Advanced Phases of Evolution of Population II Stars

Abstract

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.