Evolution of Single Stars. IV. Helium Stars

Model evolutionary calculations have been made for helium stars (X=0.0, Z=0.03) with masses of 0.5, 0.7, 1.0, 1.5, 2, 4, 8 and 16 M_odot. The initial models were chemically homogeneous. They were evolved through the core helium and core carbon burning phases (if carbon was ignited at all). The carbon ignition was never violent. It took place in the centers of models with M>=1 M_odot when the neutrino emission was ignored. The carbon ignited in the centers of massive models (M>= 4 M_odot) and in the shells in 1.5 M_odot and 2 M_odot models when the neutrino losses were taken into account. The effective temperature of massive models (M>=8 M_odot) was never lower than 10^5 K, no matter whether the neutrino emission was taken into account or not. It was possible to evolve models to the "red" supergiant region only in the case of 1 M_odot and 1.5 M_odot when the neutrino emission was taken into account. Those models spent 10^5 years in the helium shell burning phase at log T_e=3.7+/-0.05, and M_bol=-6+/-0.5. We would like to identify those models with the T CrB stars. In the absence of mixing in our models, we are unable to explain carbon overabundance of the R C rB variables. It is likely that the R C rB variables and the nuclei of planetary nebulae represent the two different evolutionary patterns for stars intermediate population.