Jhj. De Bruijne et al., A Hipparcos study of the Hyades open cluster - Improved colour-absolute magnitude and Hertzsprung-Russell diagrams, ASTRON ASTR, 367(1), 2001, pp. 111-147
Hipparcos parallaxes Ex distances to individual stars in the Hyades cluster
with an accuracy of similar to6 percent. We use the Hipparcos proper motio
ns; which have a larger relative precision than the trigonometric parallaxe
s, to derive similar to3 times more precise distance estimates, by assuming
that all members share the same space motion. An investigation of the avai
lable kinematic data confirms that the Hyades velocity field does not conta
in significant structure in the form of rotation and/or shear, but is fully
consistent with a common space motion plus a (one-dimensional) internal ve
locity dispersion of similar to0.30 km s(-1). The improved parallaxes as a
set are statistically consistent with the Hipparcos parallaxes. The maximum
expected systematic error in the proper motion-based parallaxes for stars
in the outer regions of the cluster (i.e., beyond similar to2 tidal radii s
imilar to 20 pc) is less than or similar to0.30 mas. The new parallaxes con
firm that the Hipparcos measurements are correlated on small angular scales
, consistent with the limits specified in the Hipparcos Catalogue, though w
ith significantly smaller "amplitudes" than claimed by Narayanan & Could. W
e use the Tycho-2 long time-baseline astrometric catalogue to derive a set
of independent proper motion-based parallaxes for the Hipparcos members. Th
e new parallaxes provide a uniquely sharp view of the three-dimensional str
ucture of the Hyades. The colour-absolute magnitude diagram of the cluster
based on the new parallaxes shows a well-defined main sequence with two "ga
ps"/"turn-offs".. These features provide the first direct observational sup
port of Bohm-Vitense's prediction that (the onset of) surface convection in
stars significantly affects their (B - V) colours. We present and discuss
the theoretical Hertzsprung-Russell diagram (log L versus log T-eff) for an
objectively defined set of 88 high-fidelity members of the cluster as well
as the delta Scuti star theta (2) Tau, the giants delta (1), theta (1), ep
silon, and gamma Tau, and the white dwarfs V471 Tau and HD 27483 (all of wh
ich are also members). The precision with which the new parallaxes place in
dividual Hyades in the Hertzsprung-Russell diagram is limited by (systemati
c) uncertainties related to the transformations from observed colours and a
bsolute magnitudes to effective temperatures and luminosities. The new para
llaxes provide stringent constraints on the calibration of such transformat
ions when combined with detailed theoretical stellar evolutionary modelling
, tailored to the chemical composition and age of the Hyades, over the larg
e stellar mass range of the cluster probed by Hipparcos.