Consequences of Hipparcos Parallaxes for Stellar Evolutionary Models

Three binary systems in the Hyades open cluster (51 Tau, V818 Tau, and θ2 Tau), with known metallicity and good Johnson photometric data are used to test the validity of three independent sets of stellar evolutionary tracks. A statistical method is described and applied to the colour-magnitude diagram of the six selected components, giving rise to χ2-contours in the metallicity–age plane. The effects of the Hipparcos parallaxes on these confidence regions are studied in detail for these binaries through a comparison with very accurate but older orbital parallaxes. There are no significant differences in 51 Tau and θ2 Tau, and hence the test validates the sets of theoretical tracks used. However the orbital parallax of V 818 Tau leads to metallicities too large in comparison with the observed range, while the Hipparcos parallax give consistent results, at the 1σ level. Comparisons between the predicted and measured masses are also made and show a good agreement. However, even if the Padova tracks predict correct masses for each component of V 818 Tau, there is no Padova track that fits both components simultaneously in the mass-radius diagram. Finally, since the individual masses of these binaries are accurately known, some issues about the influence of the Hipparcos parallaxes on their location in the mass-luminosity diagram are also discussed.