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 c apture determine the nuclear charge-changing rates. For very rapid exp ansions, we find that weak steady flow equilibrium normally cannot be attained. However, even when neutron-capture processes freeze out in s uch nonequilibrium conditions, abundance ratios of nuclear species in the r-process peaks might still mimic those attained in weak steady fl ow. This result suggests that the r-process yield in a regime of rapid expansion can be calculated reliably only when all neutron-capture, p hotodisintegration, and weak interaction processes are fully coupled i n a dynamical calculation. We discuss the implications of these result s for models of the r-process sited in rapidly expanding neutrino-heat ed ejecta.