Charged-particle and neutron-capture processes in the high-entropy wind of core-collapse supernovae

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Abstract

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 Ye , 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 Ye , 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. © 2010. The American Astronomical Society. All rights reserved.

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Farouqi, K., Kratz, K. L., Pfeiffer, B., Rauscher, T., Thielemann, F. K., & Truran, J. W. (2010). Charged-particle and neutron-capture processes in the high-entropy wind of core-collapse supernovae. Astrophysical Journal, 712(2), 1359–1377. https://doi.org/10.1088/0004-637X/712/2/1359

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