ALPHA-CENTAURI AND CONVECTION THEORIES

The metallicity of the alpha Centauri system, Z, suffers from uncertainities. For this reason, different methods are used to calibrate the system: calibrations performed in YALE (Edmonds et al. 1992) use a fixed value for Z: Z = 0.026 and a convection parameter for each star, while those made in Meudon and Liege (Noels et al. 1991; Neuforge 1993a) make the hypothesis of a unique convection parameter for the two components of the system and consider Z as a free parameter. We discuss these two techniques, both using models calculated with mixing length convection theory, (MLT), and we explain our solution through the behaviour of the convection parameter with chemical composition. We also compare our results with those of Lydon (1993) and find consistency. With a precise observational value of Z, of the effective temperature and of the luminosities, our results provide a test for the unicity of alpha, if, in the frame of the same physics, a precise atmosphere treatment can be used and low-temperature opacities are known with sufficient accuracy. Finally, we perform calibrations with models calculated with the convection treatment of Canuto & Mazzitelli (1991, 1992), where we use LAMBDA = z, z being the distance to the top of the convective envelope. We avoid thus problems raised by the MLT convection parameter. In this frame, satisfactory solutions can be found for 0.024 less-than-or-equal-to Z less-than-or-equal-to 0.040.