The Hipparcos HR diagram of nearby stars in the metallicity range: -1.0 <[Fe/H]< 0.3 - A new constraint on the theory of stellar interiors and model atmospheres

The Hipparcos mission has provided very high quality parallaxes of a sample of a hundred nearby disk stars, of spectral types F to K. In parallel, bol ometric fluxes, effective temperatures, and accurate Fe/H ratios of many of these stars became available through infrared photometry and detailed spec troscopic analyses. These new accurate data allow to build the Hertzsprung- Russell diagram of stars of the solar neighbourhood with the smallest error bars ever obtained. We analyse these observations by means of theoretical stellar models, compu ted with the most recent input physics. We first examine the positions of the objects versus standard theoretical i sochrones, corresponding to their chemical composition and age. For these i sochrones we have first assumed that the helium content was varying in lock steps with metallicity. The comparison becomes age-independent in the lower part of the HR diagram, where evolutionary effects are negligible. We show that for the unevolved stars, the agreement between real stars and models is fairly satisfactory for stars with metallicity within +/- 0.3 dex of the solar metallicity, but that a conflict exists for stars with metallicity l ess than [Fe/H] = -0.5. This conflict cannot be resolved by decreasing the helium abundance: values of this abundance below the primordial abundance w ould be required. On the basis of recent works, we show that the addition of two processes no t included in standard models can help solving the above discrepancy. These are (i) correcting the LTE iron abundances using a non-LTE approach and (i i) including microscopic diffusion of He and heavier elements in the stella r interior. The case of the binary star mu Gas is particularly useful to su pport this conclusion as its mass is also known from its orbit. After inclu sion of the two effects, mu Gas A falls on its expected isochrone, within t he error bars corresponding to its mass. All stars with -0.3 < [Fe/H] < 0.3 are located between the helium-scaled is ochrones corresponding to these metallicities. However five of them are not located exactly where they are expected to be for their metallicity. This may reflect a helium content lower than the metallicity-scaled value. But n ot necessarily, as a possible sedimentation of the elements might complicat e the determination of the helium content. The age of main sequence solar c omposition stars covers a large range, and the effects of sedimentation are time dependent.