Evolution of the Alpha Centauri system

Available astrometric and photometric data are examined and in some cases reanalyzed in order to derive accurate estimates for the stellar masses, luminosities, temperatures, and chemical compositions of the A and B components of Alpha Centauri. Theoretical evolutionary sequences for the A and B components are computed for two values of metallicity, Z = 0.02 and 0.04; a reference solar model with Z = 0.02 is also calculated. The observed abundances are partially reanalyzed on the basis of the equivalent measurements of French and Powell (1971), which provide a consistency check on the evolutionary models. The results indicate that: (1) the system is partially evolved since component A is 30% brighter with respect to B than would be expected for an unevolved system; (2) the system is somewhat metal-rich with respect to the sun; (3) the age of Alpha Cen is 6 billion years and the helium abundance is essentially solar for an evolutionary sequence with twice the solar metals; and (4) the components of Alpha Cen are also similar to the sun in terms of galactic orbits. The difference in Z between the sun and Alpha Cen is shown to be consistent with the idea that there is a substantial dispersion in the metallicity of the interstellar medium at any one time.