On the dynamics of proto-neutron star winds and r-process nucleosynthesis

We study here the formation of heavy r-process nuclei in the high-entropy environment of rapidly expanding neutrino-driven winds from compact objects. In particular, we explore the sensitivity of the element creation in the A greater than or similar to 130 region to the low-temperature behavior of the outflows. For this purpose we employ a simplified model of the dynamics and of the thermodynamical evolution for radiation dominated, adiabatic outflows. It consists of a first stage of fast, exponential cooling with timescale tau(dyn), followed by a second phase of slower evolution, assuming either constant density and temperature or a power-law decay of these quantities. These cases describe a strong deceleration or decreasing acceleration of the transsonic outflows, respectively, and thus are supposed to capture the most relevant effects associated with a change in the wind expansion behavior at large radii, for example because of the collision with the slower, preceding supernova ejecta and the possible presence of a wind termination shock. We find that for given entropy, expansion timescale, and proton-to-baryon ratio not only the transition temperature between the two expansion phases can make a big difference in the formation of the platinum peak, but also the detailed cooling law during the later phase. Because the nuclear photodisintegration rates between about 2 x 10(8) K and roughly 10(9) K are more sensitive to the temperature than the neutron-capture rates are to the free neutron density, a faster cooling but continuing high neutron density in this temperature regime allow the r-process path to move closer to the neutron-drip line. With low (gamma, n)- but high beta-decay rates, the r-processing does then not proceed through a (gamma, n)-(n, gamma) equilibrium but through a quasi-equilibrium of (n, gamma)- reactions and beta-decays, as recently also pointed out by Wanajo. Unless the transition temperature and corresponding ( free neutron) density become too low ( T less than or similar to 2 x 10(8) K), a lower temperature or faster temperature decline during the slow, late evolution phase therefore allow for a stronger appearance of the third abundance peak.