The Astrophysicalr‐Process: A Comparison of Calculations following Adiabatic Expansion with Classical Calculations Based on Neutron Densities and Temperatures

Abstract

The rapid neutron-capture process (r-process) encounters unstable nuclei far from β-stability. Therefore its observable features, like the abundances, witness (still uncertain) nuclear structure as well as the conditions in the appropriate astrophysical environment. With the remaining lack of a full understanding of its astrophysical origin, parameterized calculations are still needed. We consider two approaches: (1) the classical approach is based on (constant) neutron number densities n_n and temperatures T over duration timescales τ; (2) recent investigations, motivated by the neutrino wind scenario from hot neutron stars after a supernova explosion, followed the expansion of matter with initial entropies S and electron fractions Y_e over expansion timescales τ. In the latter case the freezeout of reactions with declining temperatures and densities can be taken into account explicitly. We compare the similarities and differences between the two approaches with respect to resulting abundance features and their relation to solar r-process abundances, applying for the first time different nuclear mass models in entropy-based calculations. Special emphasis is given to the questions of (a) whether the same nuclear properties far from stability lead to similar abundance patterns and possible deficiencies in (1) and (2), and (b) whether some features can also provide clear constraints on the astrophysical conditions in terms of permitted entropies, Y_e values, and expansion timescales in (2). This relates mostly to the A < 110 mass range, where a fit to solar r-abundances in high-entropy supernova scenarios seems to be hard to attain. Possible low-entropy alternatives are presented.