Structure and Evolution of Model Helium Stars.

Numerical models have been constructed that describe the evolution of stars that are initially pure helium. The iterative technique, programmed for a large computer, permits the inclusion of radiation pressure, electron degeneracy, electron-scattering opacity, gravitational contraction, and detailed composition changing brought about by triple-alpha and (a, T) reactions. The twenty homogeneous (pure helium) models, which range in mass from 0.4 to 60 Mo, form a "main sequence" on the left of the H-R diagram. Evolutionary sequences of models at 0 5,1, and 6 Mo exhaust their central helium supply in times of 80,12, and 0.8 million years. Thereafter the convective core models, which have remained near their main-sequence positions, are replaced by shell-source models of increasing luminosity. At all three masses the final composition of the helium-exhausted core is almost exclusively oxygen, although a reduced rate for C22(a, )Oi6 can lead to moderate fractions of carbon. Early stages of the computed evolutionary tracks agree with other calculated sequences of fitted models. Massive helium-star models may resemble Wolf-Rayet stars in mass, luminosity, and effective temperature.