Proto-Neutron Star Convection and the Neutrino-Driven Wind: Implications for the r-Process

TL;DR

This study investigates how convection-generated gravito-acoustic waves influence proto-neutron star winds, potentially enabling conditions suitable for the r-process nucleosynthesis even with minimal wave energy flux.

Contribution

It provides a systematic analysis of wave effects on wind conditions, showing they can promote the r-process under specific conditions, a novel insight into nucleosynthesis mechanisms.

Findings

Waves can induce favorable r-process conditions with very low energy flux.

Wave effects depend critically on the radius where shocks form.

Entropy production and wind acceleration influence nucleosynthesis outcomes.

Abstract

The neutrino-driven wind from proto-neutron stars is a proposed site for r-process nucleosynthesis, although most previous work has found that a wind heated only by neutrinos cannot produce the third r-process peak. However, several groups have noted that introducing a secondary heating source within the wind can change the hydrodynamic conditions sufficiently for a strong r-process to proceed. One possible secondary heating source is gravito-acoustic waves, generated by convection inside the proto-neutron star. As these waves propagate into the wind, they can both accelerate the wind and shock and deposit energy into the wind. Additionally, the acceleration of the wind by these waves can reduce the total number of neutrino captures and thereby reduce the final electron fraction of the wind. In neutron rich conditions, all of these effects can make conditions more favorable for r-process nucleosynthesis. Here, we present a systematic investigation of the impact of these convection-generated gravito-acoustic waves within the wind on potential nucleosynthesis. We find that wave effects in the wind can generate conditions favorable for a strong r-process, even when the energy flux in the waves is a factor of $10^{-4}$ smaller than the total neutrino energy flux and the wind is marginally neutron-rich. Nevertheless, this depends strongly on the radius at which the wave become non-linear and form shocks. We also find that both entropy production after shock formation and the acceleration of the wind due to stresses produced by the waves prior to shock formation impact the structure and nucleosynthesis of these waves.