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 qual- ity parallaxes of a sample of a hundred nearby disk stars, of spectral types F to K. In parallel, bolometric fluxes, effective temperatures, and accurate Fe/H ratios of many of these stars be- came 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 neighbour- hood with the smallest error bars ever obtained. We analyse these observations by means of theoretical stel- lar models, computed with the most recent input physics. We first examine the positions of the objects versus standard theoretical isochrones, corresponding to their chemical compo- sition and age. For these isochrones we have first assumed that the helium content was varying in locksteps 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 less than (Fe/H) = 0:5. This conflict cannot be resolved by decreasing the helium abundance: values of this abundance below the primordial abundance would be required. On the basis of recent works, we show that the addition of two processes not included in standard models can help solving the above discrepancy. These are (i) correcting the LTE iron abundances using a non-LTE approach and (ii) including mi- croscopic diffusion of He and heavier elements in the stellar interior. The case of the binary star Cas is particularly useful to support this conclusion as its mass is also known from its orbit. After inclusion of the two effects, Cas A falls on its expected isochrone, within the error bars corresponding to its mass. All stars with -0.3 < (Fe/H) < 0.3 are located between the helium-scaled isochrones 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 Send offprint requests to: Y. Lebreton content lower than the metallicity-scaled value. But not neces- sarily, as a possible sedimentation of the elements might com- plicate the determination of the helium content. The age of main sequence solar composition stars covers a large range, and the effects of sedimentation are time dependent.