The Pre - Main Sequence

Abstract

AbstractThe Pre-Main-Sequence (pre-MS) is defined as the phase following the protostellar stage and ending with the ignition of hydrogen in the stellar core. Models in hydrostatic and thermal equilibrium such as typical “Hayashi track” models are basically adequate to describe the internal structure of pre-MS stars, although for a fraction of pre-MS objects there can be phases of accretion also at late stages, which may influence the surface abundances of light elements.The interior evolution of pre-MS structures is focused in the two main stages’ of Deuterium and Lithium burning for the low mass (M ≤ 1.5M⊙) stars. The location in the HR diagram of theoretical tracks is still subject to large uncertainty, even more apparent today that new opacities and treatment of turbulent convection are available, and the results can be internally compared. Uncertainties amount to a factor ~ two for mass and age determination of individual objects, and affect the absolute location for both the D-burning and the Li-burning regions. Qualitative constraints, such as that there can not be Li-depletion at L ≥ L⊙, remain valid. Abundances of7Li at the surface of pre-MS stars, if measured at a stage which can definitely be considered previous to the possible occurrence of nuclear burning, and, consequently, previous to other depletion mechanism which may occur later on, provide constraints on galactic evolution of this element of cosmological interest.