The helium content and age of the Hyades: Constraints from five binary systems and Hipparcos parallaxes

We compare the accurate empirical mass-luminosity (M-L) relation based on f ive Hyades binary systems to predictions of stellar models calculated with various input parameters (helium, metallicity and age) or physics (mixing-l ength ratio, model atmosphere, equation of state and microscopic diffusion) . Models based on a helium content Y similar to 0.28 inferred from the Delt aY/DeltaZ enrichment law are more than 3 sigma beyond the observations, sug gesting that the Hyades initial helium abundance is lower than expected fro m its supersolar metallicity. With the photometric metallicity ([Fe=H] = 0. 144 +/-0.013 dex, Grenon 2000) we derive Y = 0.255 +/-0.009. Because of the (Y, [Fe=H]) degeneracy in the M-L plane, the uncertainty grows to DeltaY = 0.013 if the metallicity from spectroscopy is adopted ([Fe=H] = 0.14 +/-0. 05 dex, Cayrel de Strobel et al. 1997). We use these results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the (M-V, B-V) plane, b ased on the very precise Hipparcos dynamical parallaxes. Present models fit the tight observed sequence very well except at low temperatures. We show that the HR diagram does not bring further constraints on the helium abunda nce or metallicity of the cluster. In the low mass region of the HR diagram sensitive to the mixing-length parameter (alpha (MLT)), the slope of the m ain sequence (MS) suggests that alpha (MLT) could decrease from a solar (or even supersolar) value at higher mass to subsolar values at low mass, whic h is also supported by the modeling of the vB22 M-L relation. We find that the discrepancy at low temperatures ((B-V) greater than or similar to 1.2) remains, even if an improved equation of state or better model atmospheres are used. Finally, we discuss the positions of the stars at turn-off in the light of their observed rotation rates and we deduce that the maximum age of the Hyades predicted by the present models is similar to 650 Myr.