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 five Hyades binary systems to predictions of stellar models calculated with various input parameters (helium, metallicity, age) or physics (mixing-length ratio, model atmosphere, equation of state, microscopic diffusion). Models based on a helium content Ysim0.28 inferred from the dydz enrichment law are more than 3sigma beyond the observations, suggesting that the initial helium abundance is lower than expected from its supersolar metallicity. With the photometric metallicity (FeH=0.144pm0.013 dex, Grenon (2000) we derive Y=0.255\pm0.009. Because of the (Y,FeH) degeneracy in the M-L plane, the uncertainty grows to Delta Y=0.013 if the metallicity from spectroscopy is adopted (FeH=0.14pm0.05 dex, Cayrel de Strobel et al 1997). We use these results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the (Mv,B-V) plane, based on the very precise Hipparcos dynamical parallaxes. Present models fit the tight observed sequence very well except at low temperatures. In the low mass region of the HR diagram sensitive to the mixing-length parameter (aMLT), the slope of the main sequence (MS) suggests that aMLT could decrease from a solar (or even supersolar) value at higher mass to subsolar values at low mass, which is also supported by the modeling of the vB22 M-L relation. We find that the discrepancy at low temperatures (B-V\gtrsim 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 sim650 Myr.