The Evolution of Early Main-Sequence Stars.

Abstract

The early phases of evolution for three early main-sequence stars with masses 10, 5, and 2.5 times the solar mass are considered. The range of spectral types covered is from B1 to A2. Radiation pressure is taken into account. Opacity is computed by straight addition of electron scattering and Kramers absorption with modified guillotine factor. The initial homogeneous models for these stars are constructed, and their evolution is followed. It is found that the convective core retreats, creating an inhomogeneous radiative zone between itself and the outer homogeneous radiative envelope. During the early evolutionary phases all the nuclear energy is generated inside the core. These early evolutionary phases are represented here by a series of three or four inhomogeneous models for each of the three stars. For the heaviest mass two later inhomogeneous models are also computed, in which the energy generation in the intermediate zone is accounted for. It is seen that the HR diagram constructed from these models agrees well with the observed one galactic clusters. The theoretical models are also checked by computing the apsidal-motion contants for the three initial models, as well as for some of the later models. Quite reasonable agreement is found with the observations.