Evolution of low-mass metal-free stars including effects of diffusion and external pollution

We investigate the evolution of low-mass metal-free Population III stars. Emphasis is laid upon the question of internal and external sources for CNO elements, which-if present in sufficient amounts in the hydrogen-burning regions-lead to a strong modification of the stars' evolutionary behavior. For the production of carbon due to nuclear processes inside the stars, we use an extended nuclear network, demonstrating that hot p-p chains do not suffice to produce enough carbon or are less effective than the 3 alpha process. As an external source of CNO elements we test the efficiency of pollution by a nearby massive star combined with particle diffusion. For all cases investigated, the additional metals fail to reach nuclear-burning regions before deep convection on the red giant branch obliterates the previous evolution. The surface abundance history of the polluted Population III stars is presented. The possibilities to discriminate between a Population II and a polluted Population III field star are also discussed.