Population I Helium Burning Red Giants. Theory

In the preceding paper, Cannon has outlined the observational evidence for the existence of a distinct concentration of stars near the base of the red giant branch in intermediate-age galactic clusters, which he tentatively identifies with the core helium burning phase of evolution occurring after the helium flash. This paper reports preliminary results of evolutionary calculations to test this identification. Since the computation of hydrogen shell-burning evolution up the red giant branch is extremely time-consuming, the present calculations have been commenced at the stage immediately following the helium flash. It is assumed that no overall mixing occurs at the flash, so that the composition discontinuity at the hydrogen-burning shell remains sharp. The initial stellar composition was set at (X, Y, Z) = (0.68, 0.30, 0.02), corresponding to Population I material.