Evolution of Population II Stars in the Helium-Burning Phase

Abstract

Evolution of 1.2 M⊙ Population II stars in the helium-burning phase is investigated, taking into account the effect of conversion of helium into carbon and oxygen both on the mean molecular weight and on the nuclear energy generation. The stellar models consist of the following three regions: (i) a hydrogen-rich envelope, (ii) a radiative pure helium zone and (iii) a convective core in which helium burning occurs. The mass fraction of helium region is fixed respectively at 0.45, 0.50 and 0.54 and is treated as a parameter. Series of models are constructed for various helium contents in the convective core. It is found that the stars nearly in the narrow region of the horizontal branch of H-R diagram of the globular cluster M3. But the dependence of stellar radii on the nuclear reaction rates are so strong that a change by one order of magnitude of the rate for CN-cycle shifts the stars through the horizontal branch. The calculated lifetime for this phase is 3 ×107 years, which is a little smaller than that estimated by Sandage.