On the Dynamics of Proto-Neutron Star Winds and r-Process Nucleosynthesis

TL;DR

This paper investigates how the temperature evolution of neutrino-driven winds from proto-neutron stars affects the formation of heavy r-process nuclei, highlighting the importance of cooling laws and transition temperatures in nucleosynthesis outcomes.

Contribution

It introduces a simplified dynamical model to study the impact of outflow cooling behavior on r-process element synthesis, emphasizing the role of temperature transitions and decay rates.

Findings

Lower transition temperatures enhance third peak formation.

Faster cooling shifts r-process path closer to neutron drip line.

Temperature sensitivity of photodisintegration rates influences nucleosynthesis pathways.

Abstract

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>130 region to the low-temperature behavior of the outflows. For this purpose we employ a simplified model of the dynamics and thermodynamical evolution for radiation dominated, adiabatic outflows. It consists of a first stage of fast, exponential cooling, followed by a second phase of slower evolution, either assuming constant density and temperature or a power-law decay of these quantities. These cases are supposed to capture the most relevant effects of a strong deceleration or decreasing acceleration of the transsonic outflows, respectively, e.g. in a wind termination shock caused by the collision with the slower, preceding supernova ejecta. We find that 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. Unless the transition temperature and corresponding (free neutron) density become too small (T < 2*10^8 K), a lower temperature or faster temperature decline during this phase allow for a stronger appearance of the third abundance peak. Since the nuclear photodisintegration rates between ~2*10^8 K and ~10^9 K are more sensitive to the temperature than the n-capture rates are to the free neutron density, a faster cooling in this temperature regime shifts the r-process path closer to the n-drip line. With low (gamma,n)- but high beta-decay rates, the r-processing then does 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.