NEW EVOLUTIONARY TRACKS FOR VERY-LOW MASS STARS

We present new evolutionary calculations for low-mass and very low mass M dwarfs, for a metallicity range -2 less than or equal to [M/H] less than or equal to 0 down to the hydrogen-burning minimum mass (0.07 < M/M. < 0.6). We use the most recent atmosphere models calculated by Allard and Hauschildt (1995), based on synthetic spectra at finite metallicity, and gray atmosphere models based on Alexander and Ferguson (1994) Rosseland opacities. Comparisons are made with observational results down to the bottom of the main sequence, for different metallicities, in magnitude-color and color-color diagrams. We find excellent agreement between theory and observations over the whole characteristic temperature/luminosity range. This enables us to determine the mass of the faintest objects observed, which is found to be m(lim) approximate to 0.085 M. for [M/H] = 0 and -0.5, and m(lim) = 0.09 M. for [M/H] = -1.5, for an age of 10 Gyr. We also examine the effect of the age, the metallicity, and the outer boundary conditions on the evolution.