Advanced Phases of Evolution of Population II Stars

As the extension of our previous work, calculations have been made of the evolution of a population II star after the formation of a carbon core. We start from a stage when the star has an inactive hydrogen shell-source, a helium-burning shell-source and a partially degenerate isothermal carbon core. The luminosity due to shell helium-burning attains a maximum value, 1.6 X 103Le, when the core mass grows to 0.47 !vf0. After this stage, helium-burning decays rapidly while the hydrogen shell-source becomes active. In the next phase the mass of the helium zone grows through shell hydrogen-burning and the inner layers of the helium zone begin to be degenerate. The gravitational contrac tio~ of the helium zone gives rise to the heating of the helium shell-source, until the second helium flash sets in at the degenerate shell when the mass interior to the hydrogen shell source becomes 0.61 Me. The subsequent evolution is, in general, the repetition of the preceding phases. The third helium flash sets in when the mass interior to the hydrogen shell-source becomes about 0.7 Me and the luminosity is about 8X10 3 LCOJ.