Charged-Particle and Neutron-Capture Processes in the High-Entropy Wind of Core-Collapse Supernovae

The astrophysical site of the r -process is still uncertain, and a full exploration of the systematics of this process in terms of its dependence on nuclear properties from stability to the neutron drip-line within realistic stellar environments has still to be undertaken. Sufficiently high neutron-to-seed ratios can only be obtained either in very neutron-rich low-entropy environments or moderately neutron-rich high-entropy environments, related to neutron star mergers (or jets of neutron star matter) and the high-entropy wind of core-collapse supernova explosions. As chemical evolution models seem to disfavor neutron star mergers, we focus here on high-entropy environments characterized by entropy S , electron abundance Y e , and expansion velocity V exp . We investigate the termination point of charged-particle reactions, and we define a maximum entropy S final for a given V exp and Y e , beyond which the seed production of heavy elements fails due to the very small matter density. We then investigate whether an r -process subsequent to the charged-particle freeze-out can in principle be understood on the basis of the classical approach, which assumes a chemical equilibrium between neutron captures and photodisintegrations, possibly followed by a β-flow equilibrium. In particular, we illustrate how long such a chemical equilibrium approximation holds, how the freeze-out from such conditions affects the abundance pattern, and which role the late capture of neutrons originating from β-delayed neutron emission can play. Furthermore, we analyze the impact of nuclear properties from different theoretical mass models on the final abundances after these late freeze-out phases and β-decays back to stability. As only a superposition of astrophysical conditions can provide a good fit to the solar r -abundances, the question remains how such superpositions are attained, resulting in the apparently robust r -process pattern observed in low metallicity stars.