Evolution of Low‐Mass Population III Stars

We present the evolutionary models of metal-free stars in the mass range from0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to theonset of hydrogen mixing into a convection, driven by the helium flash at redgiant or asymptotic giant branch phase. The models of mass M >= 0.9 Msun undergo the central hydrogen flash,triggered by the carbon production due to the 3-alpha reactions. We find that the border of the off-center and central ignition of helium coreflash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msunexperience the hydrogen mixing at the tip of red giant branch while the modelsof M = 1.2 Msun during the helium shell flashes on the asymptotic giant branch. The equation of state for the Coulomb liquid region, where electronconduction and radiation compete, is shown to be important since it affects thethermal state in the helium core and influences the red giant branch evolution. It is also found that the non-resonant term of 3-alpha reactios plays animportant role, although it has negligible effect in the evolution of stars ofyounger populations. We compare our models with the computations by several other sets of authors,to confirm the good agreement except for one study which finds the heliumignition much closer to the center with consequences important for subsequentevolution.Comment: 23 pages, 6 figures, 2 tables, accepted for Ap