ALPHA-CENTAURI AND CONVECTION THEORIES

The metallicity of the alpha Centauri system, Z, suffers from uncertai nities. For this reason, different methods are used to calibrate the s ystem: calibrations performed in YALE (Edmonds et al. 1992) use a fixe d value for Z: Z = 0.026 and a convection parameter for each star, whi le those made in Meudon and Liege (Noels et al. 1991; Neuforge 1993a) make the hypothesis of a unique convection parameter for the two compo nents of the system and consider Z as a free parameter. We discuss the se two techniques, both using models calculated with mixing length con vection theory, (MLT), and we explain our solution through the behavio ur of the convection parameter with chemical composition. We also comp are our results with those of Lydon (1993) and find consistency. With a precise observational value of Z, of the effective temperature and o f the luminosities, our results provide a test for the unicity of alph a, if, in the frame of the same physics, a precise atmosphere treatmen t 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 w e use LAMBDA = z, z being the distance to the top of the convective en velope. We avoid thus problems raised by the MLT convection parameter. In this frame, satisfactory solutions can be found for 0.024 less-tha n-or-equal-to Z less-than-or-equal-to 0.040.