The chemical abundances are often represented by X, Y, Z, which are the mass fractions of H, He and heavy elements (all except H and He). Many models and data concern a so-called standard composition, typically X = o.7o5, Y = o.275, Z = o.o2. Recent determinations of solar abundances yield X = o.7393, Y = o.2485, Z = o.o122 according to Asplund, Grevesse and Sauval [2o]. This has to be distinguished from the initial solar abundances because of microscopic diffusion of the elements during the evolution of the Sun. The initial solar abundances giving the best fit of helioseismic data, also accounting for the neon value from Cunha et al. [151], are X = o.72o, Y = o.266, Z = o.o14 [169], see also Appendix A.3. As a function of X, Y, Z, the mean molecular weight of the fully ionized medium is

With account of the fact that X + Y + Z = 1, one has

0 0

Post a comment