Advanced Evolution of Population II Stars. I. Red Giants and the Helium Flash

Models for red-giant stars of Population II have been constructed with emphasis on the determination of the critical mass of the helium core at the onset of the helium flash and on the subsequent development of the flash and core-cooling phase which precedes horixontal-branch evolution. Stars with masses 0.85, 0.60, and 0.502 and compositions (X, Z) = (0.75, 0.001) and (0.75, 0.0001) have been considered. The effects of including relativistic degeneracy have been investigated, together with the effects of the inclusion of neutrino losses due to weakAnteraction processes. No mixing between the helium core and the hydrogen-rich envelope is found in any of the models considered. The dependence of the mass of the helium core on total mass and on Z has been determined and found to be very weak in both cases. Finally, a tentative explanation for Barnard 29 is suggested.