The long-term evolution of neutron star merger remnants - I. The impact of r-process nucleosynthesis

TL;DR

This study models the long-term evolution of neutron star merger remnants over 100 years, highlighting the effects of nuclear heating on dynamics and nucleosynthesis, and identifying robust production of heavy r-process elements and variable yields from neutrino-driven winds.

Contribution

It provides the first detailed long-term simulation including nuclear heating effects, showing their impact on remnant evolution and nucleosynthesis yields in neutron star mergers.

Findings

Nuclear heating smooths initial inhomogeneities in the ejecta.

Dynamic ejecta consistently produce heavy r-process elements with A > 130.

Neutrino-driven winds produce variable, weaker r-process contributions.

Abstract

We follow the longterm evolution of the dynamic ejecta of neutron star mergers for up to 100 years and over a density range of roughly 40 orders of magnitude. We include the nuclear energy input from the freshly synthesized, radioactively decaying nuclei in our simulations and study its effects on the remnant dynamics. Although the nuclear heating substantially alters the longterm evolution, we find that running nuclear networks over purely hydrodynamic simulations (i.e. without heating) yields actually acceptable nucleosynthesis results. The main dynamic effect of the radioactive heating is to quickly smooth out inhomogeneities in the initial mass distribution, subsequently the evolution proceeds self-similarly and after 100 years the remnant still carries the memory of the initial binary mass ratio. We also explore the nucleosynthetic yields for two mass ejection channels. The dynamic ejecta very robustly produce "strong" r-process elements with $A > 130$ with a pattern that is essentially independent of the details of the merging system. From a simple model we find that neutrino-driven winds yield "weak" r-process contributions with $50 < A < 130$ whose abundance patterns vary substantially between different merger cases. This is because their electron fraction, set by the ratio of neutrino luminosities, varies considerably from case to case. Such winds do not produce any $^{56}{\rm Ni}$, but a range of radioactive isotopes that are long-lived enough to produce a second, radioactively powered electromagnetic transient in addition to the "macronova" from the dynamic ejecta. While our wind model is very simple, it nevertheless demonstrates the potential of such neutrino-driven winds for electromagnetic transients and it motivates further, more detailed neutrino-hydrodynamic studies. The properties of the mentioned transients are discussed in more detail in a companion paper.