Impact of weak interactions of free nucleons on the r-process in dynamical ejecta from neutron-star mergers

TL;DR

This study examines how weak interactions of free nucleons influence the electron fraction and r-process nucleosynthesis in neutron-star merger ejecta, highlighting the importance of neutrino physics in modeling these astrophysical events.

Contribution

It introduces a parameterized approach to neutrino interactions in BNSM ejecta, assessing their impact on nucleosynthesis and electron fraction without full neutrino transport modeling.

Findings

Neutrino interactions can raise Y_e to 0.25-0.40, enabling successful r-process nucleosynthesis.

Large neutrino luminosities can increase Y_e above 0.40, reducing r-process yields but still matching solar abundance patterns.

Proper neutrino physics is essential for accurate modeling of kilonovae and nucleosynthesis in neutron-star mergers.

Abstract

We investigate beta-interactions of free nucleons and their impact on the electron fraction (Y_e) and r-process nucleosynthesis in ejecta characteristic of binary neutron star mergers (BNSMs). For that we employ trajectories from a relativistic BNSM model to represent the density-temperature evolutions in our parametric study. In the high-density environment, positron captures decrease the neutron richness at the high temperatures predicted by the hydrodynamic simulation. Circumventing the complexities of modelling three-dimensional neutrino transport, (anti)neutrino captures are parameterized in terms of prescribed neutrino luminosities and mean energies, guided by published results and assumed as constant in time. Depending sensitively on the adopted neutrino-antineutrino luminosity ratio, neutrino processes increase Y_e to values between 0.25 and 0.40, still allowing for a successful r-process compatible with the observed solar abundance distribution and a significant fraction of the ejecta consisting of r-process nuclei. If the electron neutrino luminosities and mean energies are relatively large compared to the antineutrino properties, the mean Y_e might reach values >0.40 so that neutrino captures seriously compromise the success of the r-process. In this case, the r-abundances remain compatible with the solar distribution, but the total amount of ejected r-material is reduced to a few percent, because the production of iron-peak elements is favored. Proper neutrino physics, in particular also neutrino absorption, have to be included in BNSM simulations before final conclusions can be drawn concerning r-processing in this environment and concerning observational consequences like kilonovae, whose peak brightness and color temperature are sensitive to the composition-dependent opacity of the ejecta.