A method to derive the absolute composition of the Sun, the solar system, and the stars

Context. The knowledge of isotopic and elemental abundances of the pristine solar system material provides a fundamental test of galactic chemical evolution models, while the composition of the solar photosphere is a reference pattern to understand stellar abundances. However, spectroscopic or meteoritic abundance determinations are only possible for an incomplete sample of the 83 elements detected in the solar system. Therefore, only relative abundances are experimentally determined, with respect to H or to Si for spectroscopic or meteoritic measurements, respectively. For this reason, the available compilations of solar abundances are obtained by combining spectroscopic and meteoritic determinations, a procedure requiring the knowledge of the chemical modification occurring in the solar photosphere. Aims. We provide a method to derive the mass fractions of the 83 elements (and their most abundant isotopes) in the early solar system material and in the present-day solar surface. Methods. By computing a solar model, we investigate physical processes responsible for the variation of the solar surface composition in the last 4.57 Gyr. An extended network, from H to U, is coupled to our stellar evolutionary code. The effects of microscopic diffusion, rotational-induced mixing in the tachocline and radiative acceleration are discussed. Results. The abundances of the 83 elements are given for both the pristine solar system and the solar photosphere. Calculations are repeated by adopting the most widely adopted compilations of solar abundances. Since for a given [Fe/H], the total metallicity depends on (Z/X)(circle dot), a 30% reduction of Z is found when passing from the classical Anders & Grevesse to the most recent Lodders compilation. Some implications are discussed, such as the increase of about 700 Myr in the estimated age of Globular Clusters. Conclusions. Within the experimental errors, the complete set of relative solar abundances, as obtained by combining meteorite and photosphere measurements, are consistent with the variations implied by the quoted physical processes. The few deviations can be easily attributed to the decay of long- lived radioactive isotopes. The large lithium depletion is only partially explained by introducing a rotation- induced mixing in the tachocline.