2007ApJ...667.1206S

We present the evolutionary models of metal-free stars in the mass range from 0.8 to 1.2 M_☉ with up-to-date input physics. The evolution is followed to the onset of hydrogen mixing into a convection, driven by the helium flash at the red giant or asymptotic giant branch phase. The models of mass M≥0.9 M_☉ undergo a central hydrogen flash, triggered by carbon production due to the triple-α reaction. We find that the border of the off-center and central ignition of the helium core flash falls between 1.1 and 1.2 M_☉; the models of mass M≤1.1 M_☉ experience hydrogen mixing at the tip of the red giant branch, while the models of M=1.2 M_☉during the helium shell flashes on the asymptotic giant branch. The equation of state for the Coulomb liquid region, where electron conduction and radiation compete, is shown to be important since it affects the thermal state in the helium core and influences the red giant branch evolution. It is also found that the nonresonant term of the triple-α reaction plays an important role, although it has negligible effect on the evolution of stars of younger populations. We compare our models with the computations by several other sets of authors and confirm good agreement, except for one study that finds helium ignition much closer to the center, with important consequences for subsequent evolution.