Evolution of Stars of Small Masses in the Pre-Main-Sequence Stages

Abstract

The structure of the outer envelope with an H-ionization zone and an H_2-dissociation zone is investigated for Population I stars of small masses (2M_{odot} ge M ge 0.05M_{odot}), which have low luminosities (L_{odot} ge L ge 10^{-3}L_{odot}) and low effective temperatures (6000 ^{circ}K ge T_e ge 2500 ^{circ}K), in order to find the surface conduction for the internal structure of these stars. The effective temperature of a star which is wholly convective and which has an H_2-dissociation zone is found to be nearly constant in the wide range of its luminosity. Using stellar models composed of a radiative core and a convective envelope together with the above surface condition, the evolution of contracting stars is calculated up to the onset of hydrogen burning and the results are compared with the observed red dwarf stars. It is found that the stars on the zero-age main-sequence have radiative cores for M > 0.26M_{odot} but they are wholly convective for 0.26M_{odot} ge M ge 0.08M_{odot}. The stars less massive than 0.08M_{odot} are found to contract toward the configurations of high electron-degeneracy without hydrogen burning.