Massive star evolution : nucleosynthesis and nuclear reaction rate uncertainties

We present a nucleosynthesis calculation of a 25 M-circle dot star of solar composition that includes all relevant isotopes up to polonium. We follow the stellar evolution from hydrogen burning till iron core collapse and simulate the explosion using a 'piston' approach. We discuss the influence of two key nuclear reaction rates, C-12(alpha, gamma)O-16 and Ne-22(alpha, n)Mg-25, on stellar evolution and nucleosynthesis. The former significantly influences the resulting core sizes (iron, silicon, oxygen) and the overall presupernova structure of the star. It thus has significant consequences for the supernova explosion itself and the compact remnant formed. The later rate considerably affects the s-process in massive stars and we demonstrate the changes that different currently suggested values for this rate cause. (C) 2002 Elsevier Science B.V. All rights reserved.