The Evolution of Population II Stars

On comparing the evolutionary paths of several double energy-source models with the horizontal branch in metal-weak Population II clusters such as M92, it is found that best fits are achieved with stellar masses Me 0.70 M0, CNO content 10- , and with two choices of envelope hydrogen content, Xe 0.65 or Xe 0.90. Models with Xe 0.65 evolve along the horizontal branch for 1.3 X 108 years compared with 4 X 10 years for models with Xe __ 0.90, a result which favors the choice Xe = 0.65. Evolutionary tracks corresponding to hydrogen-burning stages off the main sequence show that it is possible to achieve good fits with the subgiant and giant branches of M92 for Xe = 0.65, = 10- . With this composition, masses of stars along the giant branch are found to be 0.70 M0. Thus, by choosing for Population II stars an initial helium abundance usually attributed only to Population I stars, it is possible to account for several features of the horizontal branch without assuming significant mass loss between the red-giant and horizontal-branch stages. Further, satisfactory fits with the characteristic shape of the subgiant and giant portions of globular clusters are readily achieved, and cluster ages are more consistent with the Hubble time than is the case with the higher choice of Xe.