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

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 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.

Production of56Ni in black hole-neutron star merger accretion disc outflows

“…Element synthesis in the hot outflows from merger-type black hole accretion disks has previously been addressed in, e.g., [17,18,19,20,21,22]. These found neutron-rich outflows to be possible in this environment, and thus suggested that merger-type disk winds may be an important source of particularly the lighter r-process nuclei.…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…Optical and near-infrared observations of such an event, accompanying the short-duration GRB130603B have been reported [71] (see also [4]). After the first detailed nucleosynthesis predictions (following ideas of [33]) of such an event [13], many more and more sophisticated investigations have been undertaken, involving quite a number of authors [5,12,16,17,27,32,39,42,51,56,61,78] as well as the black hole accretion disk system after the formation of a central black hole [27,70,78,82].…”

“…Non-relativistic simulations (e.g. [32,61]) lead to large amounts of ejecta of the order 10 −2 M ⊙ with very small Y e -values of 0.04 and less. This causes a very strong r-process with fission cycling.…”

“…the dynamic ejecta), a hot and massive combined neutron star, forming (dependent on the equation of state) before collapsing to a black hole, evaporates a neutrino wind [39,56,61] with neutrino wind contributions from matter in more polar directions with Y e 's up to 0.4. These can contribute also the lighter r-process nuclei.…”

Origin of r-Process Elements and Galactic Chemodynamical Evolution

“…[28][29][30][31][32][33][34][35]). The ejected matter from NSMs is very neutron-rich (⟨Z/A⟩ ≡ Y e ∼ 0.1).…”