 DASGAL/URA CNRS 335, Section d'Astrophysique, Observatoire de Paris, F-92195 Meudon Cedex, France
4 Université de Lausanne, Chavannes-des-Bois, CH-1290, Switzerland
5 Observatoire de la Côte d'Azur, CERGA, Avenue Copernic, F-06130 Grasse, France

Received 20 March 1997 / Accepted 16 July 1997


We use absolute trigonometric parallaxes from the Hipparcos Catalogue to determine individual distances to members of the Hyades cluster, from which the 3-dimensional structure of the cluster can be derived. Inertially-referenced proper motions are used to rediscuss distance determinations based on convergent-point analyses. A combination of parallaxes and proper motions from Hipparcos, and radial velocities from ground-based observations, are used to determine the position and velocity components of candidate members with respect to the cluster centre, providing new information on cluster membership: 13 new candidate members within 20 pc of the cluster centre have been identified. Farther from the cluster centre there is a gradual merging between certain cluster members and field stars, both spatially and kinematically. Within the cluster, the kinematical structure is fully consistent with parallel space motion of the component stars with an internal velocity dispersion of about 0.3 km s-1. The spatial structure and mass segregation are consistent with N -body simulation results, without the need to invoke expansion, contraction, rotation, or other significant perturbations of the cluster. The quality of the individual distance determinations permits the cluster zero-age main sequence to be accurately modelled. The helium abundance for the cluster is determined to be Y =  [FORMULA] which, combined with isochrone modelling including convective overshooting, yields a cluster age of [FORMULA]  Myr. The distance to the observed centre of mass (a concept meaningful only in the restricted context of the cluster members contained in the Hipparcos Catalogue) is [FORMULA]  pc, corresponding to a distance modulus [FORMULA]  mag for the objects within 10 pc of the cluster centre (roughly corresponding to the tidal radius). This distance 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 trigonometric parallax determinations, and smaller than those found from recent studies of the cluster convergent point. These discrepancies are investigated and explained.

Key words: astrometry – parallaxes – HR Diagram – Hyades – distance scale

* Based on observations made with the ESA Hipparcos astrometry satellite. Table 2 is also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/Abstract.html

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998