## 1. IntroductionAs the Sun's nearest stellar neighbors, the two members of the
visual binary Centauri A & B (G2V
+ K1V) provide the most accurate potentiality of testing stellar
physics in conditions slightly different from the solar ones and then
deserve undivided attention for internal structure modeling and
oscillation frequency calculations. By coincidence, the masses and the
spectral types of components A/B (HD 128620/1, IDS 14328-6025 A/B,
Hipparcos 71 683/1) bracket those of the Sun. The high apparent
brightness and the large parallax imply that surface abundances and
astrometric parameters are known better than for any star (except the
Sun. The basic intent of this paper is to model
Cen A & B using updated physics.
This allows to predict the Based on the reasonable hypothesis of a common origin for both components, i.e. same initial chemical composition and age, the calibration of a binary system consists in determining a consistent evolutionary history for the double star, given (1) the positions of the two components in a H-R diagram, (2) the stellar masses and, (3) the present day surface chemical composition. The goal is to compute evolutionary models that reproduce the observations. This procedure yields estimates for the age , the initial helium mass fraction and initial metallicity (logarithm of the number abundances of iron to hydrogen relative to the solar value), which are fundamental quantities for our understanding of the galactic chemical evolution. We also derive values of the "mixing-length parameter" or "convection parameter" , ratio of the mixing-length to the pressure scale height. Once the initial masses and the physics are fixed, the modeling of the two components A & B of a binary system requires a set of five so-called modeling parameters: In most cases there are only four reliable observables namely, the effective temperatures and the luminosities or the gravity of each component. Therefore, one of the unknowns has to be fixed. Very often the mixing-length parameters are assumed to be the same for both components, even if the mass ratio differs significantly from unity. Once detailed spectroscopic analyses have been performed on the system, the precise present day surface metallicities of stars come as additional observational constraints. In this paper we attempt to reproduce the observed metallicities
and, if possible, the lithium depletion by means of models including
microscopic diffusion. We calibrate the binary system using both
Böhm-Vitense's (1958, hereafter
MLT The paper is divided as follows: in Sect. 2, we recall the main results obtained in previous theoretical works and, in Sect. 3, we emphasize the difficulties related to the choice of mixing-length parameters. In Sect. 4, we discuss the modeling of the transport processes acting beneath the convection zone. The observational material, relevant to the evolutionary status of Cen and available in the literature, is collected in Sect. 5. The method of calibration is described in Sect. 6. In Sect. 7, we present the stellar modeling procedure. In Sect. 8, we give the results with emphasis on the seismological analysis. We summarize our results and conclude in Sect. 9. © European Southern Observatory (ESO) 2000 Online publication: December 11, 2000 |