α Centauri AB

Abstract

Detailed models of α Centauri A and B based on the Hipparcos, Yale, and Söderhjelm 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 α Cen A and B that satisfy the observational constraints have an initial helium mass fraction Y_ZAMS=~0.28. The age of the system depends critically on whether or not α Cen A has a convective core. If it does (our best model), then α 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_ZAMS are accurate to +/-~10% owing to observational uncertainties. The Galactic enrichment parameter (ΔY/ΔZ) for our best model pair is less than 1. Pulsation analyses of our best models yield an average large and small spacing of 101+/-3 μHz and 4.6+/-0.4 μHz, respectively, for α Cen A, and 173+/-6 μHz and 15+/-1 μHz for α 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 α 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.