EVOLUTIONARY MODELS AND THE P-MODE OSCILLATION SPECTRUM OF ALPHA-CENTAURI-A AND CENTAURI-B

Detailed evolutionary models of the stars alpha-Cen A and B have been constructed. The opacities were calculated specifically with the Los Alamos Opacity Library using recently determined abundances of alpha-Cen A. Consistent with the measured masses and a common age and composition for both alpha-Cen A and B, we have derived the initial helium abundance Y = 0.300 +/- 0.005 (corresponding to Z = 0.026 +/- 0.003) and age = 4.6 +/- 0.4 Gyr, where we have ignored errors intrinsic to the models. The effects of helium diffusion in the interior were also included in the evolutionary sequences. Helium diffusion lowers the inferred age of the system by 2% (from 4.7 to 4.6 Gyr) and increases the mixing length needed to reproduce the radius of alpha-Cen A. Thus the inclusion of helium diffusion effectively decreases the need for a smaller mixing length in alpha-Cen A than in the Sun. The predicted first-order spacing of p-modes averages to [DELTA-v] = 108-mu-Hz for l = 0 for alpha-Cen A and [DELTA-v] = 179-mu-Hz for l = 0 for alpha-Cen B. The second-order splitting averages to [delta-v] = 6.2-mu-Hz for l = 0 for alpha-Cen A and 12.6-mu-Hz at l = 0 for alpha-Cen B. The reduction in Y at the surface of alpha-Cen A due to diffusion has no detectable effect on the frequencies of the low-l p-modes.