The Evolution of Low-Mass Stars

Abstract

We present evolutionary models for low-mass stars using the best currently available physics for the equation of state, the Rosseland mean opacities and the screening factors of thermonuclear reaction rates. We follow the evolution of stars of mass M ≤ 0. 6 M_⊙ down to the hydrogen burning minimum mass, with initial metallicity Z=0, 10^-3 and 0. 02. We analyze the validity of the instantaneous mixing approximation for deuterium in central convective zones of low-mass stars evolving toward the initial main-sequence. Preliminary results are given based on an approximative treatment of the atmosphere. We show in this contribution the importance of the outer boundary conditions for such stars. We also point out the difficulty of comparing models with observations. The transformation of theoretical predictions (luminosity and effective temperature) in observable quantities (magnitudes and colours) should indeed be taken with caution.