Dynamical ejecta of neutron star mergers with nucleonic weak processes I: nucleosynthesis

We present a coherent study of the impact of neutrino interactions on the r-process element nucleosynthesis and the heating rate produced by the radioactive elements synthesized in the dynamical ejecta of neutron star-neutron star (NS-NS) mergers. We have studied the material ejected from four NS-NS merger systems based on hydrodynamical simulations which handle neutrino effects in an elaborate way by including neutrino equilibration with matter in optically thick regions and re-absorption in optically thin regions. We find that the neutron richness of the dynamical ejecta is significantly affected by the neutrinos emitted by the post-merger remnant, in particular when compared to a case neglecting all neutrino interactions. Our nucleosynthesis results show that a solar-like distribution of r-process elements with mass numbers A greater than or similar to 90 is produced, including a significant enrichment in Sr and a reduced production of actinides compared to simulations without inclusion of the nucleonic weak processes. The composition of the dynamically ejected matter as well as the corresponding rate of radioactive decay heating are found to be rather independent of the system mass asymmetry and the adopted equation of state. This approximate degeneracy in abundance pattern and heating rates can be favourable for extracting the ejecta properties from kilonova observations, at least if the dynamical component dominates the overall ejecta. Part II of this work will study the light curve produced by the dynamical ejecta of our four NS merger models.

Automated summary

This study investigates the impact of neutrino interactions on r-process element nucleosynthesis and heating rate in the dynamical ejecta of neutron star-neutron star mergers. The material ejected from four merger systems is studied using hydrodynamical simulations that consider neutrino effects. The results show that neutrinos emitted by the post-merger remnant significantly affect the neutron richness of the dynamical ejecta and the nucleosynthesis of r-process elements. The composition of the ejecta and the rate of radioactive decay heating are found to be relatively independent of system parameters.