The -process nucleosynthesis in core-collapse supernovae I. A novel analysis of -process yields in massive stars

Context. The gamma-process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average yields of gamma-process nucleosynthesis from massive stars are still insufficient to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are a factor of ten lower than the yields of the other gamma-process nuclei. Aims. We investigate the gamma-process in five sets of core-collapse supernova models published in the literature with initial masses of 15, 20, and 25 M-circle dot at solar metallicity. Methods. We compared the gamma-process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass relative to their solar values. Further, we investigated the contribution of C-O shell mergers in the supernova progenitors as an additional site of the gamma-process. Results. Our analysis shows that a large scatter among the different models exists for both the gamma-process integrated yields and the isotopic ratios. We find only ten ratios that agree with their solar values, all the others differ by at least a factor of three from the solar values in all the considered sets of models. The gamma-process within C-O shell mergers mostly influences the isotopic ratios that involve intermediate and heavy proton-rich isotopes with A > 100. Conclusions. We conclude that there are large discrepancies both among the different data sets and between the model predictions and the solar abundance distribution. More calculations are needed; particularly updating the nuclear network, because the majority of the models considered in this work do not use the latest reaction rates for the gamma-process nucleosynthesis. Moreover, the role of C-O shell mergers requires further investigation.