Fusion Hindrance for a Positive- Q -Value System Mg 24 + Si 30

Measurements of the excitation function for the fusion of Mg-24 + Si-30 (Q = 17.89 MeV)have been extended toward lower energies with respect to previous experimental data. The S-factor maximum observed in this large, positive-Q-value system is the most pronounced among such systems studied thus far. The significance and the systematics of an S-factor maximum in systems with positive fusion Q values are discussed. This result would strongly impact the extrapolated cross sections and reaction rates in the carbon and oxygen burnings and, thus, the study of the history of stellar evolution.