Helium flashes and hydrogen mixing in low-mass population III stars

A one solar mass, zero-metal star is evolved through the peak of the major core helium flash and compared with models for which the metal abundance is Z = 0.0001 and 0.02. Before central hydrogen is exhausted, and long before the major core helium flash occurs, the Z = 0 model experiences a helium-burning thermal runaway at the center. As this model evolves along the giant branch, recurrent helium-burning thermal runaways are initiated at the base of the hydrogen-burning shell. The thermal structure of the electron-degenerate core of the Z = 0 red giant is completely different from that of the higher Z red giants. As a result of this structure, the major flash begins at a location much farther from the center than in the higher Z cases. Most significantly, during the flash in the Z = 0 model, the outer edge of the convective shell formed in the helium zone extends into layer containing hydrogen, which does not occur in the Z = 0.0001 or Z = 0.02 cases.