A Method to Derive the Absolute Composition of the Sun, the Solar System and the Stars

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 occurred in the solar photosphere. We provide a method to derive the mass fractions of all the 83 elements (and their most abundant isotopes) in the early solar system material and in the present-day solar surface. Calculations are repeated by adopting the most widely adopted compilations of solar abundances. Since for a given [Fe/H], the total metallicity depends on solar (Z/X), a 30% reduction of Z is found when passing from the classical Anders&Grevesse to the most recent Lodders compilation. Some implications are discussed, as, in particular, an increase of about 700 Myr of the estimated age of Globular Clusters. Within the experimental errors, the complete set of relative solar abundances, as obtained by combining meteoritic and photospheric measurements, are consistent with the variations implied by the quoted physical processes. Few deviations can be easily attributed to the decay of long-lived radioactive isotopes. The huge lithium depletion is only partially explained by introducing a rotational-induced mixing in the tachocline.