HELIUM FLASHES AND HYDROGEN MIXING IN LOW-MASS POPULATION-III STARS

We evolve a 1 M⊙, zero-metal star through the peak of the major core helium flash and compare it with models for which the metal abundance is Z = 10-4 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. These runaways are assisted by CN cycle reactions which occur because of the local production of 12C by triple-α reactions, and they are quenched by the exhaustion of hydrogen in the region of instability. 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 layers containing hydrogen, which does not occur in the Z = 10-4 or Z = 0.02 cases. Hydrogen mixing occurs in the Z = 0 model due to the low entropy in the hydrogen-burning shell and in the envelope of this low-luminosity model. We speculate about the consequences of hydrogen mixing and discuss the possiblity that the ultrametal-deficient star CD-38°245 may, after all, be a Population III object (formed out of material with Z < 10-10). We analyze the physics of the major core flash and conclude that those hydrodynamical studies which (contrary to the results of quasistatic studies) suggest that the core flash evolves into a hydrodynamical event and that (even for high-Z models) nucleosynthetic products of the flash are mixed into hydrogen-rich material do so due to a neglect and/or improper treatment of an all important gravothermal mechanism: the reduction of pressure and gravity due to the expansion of the burning zone and of the interior.