NEW PRE-MAIN-SEQUENCE TRACKS FOR M-LESS-THAN-OR-EQUAL-TO-2.5M-CIRCLE-DOT AS TESTS OF OPACITIES AND CONVECTION MODEL

We present tabular and graphic results on the computation of pre-main-sequence evolutionary tracks of Population I stellar structures from 2.5 to approximately 0.0 1 5 M.. Deuterium and lithium burning are followed in detail. The chosen input physics gives M approximately 0.018 M. as minimum mass for deuterium burning and M approximately 0.065 M. as minimum mass for lithium burning. While we adopt the approximations of hydrostatic equilibrium, no mass accretion and no mass loss, we have taken care to include several updates in the input physics, among them two different sets of the more recent available low-temperature opacities, and we test two different models of overadiabatic convection (the mixing-length theory [MLT] with the mixing-length scale calibrated on the solar model, and the recent Canuto & Mazzitelli [CM] model). The H-R diagram location of tracks turns out to be largely model-dependent, especially for M less-than-or-equal-to 0.6 M., and we are able to relate the cause of the large differences (up to 0.04 dex in T(eff) at 0.3 M.) with opacity and with the details of the convection model adopted. Since we are not able to provide ''first-principle'' physical reasons to choose among models, we consider these tracks as ''tests,'' in the hope that significant comparisons with observations can exclude some models or provide hints toward a better understanding of convection. Nevertheless, we feel obliged to call the reader's attention to the fact that theoretical T(eff)'s, especially in the red, are intrinsically ill-determined, and no sound observational interpretation critically depending on the T(eff)'s can be presently performed, contrary to the current habit due to an exceedingly ''faithful'' use of the MLT.