NUCLEOSYNTHESIS IN MASSIVE STARS AND THE C-12(ALPHA,GAMMA)O-16 REACTION-RATE

The evolution of a grid of massive stars ranging from 12 to 40 M. has been followed through all stages of nuclear burning up to the point of iron core collapse. The critical and highly uncertain rate for the reaction C-12(alpha, gamma) O-16 has been varied over a range from 0.5 to 3.0 times that given by Caughlan and Fowler and two different prescriptions for semiconvection have been explored. The nucleosynthesis resulting from integrating the yields of these models over plausible initial stellar mass distributions is found to be in excellent agreement with the observed solar abundances of virtually all the intermediate mass isotopes (16 less-than-or-equal-to A less-than-or-equal-to 32) if, and only if, the rate of the C-12(alpha, gamma)O-16 reaction is taken to be 1.7+/-0.5 times that given by Caughlan and Fowler. This range is a small subset of what is allowed by current experimental measurements and can be taken as a nucleosynthetic ''prediction'' of the value that this rate needs to have in order to prevent 5- to 100-fold deviations from the observed relative abundances of key isotopes. These results are insensitive to the assumed slope of the initial stellar mass distribution within observational limits, and relatively insensitive to the theory of semiconvection (except for the apparent excessive production of O-18 when semiconvective mixing is suppressed). Three of the stars have been followed through simulated explosions to obtain the explosive modifications to their nucleosynthesis (including the ''neutrino process''), which for most isotopes is relatively small. Isotopic yields of both stable and radioactive products are tabulated as are the calculated iron core masses of the presupernova stars.