Weak charge-changing flow in expanding r-process environments

We assess the prospects for attaining steady nuclear-flow equilibrium in expanding r-process environments where beta decay and/or neutrino capture determine the nuclear charge-changing rates. For very rapid expansions, we find that weak steady flow equilibrium normally cannot be attained. However, even when neutron-capture processes freeze out in such nonequilibrium conditions, abundance ratios of nuclear species in the r-process peaks might still mimic those attained in weak steady flow. This result suggests that the r-process yield in a regime of rapid expansion can be calculated reliably only when all neutron-capture, photodisintegration, and weak interaction processes are fully coupled in a dynamical calculation. We discuss the implications of these results for models of the r-process sited in rapidly expanding neutrino-heated ejecta. copyright 1997 The American Astronomical Society