[alpha] Centauri AB

Abstract

Detailed models of [alpha] Centauri A and B based on the Hipparcos, Yale, and Soderhjelm parallaxes are compared. The consequences of the uncertainty in mass, luminosity, surface temperature, and composition on the structure and the p-mode pulsation spectrum of the models are presented. All of the models were constructed using the most current stellar structure physics available to us, including helium and heavy-element diffusion, OPAL (Lawrence Livermore Opacity Library) equation of state, and OPAL and Alexander opacities. Self-consistent models of [alpha] Cen A and B that satisfy the observational constraints have an initial helium mass fraction Y[subscript ZAMS] = ~0.28. The age of the system depends critically on whether or not [alpha] Cen A has a convective core. If it does (our best model), then [alpha] Cen AB is ~7.6 Gyr old, and if it does not, then the binary system is ~6.8 Gyr old. Both ages and Y[subscript ZAMS] are accurate to [plus or minus] ~10% owing to observational uncertainties. The Galactic enrichment parameter ([delta]Y /[delta]Z) for our best model pair is less than 1. Pulsation analyses of our best models yield an average large and small spacing of 101 [plus or minus] 3 [microhertz] and 4.6 [plus or minus] 0.4 [microhertz], respectively, for [alpha] Cen A, and 173 [plus or minus] 6 [microhertz] and 15 [plus or minus] 1 [microhertz] for [alpha] Cen B. Some methodologies that use p-mode frequency observations to constrain the system further are outlined. We include a simple test to determine whether or not [alpha] Cen A has a convective core and introduce a method to use the small frequency spacing to determine the age of system, overcoming the limitation that it is also sensitive to composition.